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I've never really understood why C++ needs a separate header file with the same functions as in the .cpp file. It makes creating classes and refactoring them very difficult, and it adds unnecessary files to the project. And then there is the problem with having to include header files, but having to explicitly check if it has already been included.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Follow up question:
How does the compiler find the .cpp file with the code in it, when all I include is the .h file? Does it assume that the .cpp file has the same name as the .h file, or does it actually look through all the files in the directory tree?
Some people consider header files an advantage:
It is claimed that it enables/enforces/allows separation of interface and implementation -- but usually, this is not the case. Header files are full of implementation details (for example member variables of a class have to be specified in the header, even though they're not part of the public interface), and functions can, and often are, defined inline in the class declaration in the header, again destroying this separation.
It is sometimes said to improve compile-time because each translation unit can be processed independently. And yet C++ is probably the slowest language in existence when it comes to compile-times. A part of the reason is the many many repeated inclusions of the same header. A large number of headers are included by multiple translation units, requiring them to be parsed multiple times.
Ultimately, the header system is an artifact from the 70's when C was designed. Back then, computers had very little memory, and keeping the entire module in memory just wasn't an option. A compiler had to start reading the file at the top, and then proceed linearly through the source code. The header mechanism enables this. The compiler doesn't have to consider other translation units, it just has to read the code from top to bottom.
And C++ retained this system for backwards compatibility.
Today, it makes no sense. It is inefficient, error-prone and overcomplicated. There are far better ways to separate interface and implementation, if that was the goal.
However, one of the proposals for C++0x was to add a proper module system, allowing code to be compiled similar to .NET or Java, into larger modules, all in one go and without headers. This proposal didn't make the cut in C++0x, but I believe it's still in the "we'd love to do this later" category. Perhaps in a TR2 or similar.
You seem to be asking about separating definitions from declarations, although there are other uses for header files.
The answer is that C++ doesn't "need" this. If you mark everything inline (which is automatic anyway for member functions defined in a class definition), then there is no need for the separation. You can just define everything in the header files.
The reasons you might want to separate are:
To improve build times.
To link against code without having the source for the definitions.
To avoid marking everything "inline".
If your more general question is, "why isn't C++ identical to Java?", then I have to ask, "why are you writing C++ instead of Java?" ;-p
More seriously, though, the reason is that the C++ compiler can't just reach into another translation unit and figure out how to use its symbols, in the way that javac can and does. The header file is needed to declare to the compiler what it can expect to be available at link time.
So #include is a straight textual substitution. If you define everything in header files, the preprocessor ends up creating an enormous copy and paste of every source file in your project, and feeding that into the compiler. The fact that the C++ standard was ratified in 1998 has nothing to do with this, it's the fact that the compilation environment for C++ is based so closely on that of C.
Converting my comments to answer your follow-up question:
How does the compiler find the .cpp file with the code in it
It doesn't, at least not at the time it compiles the code that used the header file. The functions you're linking against don't even need to have been written yet, never mind the compiler knowing what .cpp file they'll be in. Everything the calling code needs to know at compile time is expressed in the function declaration. At link time you will provide a list of .o files, or static or dynamic libraries, and the header in effect is a promise that the definitions of the functions will be in there somewhere.
C++ does it that way because C did it that way, so the real question is why did C do it that way? Wikipedia speaks a little to this.
Newer compiled languages (such as
Java, C#) do not use forward
declarations; identifiers are
recognized automatically from source
files and read directly from dynamic
library symbols. This means header
files are not needed.
To my (limited - I'm not a C developer normally) understanding, this is rooted in C. Remember that C does not know what classes or namespaces are, it's just one long program. Also, functions have to be declared before you use them.
For example, the following should give a compiler error:
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
The error should be that "SomeOtherFunction is not declared" because you call it before it's declaration. One way of fixing this is by moving SomeOtherFunction above SomeFunction. Another approach is to declare the functions signature first:
void SomeOtherFunction();
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
This lets the compiler know: Look somewhere in the code, there is a function called SomeOtherFunction that returns void and does not take any parameters. So if you encouter code that tries to call SomeOtherFunction, do not panic and instead go looking for it.
Now, imagine you have SomeFunction and SomeOtherFunction in two different .c files. You then have to #include "SomeOther.c" in Some.c. Now, add some "private" functions to SomeOther.c. As C does not know private functions, that function would be available in Some.c as well.
This is where .h Files come in: They specify all the functions (and variables) that you want to 'Export' from a .c file that can be accessed in other .c files. That way, you gain something like a Public/Private scope. Also, you can give this .h file to other people without having to share your source code - .h files work against compiled .lib files as well.
So the main reason is really for convenience, for source code protection and to have a bit of decoupling between the parts of your application.
That was C though. C++ introduced Classes and private/public modifiers, so while you could still ask if they are needed, C++ AFAIK still requires declaration of functions before using them. Also, many C++ Developers are or were C devleopers as well and took over their concepts and habits to C++ - why change what isn't broken?
First advantage: If you don't have header files, you would have to include source files in other source files. This would cause the including files to be compiled again when the included file changes.
Second advantage: It allows sharing the interfaces without sharing the code between different units (different developers, teams, companies etc..)
The need for header files results from the limitations that the compiler has for knowing about the type information for functions and or variables in other modules. The compiled program or library does not include the type information required by the compiler to bind to any objects defined in other compilation units.
In order to compensate for this limitation, C and C++ allow for declarations and these declarations can be included into modules that use them with the help of the preprocessor's #include directive.
Languages like Java or C# on the other hand include the information necessary for binding in the compiler's output (class-file or assembly). Hence, there is no longer a need for maintaining standalone declarations to be included by clients of a module.
The reason for the binding information not being included in the compiler output is simple: it is not needed at runtime (any type checking occurs at compile time). It would just waste space. Remember that C/C++ come from a time where the size of an executable or library did matter quite a bit.
Well, C++ was ratified in 1998, but it had been in use for a lot longer than that, and the ratification was primarily setting down current usage rather than imposing structure. And since C++ was based on C, and C has header files, C++ has them too.
The main reason for header files is to enable separate compilation of files, and minimize dependencies.
Say I have foo.cpp, and I want to use code from the bar.h/bar.cpp files.
I can #include "bar.h" in foo.cpp, and then program and compile foo.cpp even if bar.cpp doesn't exist. The header file acts as a promise to the compiler that the classes/functions in bar.h will exist at run-time, and it has everything it needs to know already.
Of course, if the functions in bar.h don't have bodies when I try to link my program, then it won't link and I'll get an error.
A side-effect is that you can give users a header file without revealing your source code.
Another is that if you change the implementation of your code in the *.cpp file, but do not change the header at all, you only need to compile the *.cpp file instead of everything that uses it. Of course, if you put a lot of implementation into the header file, then this becomes less useful.
C++ was designed to add modern programming language features to the C infrastructure, without unnecessarily changing anything about C that wasn't specifically about the language itself.
Yes, at this point (10 years after the first C++ standard and 20 years after it began seriously growing in usage) it is easy to ask why doesn't it have a proper module system. Obviously any new language being designed today would not work like C++. But that isn't the point of C++.
The point of C++ is to be evolutionary, a smooth continuation of existing practise, only adding new capabilities without (too often) breaking things that work adequately for its user community.
This means that it makes some things harder (especially for people starting a new project), and some things easier (especially for those maintaining existing code) than other languages would do.
So rather than expecting C++ to turn into C# (which would be pointless as we already have C#), why not just pick the right tool for the job? Myself, I endeavour to write significant chunks of new functionality in a modern language (I happen to use C#), and I have a large amount of existing C++ that I am keeping in C++ because there would be no real value in re-writing it all. They integrate very nicely anyway, so it's largely painless.
It doesn't need a separate header file with the same functions as in main. It only needs it if you develop an application using multiple code files and if you use a function that was not previously declared.
It's really a scope problem.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Actually header files become very useful when examining programs for the first time, checking out header files(using only a text editor) gives you an overview of the architecture of the program, unlike other languages where you have to use sophisticated tools to view classes and their member functions.
If you want the compiler to find out symbols defined in other files automatically, you need to force programmer to put those files in predefined locations (like Java packages structure determines folders structure of the project). I prefer header files. Also you would need either sources of libraries you use or some uniform way to put information needed by compiler in binaries.
I think the real (historical) reason behind header files was making like easier for compiler developers... but then, header files do give advantages.
Check this previous post for more discussions...
Well, you can perfectly develop C++ without header files. In fact some libraries that intensively use templates does not use the header/code files paradigm (see boost). But In C/C++ you can not use something that is not declared. One practical way to
deal with that is to use header files. Plus, you gain the advantage of sharing interface whithout sharing code/implementation. And I think it was not envisionned by the C creators : When you use shared header files you have to use the famous :
#ifndef MY_HEADER_SWEET_GUARDIAN
#define MY_HEADER_SWEET_GUARDIAN
// [...]
// my header
// [...]
#endif // MY_HEADER_SWEET_GUARDIAN
that is not really a language feature but a practical way to deal with multiple inclusion.
So, I think that when C was created, the problems with forward declaration was underestimated and now when using a high level language like C++ we have to deal with this sort of things.
Another burden for us poor C++ users ...
I've never really understood why C++ needs a separate header file with the same functions as in the .cpp file. It makes creating classes and refactoring them very difficult, and it adds unnecessary files to the project. And then there is the problem with having to include header files, but having to explicitly check if it has already been included.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Follow up question:
How does the compiler find the .cpp file with the code in it, when all I include is the .h file? Does it assume that the .cpp file has the same name as the .h file, or does it actually look through all the files in the directory tree?
Some people consider header files an advantage:
It is claimed that it enables/enforces/allows separation of interface and implementation -- but usually, this is not the case. Header files are full of implementation details (for example member variables of a class have to be specified in the header, even though they're not part of the public interface), and functions can, and often are, defined inline in the class declaration in the header, again destroying this separation.
It is sometimes said to improve compile-time because each translation unit can be processed independently. And yet C++ is probably the slowest language in existence when it comes to compile-times. A part of the reason is the many many repeated inclusions of the same header. A large number of headers are included by multiple translation units, requiring them to be parsed multiple times.
Ultimately, the header system is an artifact from the 70's when C was designed. Back then, computers had very little memory, and keeping the entire module in memory just wasn't an option. A compiler had to start reading the file at the top, and then proceed linearly through the source code. The header mechanism enables this. The compiler doesn't have to consider other translation units, it just has to read the code from top to bottom.
And C++ retained this system for backwards compatibility.
Today, it makes no sense. It is inefficient, error-prone and overcomplicated. There are far better ways to separate interface and implementation, if that was the goal.
However, one of the proposals for C++0x was to add a proper module system, allowing code to be compiled similar to .NET or Java, into larger modules, all in one go and without headers. This proposal didn't make the cut in C++0x, but I believe it's still in the "we'd love to do this later" category. Perhaps in a TR2 or similar.
You seem to be asking about separating definitions from declarations, although there are other uses for header files.
The answer is that C++ doesn't "need" this. If you mark everything inline (which is automatic anyway for member functions defined in a class definition), then there is no need for the separation. You can just define everything in the header files.
The reasons you might want to separate are:
To improve build times.
To link against code without having the source for the definitions.
To avoid marking everything "inline".
If your more general question is, "why isn't C++ identical to Java?", then I have to ask, "why are you writing C++ instead of Java?" ;-p
More seriously, though, the reason is that the C++ compiler can't just reach into another translation unit and figure out how to use its symbols, in the way that javac can and does. The header file is needed to declare to the compiler what it can expect to be available at link time.
So #include is a straight textual substitution. If you define everything in header files, the preprocessor ends up creating an enormous copy and paste of every source file in your project, and feeding that into the compiler. The fact that the C++ standard was ratified in 1998 has nothing to do with this, it's the fact that the compilation environment for C++ is based so closely on that of C.
Converting my comments to answer your follow-up question:
How does the compiler find the .cpp file with the code in it
It doesn't, at least not at the time it compiles the code that used the header file. The functions you're linking against don't even need to have been written yet, never mind the compiler knowing what .cpp file they'll be in. Everything the calling code needs to know at compile time is expressed in the function declaration. At link time you will provide a list of .o files, or static or dynamic libraries, and the header in effect is a promise that the definitions of the functions will be in there somewhere.
C++ does it that way because C did it that way, so the real question is why did C do it that way? Wikipedia speaks a little to this.
Newer compiled languages (such as
Java, C#) do not use forward
declarations; identifiers are
recognized automatically from source
files and read directly from dynamic
library symbols. This means header
files are not needed.
To my (limited - I'm not a C developer normally) understanding, this is rooted in C. Remember that C does not know what classes or namespaces are, it's just one long program. Also, functions have to be declared before you use them.
For example, the following should give a compiler error:
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
The error should be that "SomeOtherFunction is not declared" because you call it before it's declaration. One way of fixing this is by moving SomeOtherFunction above SomeFunction. Another approach is to declare the functions signature first:
void SomeOtherFunction();
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
This lets the compiler know: Look somewhere in the code, there is a function called SomeOtherFunction that returns void and does not take any parameters. So if you encouter code that tries to call SomeOtherFunction, do not panic and instead go looking for it.
Now, imagine you have SomeFunction and SomeOtherFunction in two different .c files. You then have to #include "SomeOther.c" in Some.c. Now, add some "private" functions to SomeOther.c. As C does not know private functions, that function would be available in Some.c as well.
This is where .h Files come in: They specify all the functions (and variables) that you want to 'Export' from a .c file that can be accessed in other .c files. That way, you gain something like a Public/Private scope. Also, you can give this .h file to other people without having to share your source code - .h files work against compiled .lib files as well.
So the main reason is really for convenience, for source code protection and to have a bit of decoupling between the parts of your application.
That was C though. C++ introduced Classes and private/public modifiers, so while you could still ask if they are needed, C++ AFAIK still requires declaration of functions before using them. Also, many C++ Developers are or were C devleopers as well and took over their concepts and habits to C++ - why change what isn't broken?
First advantage: If you don't have header files, you would have to include source files in other source files. This would cause the including files to be compiled again when the included file changes.
Second advantage: It allows sharing the interfaces without sharing the code between different units (different developers, teams, companies etc..)
The need for header files results from the limitations that the compiler has for knowing about the type information for functions and or variables in other modules. The compiled program or library does not include the type information required by the compiler to bind to any objects defined in other compilation units.
In order to compensate for this limitation, C and C++ allow for declarations and these declarations can be included into modules that use them with the help of the preprocessor's #include directive.
Languages like Java or C# on the other hand include the information necessary for binding in the compiler's output (class-file or assembly). Hence, there is no longer a need for maintaining standalone declarations to be included by clients of a module.
The reason for the binding information not being included in the compiler output is simple: it is not needed at runtime (any type checking occurs at compile time). It would just waste space. Remember that C/C++ come from a time where the size of an executable or library did matter quite a bit.
Well, C++ was ratified in 1998, but it had been in use for a lot longer than that, and the ratification was primarily setting down current usage rather than imposing structure. And since C++ was based on C, and C has header files, C++ has them too.
The main reason for header files is to enable separate compilation of files, and minimize dependencies.
Say I have foo.cpp, and I want to use code from the bar.h/bar.cpp files.
I can #include "bar.h" in foo.cpp, and then program and compile foo.cpp even if bar.cpp doesn't exist. The header file acts as a promise to the compiler that the classes/functions in bar.h will exist at run-time, and it has everything it needs to know already.
Of course, if the functions in bar.h don't have bodies when I try to link my program, then it won't link and I'll get an error.
A side-effect is that you can give users a header file without revealing your source code.
Another is that if you change the implementation of your code in the *.cpp file, but do not change the header at all, you only need to compile the *.cpp file instead of everything that uses it. Of course, if you put a lot of implementation into the header file, then this becomes less useful.
C++ was designed to add modern programming language features to the C infrastructure, without unnecessarily changing anything about C that wasn't specifically about the language itself.
Yes, at this point (10 years after the first C++ standard and 20 years after it began seriously growing in usage) it is easy to ask why doesn't it have a proper module system. Obviously any new language being designed today would not work like C++. But that isn't the point of C++.
The point of C++ is to be evolutionary, a smooth continuation of existing practise, only adding new capabilities without (too often) breaking things that work adequately for its user community.
This means that it makes some things harder (especially for people starting a new project), and some things easier (especially for those maintaining existing code) than other languages would do.
So rather than expecting C++ to turn into C# (which would be pointless as we already have C#), why not just pick the right tool for the job? Myself, I endeavour to write significant chunks of new functionality in a modern language (I happen to use C#), and I have a large amount of existing C++ that I am keeping in C++ because there would be no real value in re-writing it all. They integrate very nicely anyway, so it's largely painless.
It doesn't need a separate header file with the same functions as in main. It only needs it if you develop an application using multiple code files and if you use a function that was not previously declared.
It's really a scope problem.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Actually header files become very useful when examining programs for the first time, checking out header files(using only a text editor) gives you an overview of the architecture of the program, unlike other languages where you have to use sophisticated tools to view classes and their member functions.
If you want the compiler to find out symbols defined in other files automatically, you need to force programmer to put those files in predefined locations (like Java packages structure determines folders structure of the project). I prefer header files. Also you would need either sources of libraries you use or some uniform way to put information needed by compiler in binaries.
I think the real (historical) reason behind header files was making like easier for compiler developers... but then, header files do give advantages.
Check this previous post for more discussions...
Well, you can perfectly develop C++ without header files. In fact some libraries that intensively use templates does not use the header/code files paradigm (see boost). But In C/C++ you can not use something that is not declared. One practical way to
deal with that is to use header files. Plus, you gain the advantage of sharing interface whithout sharing code/implementation. And I think it was not envisionned by the C creators : When you use shared header files you have to use the famous :
#ifndef MY_HEADER_SWEET_GUARDIAN
#define MY_HEADER_SWEET_GUARDIAN
// [...]
// my header
// [...]
#endif // MY_HEADER_SWEET_GUARDIAN
that is not really a language feature but a practical way to deal with multiple inclusion.
So, I think that when C was created, the problems with forward declaration was underestimated and now when using a high level language like C++ we have to deal with this sort of things.
Another burden for us poor C++ users ...
I've never really understood why C++ needs a separate header file with the same functions as in the .cpp file. It makes creating classes and refactoring them very difficult, and it adds unnecessary files to the project. And then there is the problem with having to include header files, but having to explicitly check if it has already been included.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Follow up question:
How does the compiler find the .cpp file with the code in it, when all I include is the .h file? Does it assume that the .cpp file has the same name as the .h file, or does it actually look through all the files in the directory tree?
Some people consider header files an advantage:
It is claimed that it enables/enforces/allows separation of interface and implementation -- but usually, this is not the case. Header files are full of implementation details (for example member variables of a class have to be specified in the header, even though they're not part of the public interface), and functions can, and often are, defined inline in the class declaration in the header, again destroying this separation.
It is sometimes said to improve compile-time because each translation unit can be processed independently. And yet C++ is probably the slowest language in existence when it comes to compile-times. A part of the reason is the many many repeated inclusions of the same header. A large number of headers are included by multiple translation units, requiring them to be parsed multiple times.
Ultimately, the header system is an artifact from the 70's when C was designed. Back then, computers had very little memory, and keeping the entire module in memory just wasn't an option. A compiler had to start reading the file at the top, and then proceed linearly through the source code. The header mechanism enables this. The compiler doesn't have to consider other translation units, it just has to read the code from top to bottom.
And C++ retained this system for backwards compatibility.
Today, it makes no sense. It is inefficient, error-prone and overcomplicated. There are far better ways to separate interface and implementation, if that was the goal.
However, one of the proposals for C++0x was to add a proper module system, allowing code to be compiled similar to .NET or Java, into larger modules, all in one go and without headers. This proposal didn't make the cut in C++0x, but I believe it's still in the "we'd love to do this later" category. Perhaps in a TR2 or similar.
You seem to be asking about separating definitions from declarations, although there are other uses for header files.
The answer is that C++ doesn't "need" this. If you mark everything inline (which is automatic anyway for member functions defined in a class definition), then there is no need for the separation. You can just define everything in the header files.
The reasons you might want to separate are:
To improve build times.
To link against code without having the source for the definitions.
To avoid marking everything "inline".
If your more general question is, "why isn't C++ identical to Java?", then I have to ask, "why are you writing C++ instead of Java?" ;-p
More seriously, though, the reason is that the C++ compiler can't just reach into another translation unit and figure out how to use its symbols, in the way that javac can and does. The header file is needed to declare to the compiler what it can expect to be available at link time.
So #include is a straight textual substitution. If you define everything in header files, the preprocessor ends up creating an enormous copy and paste of every source file in your project, and feeding that into the compiler. The fact that the C++ standard was ratified in 1998 has nothing to do with this, it's the fact that the compilation environment for C++ is based so closely on that of C.
Converting my comments to answer your follow-up question:
How does the compiler find the .cpp file with the code in it
It doesn't, at least not at the time it compiles the code that used the header file. The functions you're linking against don't even need to have been written yet, never mind the compiler knowing what .cpp file they'll be in. Everything the calling code needs to know at compile time is expressed in the function declaration. At link time you will provide a list of .o files, or static or dynamic libraries, and the header in effect is a promise that the definitions of the functions will be in there somewhere.
C++ does it that way because C did it that way, so the real question is why did C do it that way? Wikipedia speaks a little to this.
Newer compiled languages (such as
Java, C#) do not use forward
declarations; identifiers are
recognized automatically from source
files and read directly from dynamic
library symbols. This means header
files are not needed.
To my (limited - I'm not a C developer normally) understanding, this is rooted in C. Remember that C does not know what classes or namespaces are, it's just one long program. Also, functions have to be declared before you use them.
For example, the following should give a compiler error:
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
The error should be that "SomeOtherFunction is not declared" because you call it before it's declaration. One way of fixing this is by moving SomeOtherFunction above SomeFunction. Another approach is to declare the functions signature first:
void SomeOtherFunction();
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
This lets the compiler know: Look somewhere in the code, there is a function called SomeOtherFunction that returns void and does not take any parameters. So if you encouter code that tries to call SomeOtherFunction, do not panic and instead go looking for it.
Now, imagine you have SomeFunction and SomeOtherFunction in two different .c files. You then have to #include "SomeOther.c" in Some.c. Now, add some "private" functions to SomeOther.c. As C does not know private functions, that function would be available in Some.c as well.
This is where .h Files come in: They specify all the functions (and variables) that you want to 'Export' from a .c file that can be accessed in other .c files. That way, you gain something like a Public/Private scope. Also, you can give this .h file to other people without having to share your source code - .h files work against compiled .lib files as well.
So the main reason is really for convenience, for source code protection and to have a bit of decoupling between the parts of your application.
That was C though. C++ introduced Classes and private/public modifiers, so while you could still ask if they are needed, C++ AFAIK still requires declaration of functions before using them. Also, many C++ Developers are or were C devleopers as well and took over their concepts and habits to C++ - why change what isn't broken?
First advantage: If you don't have header files, you would have to include source files in other source files. This would cause the including files to be compiled again when the included file changes.
Second advantage: It allows sharing the interfaces without sharing the code between different units (different developers, teams, companies etc..)
The need for header files results from the limitations that the compiler has for knowing about the type information for functions and or variables in other modules. The compiled program or library does not include the type information required by the compiler to bind to any objects defined in other compilation units.
In order to compensate for this limitation, C and C++ allow for declarations and these declarations can be included into modules that use them with the help of the preprocessor's #include directive.
Languages like Java or C# on the other hand include the information necessary for binding in the compiler's output (class-file or assembly). Hence, there is no longer a need for maintaining standalone declarations to be included by clients of a module.
The reason for the binding information not being included in the compiler output is simple: it is not needed at runtime (any type checking occurs at compile time). It would just waste space. Remember that C/C++ come from a time where the size of an executable or library did matter quite a bit.
Well, C++ was ratified in 1998, but it had been in use for a lot longer than that, and the ratification was primarily setting down current usage rather than imposing structure. And since C++ was based on C, and C has header files, C++ has them too.
The main reason for header files is to enable separate compilation of files, and minimize dependencies.
Say I have foo.cpp, and I want to use code from the bar.h/bar.cpp files.
I can #include "bar.h" in foo.cpp, and then program and compile foo.cpp even if bar.cpp doesn't exist. The header file acts as a promise to the compiler that the classes/functions in bar.h will exist at run-time, and it has everything it needs to know already.
Of course, if the functions in bar.h don't have bodies when I try to link my program, then it won't link and I'll get an error.
A side-effect is that you can give users a header file without revealing your source code.
Another is that if you change the implementation of your code in the *.cpp file, but do not change the header at all, you only need to compile the *.cpp file instead of everything that uses it. Of course, if you put a lot of implementation into the header file, then this becomes less useful.
C++ was designed to add modern programming language features to the C infrastructure, without unnecessarily changing anything about C that wasn't specifically about the language itself.
Yes, at this point (10 years after the first C++ standard and 20 years after it began seriously growing in usage) it is easy to ask why doesn't it have a proper module system. Obviously any new language being designed today would not work like C++. But that isn't the point of C++.
The point of C++ is to be evolutionary, a smooth continuation of existing practise, only adding new capabilities without (too often) breaking things that work adequately for its user community.
This means that it makes some things harder (especially for people starting a new project), and some things easier (especially for those maintaining existing code) than other languages would do.
So rather than expecting C++ to turn into C# (which would be pointless as we already have C#), why not just pick the right tool for the job? Myself, I endeavour to write significant chunks of new functionality in a modern language (I happen to use C#), and I have a large amount of existing C++ that I am keeping in C++ because there would be no real value in re-writing it all. They integrate very nicely anyway, so it's largely painless.
It doesn't need a separate header file with the same functions as in main. It only needs it if you develop an application using multiple code files and if you use a function that was not previously declared.
It's really a scope problem.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Actually header files become very useful when examining programs for the first time, checking out header files(using only a text editor) gives you an overview of the architecture of the program, unlike other languages where you have to use sophisticated tools to view classes and their member functions.
If you want the compiler to find out symbols defined in other files automatically, you need to force programmer to put those files in predefined locations (like Java packages structure determines folders structure of the project). I prefer header files. Also you would need either sources of libraries you use or some uniform way to put information needed by compiler in binaries.
I think the real (historical) reason behind header files was making like easier for compiler developers... but then, header files do give advantages.
Check this previous post for more discussions...
Well, you can perfectly develop C++ without header files. In fact some libraries that intensively use templates does not use the header/code files paradigm (see boost). But In C/C++ you can not use something that is not declared. One practical way to
deal with that is to use header files. Plus, you gain the advantage of sharing interface whithout sharing code/implementation. And I think it was not envisionned by the C creators : When you use shared header files you have to use the famous :
#ifndef MY_HEADER_SWEET_GUARDIAN
#define MY_HEADER_SWEET_GUARDIAN
// [...]
// my header
// [...]
#endif // MY_HEADER_SWEET_GUARDIAN
that is not really a language feature but a practical way to deal with multiple inclusion.
So, I think that when C was created, the problems with forward declaration was underestimated and now when using a high level language like C++ we have to deal with this sort of things.
Another burden for us poor C++ users ...
I've never really understood why C++ needs a separate header file with the same functions as in the .cpp file. It makes creating classes and refactoring them very difficult, and it adds unnecessary files to the project. And then there is the problem with having to include header files, but having to explicitly check if it has already been included.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Follow up question:
How does the compiler find the .cpp file with the code in it, when all I include is the .h file? Does it assume that the .cpp file has the same name as the .h file, or does it actually look through all the files in the directory tree?
Some people consider header files an advantage:
It is claimed that it enables/enforces/allows separation of interface and implementation -- but usually, this is not the case. Header files are full of implementation details (for example member variables of a class have to be specified in the header, even though they're not part of the public interface), and functions can, and often are, defined inline in the class declaration in the header, again destroying this separation.
It is sometimes said to improve compile-time because each translation unit can be processed independently. And yet C++ is probably the slowest language in existence when it comes to compile-times. A part of the reason is the many many repeated inclusions of the same header. A large number of headers are included by multiple translation units, requiring them to be parsed multiple times.
Ultimately, the header system is an artifact from the 70's when C was designed. Back then, computers had very little memory, and keeping the entire module in memory just wasn't an option. A compiler had to start reading the file at the top, and then proceed linearly through the source code. The header mechanism enables this. The compiler doesn't have to consider other translation units, it just has to read the code from top to bottom.
And C++ retained this system for backwards compatibility.
Today, it makes no sense. It is inefficient, error-prone and overcomplicated. There are far better ways to separate interface and implementation, if that was the goal.
However, one of the proposals for C++0x was to add a proper module system, allowing code to be compiled similar to .NET or Java, into larger modules, all in one go and without headers. This proposal didn't make the cut in C++0x, but I believe it's still in the "we'd love to do this later" category. Perhaps in a TR2 or similar.
You seem to be asking about separating definitions from declarations, although there are other uses for header files.
The answer is that C++ doesn't "need" this. If you mark everything inline (which is automatic anyway for member functions defined in a class definition), then there is no need for the separation. You can just define everything in the header files.
The reasons you might want to separate are:
To improve build times.
To link against code without having the source for the definitions.
To avoid marking everything "inline".
If your more general question is, "why isn't C++ identical to Java?", then I have to ask, "why are you writing C++ instead of Java?" ;-p
More seriously, though, the reason is that the C++ compiler can't just reach into another translation unit and figure out how to use its symbols, in the way that javac can and does. The header file is needed to declare to the compiler what it can expect to be available at link time.
So #include is a straight textual substitution. If you define everything in header files, the preprocessor ends up creating an enormous copy and paste of every source file in your project, and feeding that into the compiler. The fact that the C++ standard was ratified in 1998 has nothing to do with this, it's the fact that the compilation environment for C++ is based so closely on that of C.
Converting my comments to answer your follow-up question:
How does the compiler find the .cpp file with the code in it
It doesn't, at least not at the time it compiles the code that used the header file. The functions you're linking against don't even need to have been written yet, never mind the compiler knowing what .cpp file they'll be in. Everything the calling code needs to know at compile time is expressed in the function declaration. At link time you will provide a list of .o files, or static or dynamic libraries, and the header in effect is a promise that the definitions of the functions will be in there somewhere.
C++ does it that way because C did it that way, so the real question is why did C do it that way? Wikipedia speaks a little to this.
Newer compiled languages (such as
Java, C#) do not use forward
declarations; identifiers are
recognized automatically from source
files and read directly from dynamic
library symbols. This means header
files are not needed.
To my (limited - I'm not a C developer normally) understanding, this is rooted in C. Remember that C does not know what classes or namespaces are, it's just one long program. Also, functions have to be declared before you use them.
For example, the following should give a compiler error:
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
The error should be that "SomeOtherFunction is not declared" because you call it before it's declaration. One way of fixing this is by moving SomeOtherFunction above SomeFunction. Another approach is to declare the functions signature first:
void SomeOtherFunction();
void SomeFunction() {
SomeOtherFunction();
}
void SomeOtherFunction() {
printf("What?");
}
This lets the compiler know: Look somewhere in the code, there is a function called SomeOtherFunction that returns void and does not take any parameters. So if you encouter code that tries to call SomeOtherFunction, do not panic and instead go looking for it.
Now, imagine you have SomeFunction and SomeOtherFunction in two different .c files. You then have to #include "SomeOther.c" in Some.c. Now, add some "private" functions to SomeOther.c. As C does not know private functions, that function would be available in Some.c as well.
This is where .h Files come in: They specify all the functions (and variables) that you want to 'Export' from a .c file that can be accessed in other .c files. That way, you gain something like a Public/Private scope. Also, you can give this .h file to other people without having to share your source code - .h files work against compiled .lib files as well.
So the main reason is really for convenience, for source code protection and to have a bit of decoupling between the parts of your application.
That was C though. C++ introduced Classes and private/public modifiers, so while you could still ask if they are needed, C++ AFAIK still requires declaration of functions before using them. Also, many C++ Developers are or were C devleopers as well and took over their concepts and habits to C++ - why change what isn't broken?
First advantage: If you don't have header files, you would have to include source files in other source files. This would cause the including files to be compiled again when the included file changes.
Second advantage: It allows sharing the interfaces without sharing the code between different units (different developers, teams, companies etc..)
The need for header files results from the limitations that the compiler has for knowing about the type information for functions and or variables in other modules. The compiled program or library does not include the type information required by the compiler to bind to any objects defined in other compilation units.
In order to compensate for this limitation, C and C++ allow for declarations and these declarations can be included into modules that use them with the help of the preprocessor's #include directive.
Languages like Java or C# on the other hand include the information necessary for binding in the compiler's output (class-file or assembly). Hence, there is no longer a need for maintaining standalone declarations to be included by clients of a module.
The reason for the binding information not being included in the compiler output is simple: it is not needed at runtime (any type checking occurs at compile time). It would just waste space. Remember that C/C++ come from a time where the size of an executable or library did matter quite a bit.
Well, C++ was ratified in 1998, but it had been in use for a lot longer than that, and the ratification was primarily setting down current usage rather than imposing structure. And since C++ was based on C, and C has header files, C++ has them too.
The main reason for header files is to enable separate compilation of files, and minimize dependencies.
Say I have foo.cpp, and I want to use code from the bar.h/bar.cpp files.
I can #include "bar.h" in foo.cpp, and then program and compile foo.cpp even if bar.cpp doesn't exist. The header file acts as a promise to the compiler that the classes/functions in bar.h will exist at run-time, and it has everything it needs to know already.
Of course, if the functions in bar.h don't have bodies when I try to link my program, then it won't link and I'll get an error.
A side-effect is that you can give users a header file without revealing your source code.
Another is that if you change the implementation of your code in the *.cpp file, but do not change the header at all, you only need to compile the *.cpp file instead of everything that uses it. Of course, if you put a lot of implementation into the header file, then this becomes less useful.
C++ was designed to add modern programming language features to the C infrastructure, without unnecessarily changing anything about C that wasn't specifically about the language itself.
Yes, at this point (10 years after the first C++ standard and 20 years after it began seriously growing in usage) it is easy to ask why doesn't it have a proper module system. Obviously any new language being designed today would not work like C++. But that isn't the point of C++.
The point of C++ is to be evolutionary, a smooth continuation of existing practise, only adding new capabilities without (too often) breaking things that work adequately for its user community.
This means that it makes some things harder (especially for people starting a new project), and some things easier (especially for those maintaining existing code) than other languages would do.
So rather than expecting C++ to turn into C# (which would be pointless as we already have C#), why not just pick the right tool for the job? Myself, I endeavour to write significant chunks of new functionality in a modern language (I happen to use C#), and I have a large amount of existing C++ that I am keeping in C++ because there would be no real value in re-writing it all. They integrate very nicely anyway, so it's largely painless.
It doesn't need a separate header file with the same functions as in main. It only needs it if you develop an application using multiple code files and if you use a function that was not previously declared.
It's really a scope problem.
C++ was ratified in 1998, so why is it designed this way? What advantages does having a separate header file have?
Actually header files become very useful when examining programs for the first time, checking out header files(using only a text editor) gives you an overview of the architecture of the program, unlike other languages where you have to use sophisticated tools to view classes and their member functions.
If you want the compiler to find out symbols defined in other files automatically, you need to force programmer to put those files in predefined locations (like Java packages structure determines folders structure of the project). I prefer header files. Also you would need either sources of libraries you use or some uniform way to put information needed by compiler in binaries.
I think the real (historical) reason behind header files was making like easier for compiler developers... but then, header files do give advantages.
Check this previous post for more discussions...
Well, you can perfectly develop C++ without header files. In fact some libraries that intensively use templates does not use the header/code files paradigm (see boost). But In C/C++ you can not use something that is not declared. One practical way to
deal with that is to use header files. Plus, you gain the advantage of sharing interface whithout sharing code/implementation. And I think it was not envisionned by the C creators : When you use shared header files you have to use the famous :
#ifndef MY_HEADER_SWEET_GUARDIAN
#define MY_HEADER_SWEET_GUARDIAN
// [...]
// my header
// [...]
#endif // MY_HEADER_SWEET_GUARDIAN
that is not really a language feature but a practical way to deal with multiple inclusion.
So, I think that when C was created, the problems with forward declaration was underestimated and now when using a high level language like C++ we have to deal with this sort of things.
Another burden for us poor C++ users ...
For whatever reason, our company has a coding guideline that states:
Each class shall have it's own header and implementation file.
So if we wrote a class called MyString we would need an associated MyStringh.h and MyString.cxx.
Does anyone else do this? Has anyone seen any compiling performance repercussions as a result? Does 5000 classes in 10000 files compile just as quickly as 5000 classes in 2500 files? If not, is the difference noticeable?
[We code C++ and use GCC 3.4.4 as our everyday compiler]
The term here is translation unit and you really want to (if possible) have one class per translation unit ie, one class implementation per .cpp file, with a corresponding .h file of the same name.
It's usually more efficient (from a compile/link) standpoint to do things this way, especially if you're doing things like incremental link and so forth. The idea being, translation units are isolated such that, when one translation unit changes, you don't have to rebuild a lot of stuff, as you would have to if you started lumping many abstractions into a single translation unit.
Also you'll find many errors/diagnostics are reported via file name ("Error in Myclass.cpp, line 22") and it helps if there's a one-to-one correspondence between files and classes. (Or I suppose you could call it a 2 to 1 correspondence).
Overwhelmed by thousands lines of code?
Having one set of header/source files per class in a directory can seem overkill. And if the number of classes goes toward 100 or 1000, it can even be frightening.
But having played with sources following the philosophy "let's put together everything", the conclusion is that only the one who wrote the file has any hope to not be lost inside. Even with an IDE, it is easy to miss things because when you're playing with a source of 20,000 lines, you just close your mind for anything not exactly referring to your problem.
Real life example: the class hierarchy defined in those thousand lines sources closed itself into a diamond-inheritance, and some methods were overridden in child classes by methods with exactly the same code. This was easily overlooked (who wants to explore/check a 20,000 lines source code?), and when the original method was changed (bug correction), the effect was not as universal as excepted.
Dependancies becoming circular?
I had this problem with templated code, but I saw similar problems with regular C++ and C code.
Breaking down your sources into 1 header per struct/class lets you:
Speed up compilation because you can use symbol forward-declaration instead of including whole objects
Have circular dependencies between classes (§) (i.e. class A has a pointer to B, and B has a pointer to A)
In source-controlled code, class dependencies could lead to regular moving of classes up and down the file, just to make the header compile. You don't want to study the evolution of such moves when comparing the same file in different versions.
Having separate headers makes the code more modular, faster to compile, and makes it easier to study its evolution through different versions diffs
For my template program, I had to divide my headers into two files: The .HPP file containing the template class declaration/definition, and the .INL file containing the definitions of the said class methods.
Putting all this code inside one and only one unique header would mean putting class definitions at the beginning of this file, and the method definitions at the end.
And then, if someone needed only a small part of the code, with the one-header-only solution, they still would have to pay for the slower compilation.
(§) Note that you can have circular dependencies between classes if you know which class owns which. This is a discussion about classes having knowledge of the existence of other classes, not shared_ptr circular dependencies antipattern.
One last word: Headers should be self-sufficients
One thing, though, that must be respected by a solution of multiple headers and multiple sources.
When you include one header, no matter which header, your source must compile cleanly.
Each header should be self-sufficient. You're supposed to develop code, not treasure-hunting by greping your 10,000+ source files project to find which header defines the symbol in the 1,000 lines header you need to include just because of one enum.
This means that either each header defines or forward-declare all the symbols it uses, or include all the needed headers (and only the needed headers).
Question about circular dependencies
underscore-d asks:
Can you explain how using separate headers makes any difference to circular dependencies? I don't think it does. We can trivially create a circular dependency even if both classes are fully declared in the same header, simply by forward-declaring one in advance before we declare a handle to it in the other. Everything else seems to be great points, but the idea that separate headers facilitate circular dependencies seems way off
underscore_d, Nov 13 at 23:20
Let's say you have 2 class templates, A and B.
Let's say the definition of class A (resp. B) has a pointer to B (resp. A). Let's also say the methods of class A (resp. B) actually call methods from B (resp. A).
You have a circular dependency both in the definition of the classes, and the implementations of their methods.
If A and B were normal classes, and A and B's methods were in .CPP files, there would be no problem: You would use a forward declaration, have a header for each class definitions, then each CPP would include both HPP.
But as you have templates, you actually have to reproduce that patterns above, but with headers only.
This means:
a definition header A.def.hpp and B.def.hpp
an implementation header A.inl.hpp and B.inl.hpp
for convenience, a "naive" header A.hpp and B.hpp
Each header will have the following traits:
In A.def.hpp (resp. B.def.hpp), you have a forward declaration of class B (resp. A), which will enable you to declare a pointer/reference to that class
A.inl.hpp (resp. B.inl.hpp) will include both A.def.hpp and B.def.hpp, which will enable methods from A (resp. B) to use the class B (resp. A).
A.hpp (resp. B.hpp) will directly include both A.def.hpp and A.inl.hpp (resp. B.def.hpp and B.inl.hpp)
Of course, all headers need to be self sufficient, and protected by header guards
The naive user will include A.hpp and/or B.hpp, thus ignoring the whole mess.
And having that organization means the library writer can solve the circular dependencies between A and B while keeping both classes in separate files, easy to navigate once you understand the scheme.
Please note that it was an edge case (two templates knowing each other). I expect most code to not need that trick.
We do that at work, its just easier to find stuff if the class and files have the same name. As for performance, you really shouldn't have 5000 classes in a single project. If you do, some refactoring might be in order.
That said, there are instances when we have multiple classes in one file. And that is when it's just a private helper class for the main class of the file.
+1 for separation. I just came onto a project where some classes are in files with a different name, or lumped in with another class, and it is impossible to find these in a quick and efficient manner. You can throw more resources at a build - you can't make up lost programmer time because (s)he can't find the right file to edit.
In addition to simply being "clearer", separating classes into separate files makes it easier for multiple developers not to step on each others toes. There will be less merging when it comes time to commit changes to your version control tool.
Most places where I have worked have followed this practice. I've actually written coding standards for BAE (Aust.) along with the reasons why instead of just carving something in stone with no real justification.
Concerning your question about source files, it's not so much time to compile but more an issue of being able to find the relevant code snippet in the first place. Not everyone is using an IDE. And knowing that you just look for MyClass.h and MyClass.cpp really saves time compared to running "grep MyClass *.(h|cpp)" over a bunch of files and then filtering out the #include MyClass.h statements...
Mind you there are work-arounds for the impact of large numbers of source files on compile times. See Large Scale C++ Software Design by John Lakos for an interesting discussion.
You might also like to read Code Complete by Steve McConnell for an excellent chapter on coding guidelines. Actualy, this book is a great read that I keep coming back to regularly.
N.B. You need the first edition of Code Complete that is easily available online for a copy. The interesting section on coding and naming guidelines didn't make it into Code Complete 2.
It's common practice to do this, especially to be able to include .h in the files that need it. Of course the performance is affected but try not to think about this problem until it arises :).
It's better to start with the files separated and after that try to merge the .h's that are commonly used together to improve performance if you really need to. It all comes down to dependencies between files and this is very specific to each project.
The best practice, as others have said, is to place each class in its own translation unit from a code maintenance and understandability perspective. However on large scale systems this is sometimes not advisable - see the section entitled "Make Those Source Files Bigger" in this article by Bruce Dawson for a discussion of the tradeoffs.
The same rule applies here, but it notes a few exceptions where it is allowed Like so:
Inheritance trees
Classes that are only used within a very limited scope
Some Utilities are simply placed in a general 'utils.h'
It is very helpful to have only have one class per file, but if you do your building via bulkbuild files which include all the individual C++ files, it makes for faster compilations since startup time is relatively large for many compilers.
I found these guidelines particularly useful when it comes to header files :
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Header_Files
Two words: Ockham's Razor. Keep one class per file with the corresponding header in a separate file. If you do otherwise, like keeping a piece of functionality per file, then you have to create all kinds of rules about what constitutes a piece of functionality. There is much more to gain by keeping one class per file. And, even half decent tools can handle large quantities of files. Keep it simple my friend.
I'm surprised that almost everyone is in favor of having one file per class. The problem with that is that in the age of 'refactoring' one may have a hard time keeping the file and class names in synch. Everytime you change a class name, you then have to change the file name too, which means that you have to also make a change everywhere the file is included.
I personally group related classes into a single files and then give such a file a meaningful name that won't have to change even if a class name changes. Having fewer files also makes scrolling through a file tree easier.
I use Visual Studio on Windows and Eclipse CDT on Linux, and both have shortcut keys that take you straight to a class declaration, so finding a class declaration is easy and quick.
Having said that, I think once a project is completed, or its structure has 'solidified', and name changes become rare, it may make sense to have one class per file. I wish there was a tool that could extract classes and place them in distinct .h and .cpp files. But I don't see this as essential.
The choice also depends on the type of project one works on. In my opinion the issue doesn't deserve a black and white answer since either choice has pros and cons.