According to Why can't I multi-declare a class
class A; is a declaration while
class A { ... } is a definition
Typically, in header files, we define the class and we implement its member functions in the .cpp. But wouldn't defining classes in the header file violate the One Definition Rule?
According to https://www.learncpp.com/cpp-tutorial/89-class-code-and-header-files/
Doesn’t defining a class in a header file violate the one-definition rule?
It shouldn’t. If your header file has proper header guards, it
shouldn’t be possible to include the class definition more than once
into the same file.
Types (which include classes), are exempt from the
part of the one-definition rule that says you can only have one
definition per program. Therefore, there isn’t an issue #including
class definitions into multiple code files (if there was, classes
wouldn’t be of much use).
While the first part is obviously true, header guards will prevent multiple definitions in the same file, but I am confused about the second part of the answer that addresses my question.
If a header file has a definition of a class, for example ThisClass, and that header file is included in two other files for example a.cpp and b.cpp. Why wouldn't it violate the One Definition Rule? If a ThisClass object is created in either file, which definition would be called?
If a header file has a definition of a class, for example ThisClass, and that header file is included in two other files for example a.cpp and b.cpp. Why wouldn't it violate the One Definition Rule?
Because, as you quoted, the one definition rule specifically allows for this.
How can it? Read on…
If a ThisClass object is created in either file, which definition would be called?
It doesn't matter, because the definitions are required to be absolutely, lexically identical.
If they're not, your program has undefined behaviour and you can expect all manner of weirdness to ensue.
Based on what I'm reading, it might be important to use Wikipedia's brief explanation of what the One Definition Rule actually is:
The One Definition Rule (ODR) is an important rule of the C++ programming language that prescribes that objects and non-inline functions cannot have more than one definition in the entire program and template and types cannot have more than one definition by translation unit.
This means that when the program is compiled, there can be one, and only one definition of a function or object. This might be confusing, however a class or type shouldn't be thought of as objects, they're really just blueprints for an object.
In your question, I think there's a misunderstanding of what #include does and how the ODR applies to classes. When the program is compiling, the .cpp files look to their #include'd class definitions as a way to link together, that is, the class.h blueprint must know where to look when calling it's member functions. Having multiple #include's of the same class.h is not the same as having multiple definitions of class.h, and thus, it cannot violate the One Definition Rule.
It is okay to have a class defined in a header file because the compiler treats the functions declared in the class in such a case as an inline functions.
So there are two options to happen when you call this class functions:
Or it wiil inline the function calls and in such a case there is no problem at all of multiple definition.
Or it wont inline them and in such a case he will create the symbols related to the functions as weak symbols. Then the linker will choose one of the symbols to be the symbol representing the function.
Of course if there will be different defintions in two or more cpp files there will be undefined behaviour.
Related
For example you have such situation:
1)Header file with some class definition that contain some member function (def.h).
2)That header is included into .cpp files with the same name that resides in a separate folders.
3)That .cpp files impements same function from def.h but in different way.
3)Then we include that header into some main.cpp and call function with multiple impementation and compile that whole project.
Question: will it compile without errors? What function implementation will be chosen?
Defining a function in two separate places violates One Definition Rule.
One and only one definition of every non-inline function or variable that is odr-used (see below) is required to appear in the entire program (including any standard and user-defined libraries). The compiler is not required to diagnose this violation, but the behavior of the program that violates it is undefined.
To have well-defined behavior, you should either:
Link just one of the definitions in the final program
Use overloads instead
Use polymorphism/inheritance instead
I'm working my way through a C++ tutorial, and in one of the articles on the preprocessor/header files the author creates 2 'header files' (not the technical term, of course), example.h and example.cpp. In example.h he puts the forward declarations for the functions and the header guard, and in example.cpp the actual body of the functions. Why is this? I tried putting the body of the function in the example.h file and it worked just fine, so why does he put it separately? Is it customary, or does it pose problems in bigger problems? Or what?
Can someone please enlighten me on this...
C++ has a rule called the one-definition rule that says that every function needs to be defined once and only once (there are a few exceptions to this rule, but we'll ignore them for now.) The function prototype typically included in a header file is a declaration saying that the function exists, and the implementation in the .cpp file is the definition giving the code for the function.
If you put the definition of the function in a header file and then include the header file in multiple places, you'll get linker errors because you're breaking the one definition rule - each .cpp file that's compiled will have its own copy of the definition of the function. On the other hand, if you just put the declaration in the header and then put the definition in a single source file, then there's only one definition and nothing will break.
There are a few exceptions for the one definition rule. First, all inline functions are exempt from ODR, so you could potentially put function definitions in a header file if you mark all the functions inline, though this is generally not considered a good idea for all but the smallest functions. Second, template functions are exempt from the ODR, which (among other things) is one of the reasons you see template functions defined in headers.
Put the function body in example.h. Then, when you write a second .cpp file that also includes example.h, and link it with example.cpp, then you'll get a linker error. The linker will see 2 definitions of the function in example.h.
Becouse in the header file you only declare functions that is really important when you want to use a custom linking configuration. For example if a function is defined in a file object and is used in an another object the linker must have the same reference for this function to link the two different files.
After reading this question I thought I understood everything, but then I saw this file from a popular header-only library.
The library uses the #ifndef line, but the SO question points out that this is NOT adequate protection against multiple definition errors in multiple TUs.
So one of the following must be true:
It is possible to avoid multiple definition linker errors in ways other than described in the SO question. Perhaps the library is using techniques not mentioned in to the other SO question that are worthy of additional explanation.
The library assumes you won't include its header files in more than translation unit -- this seems fragile since a robust library shouldn't make this assumption on its users.
I'd appreciate having some light shed on this seemingly simple curiosity.
A header that causes linking problems when included in multiple translation units is one that will (attempt to) define some object (not just, for an obvious example, a type) in each source file where it's included.
For example, if you had something like: int f = 0; in a header, then each source file into which it was included would attempt to define f, and when you tried to link the object files together you'd get a complaint about multiple definitions of f.
The "technique" used in this header is simple: it doesn't attempt to define any actual objects. Rather, it includes some typedefs, and the definition of one fairly large class--but not any instances of that class or any instance of anything else either. That class includes a number of member functions, but they're all defined inside the function definition, which implicitly defines them as inline functions (so defining separately in each translation unit in which they're used is not only allowed, but required).
In short, the header only defines types, not objects, so there's nothing there to cause linker collisions when it's included in multiple source files that are linked together.
If the header defines items, as opposed to just declaring them, then it's possible to include it in more than one translation unit (i.e. cpp file) and have multiple definitions and hence linker errors.
I've used boost's unit test framework which is header only. I include a specified header in only one of my own cpp files to get my project to compile. But I include other unit test headers in other cpp files which presumably use the items that are defined in the specified header.
Headers only include libraries like Boost C++ Libraries use (mostly) stand-alone templates and as so are compiled at compile-time and don't require any linkage to binary libraries (that would need separate compilation). One designed to never need linkage is the great Catch
Templates are a special case in C++ regarding multiple definitions, as long as they're the same. See the "One Definition Rule" section of the C++ standard:
There can be more than one definition of a class type (Clause 9),
enumeration type (7.2), inline function with external linkage (7.1.2),
class template (Clause 14), non-static function template (14.5.6),
static data member of a class template (14.5.1.3), member function of
a class template (14.5.1.1), or template specialization for which some
template parameters are not specified (14.7, 14.5.5) in a program
provided that each definition appears in a different translation unit,
and provided the definitions satisfy the following requirements. ....
This is then followed by a list of conditions that make sure the template definitions are identical across translation units.
This specific quote is from the 2014 working draft, section 3.2 ("One Definition Rule"), subsection 6.
This header file can indeed be included in difference source files without causing "multiple symbol definition" errors.
This happens because it is fine to have multiple identically named symbols in different object files as long as these symbols are either weak or local.
Let's take a closer look at the header file. It (potentially) defines several objects like this helper:
static int const helper[] = {0,7,8,13};
Each translation unit that includes this header file will have this helper in it. However, there will be no "multiple symbol definition" errors, since helper is static and thus has internal linkage. The symbols created for helpers will be local and linker will just happily put them all in the resulting executable.
The header file also defines a class template connection. But it is also okay. Class templates can be defined multiple times in different translation units.
In fact, even regular class types can be defined multiple times (I've noticed that you've asked about this in the comments). Symbols created for member functions are usually weak symbols. Once again, weak symbols don't cause "multiple symbol definition" errors, because they can be redefined. Linker will just keep redefining weak symbols with names he has already seen until there will be just one symbol per member function left.
There are also other cases, where certain things (like inline functions and enumerations) can be defined several times in different translation units (see §3.2). And mechanisms of achieving this can be different (see class templates and inline functions). But the general rule is not to place stuff with global linkage in header files. As long as you follow this rule, you're really unlikely to stumble upon multiple symbol definitions problems.
And yes, include guards have nothing to do with this.
Okay, up until now, I thought that functions defined in header files are treated like inline functions, just like template stuff, defined once, and all that.
I also use inclusion guards, yet I still got linker errors of multiple defined objects, and I know that is because of all those different units duplicating stuff the linker tries to pick out which item is the right one.
I also know that inline is merely a suggestion, and might not even get used by the compiler, etc.
Yet I have to explicitly define all those small functions in that little header only toolset I wrote.
Even if the functions were huge, I'd have to declare them inline, and the compiler would still possibly disregard the hint.
Yet I have to define them so anyway.
Example:
#ifndef texture_math_h__
#define texture_math_h__
float TexcoordToPixel(float coord, float dimension)
{
return coord * dimension;
}
float PixelToTexcoord(float pixel, float dimension)
{
return pixel / dimension;
}
float RecalcTexcoord(float coord,float oldDimension, float newDimension)
{
return PixelToTexcoord(TexcoordToPixel(coord,oldDimension),newDimension);
}
#endif // texture_math_h__
Errors are , blabla already defined in xxx.obj, for each unit that includes the file
When I declare all of those inline, it links correctly.
What's the reason for that? It's not a huge problem, and heck, optimizations probably inline stuff found in cpp, too, right?
I'm just curious about the why here, hope it's not too much of a duplicate and thank you for your time.
Inclusion guards only guard against the code being included twice in the same translation unit. So if you have multiple files that include the same header, the code is included multiple times. Functions defined in a header are not inline by default, so this will give you linker errors - you need to define those function with the inline keyword, unless they are member functions of a class.
Also, note that names that contain double-underscores are reserved for the purposes of the C++ implementation - you are not allowed to create such names in your own code.
Member functions are potencially inlined - you can't force inlining! - if (a) they are defined inside the class or if (b) you use the inline-clause in the definition. Note if you are using the inline-clause you shouldn't define the function in the header - the only exception would be templates as these are "special".
As you just updated the question:
Every user of this header will have a definition of the functions -> multiple defintions. You need to separate definition and declaration!
It's all about the one definition rule. This states that you can only have one definition for each non-inline function (along with various other types of entity) in a C++ program across all the translation units that you link in to make the program.
Marking a function inline enables an exception to the usual one definition rule. It states (paraphrased) that you can have one definition of an inline function per translation unit provided that all the definitions match and that a definition is provided in each translation in which the inline function is used.
Include guards will prevent you from accidentally providing more than one definition per translation unit by including the header file containing the definitions multiple times.
To satisfy the one definition rule for a non-inline function you would still have to ensure that there is only one translation unit containing the function definitions. The usualy way to do this is by only declaring the functions in the header file and having a single source file containing the definitions.
When we design classes in Java, Vala, or C# we put the definition and declaration in the same source file. But in C++ it is traditionally preferred to separate the definition and declaration in two or more files.
What happens if I just use a header file and put everything into it, like Java?
Is there a performance penalty or something?
The answer depends on what kind of class you're creating.
C++'s compilation model dates back to the days of C, and so its method of importing data from one source file into another is comparatively primitive. The #include directive literally copies the contents of the file you're including into the source file, then treats the result as though it was the file you had written all along. You need to be careful about this because of a C++ policy called the one definition rule (ODR) which states, unsurprisingly, that every function and class should have at most one definition. This means that if you declare a class somewhere, all of that class's member functions should be either not defined at all or defined exactly once in exactly one file. There are some exceptions (I'll get to them in a minute), but for now just treat this rule as if it's a hard-and-fast, no-exceptions rule.
If you take a non-template class and put both the class definition and the implementation into a header file, you might run into trouble with the one definition rule. In particular, suppose that I have two different .cpp files that I compile, both of which #include your header containing both the implementation and the interface. In this case, if I try linking those two files together, the linker will find that each one contains a copy of the implementation code for the class's member functions. At this point, the linker will report an error because you have violated the one definition rule: there are two different implementations of all the class's member functions.
To prevent this, C++ programmers typically split classes up into a header file which contains the class declaration, along with the declarations of its member functions, without the implementations of those functions. The implementations are then put into a separate .cpp file which can be compiled and linked separately. This allows your code to avoid running into trouble with the ODR. Here's how. First, whenever you #include the class header file into multiple different .cpp files, each of them just gets a copy of the declarations of the member functions, not their definitions, and so none of your class's clients will end up with the definitions. This means that any number of clients can #include your header file without running into trouble at link-time. Since your own .cpp file with the implementation is the sole file that contains the implementations of the member functions, at link time you can merge it with any number of other client object files without a hassle. This is the main reason that you split the .h and .cpp files apart.
Of course, the ODR has a few exceptions. The first of these comes up with template functions and classes. The ODR explicitly states that you can have multiple different definitions for the same template class or function, provided that they're all equivalent. This is primarily to make it easier to compile templates - each C++ file can instantiate the same template without colliding with any other files. For this reason, and a few other technical reasons, class templates tend to just have a .h file without a matching .cpp file. Any number of clients can #include the file without trouble.
The other major exception to the ODR involves inline functions. The spec specifically states that the ODR does not apply to inline functions, so if you have a header file with an implementation of a class member function that's marked inline, that's perfectly fine. Any number of files can #include this file without breaking the ODR. Interestingly, any member function that's declared and defined in the body of a class is implicitly inline, so if you have a header like this:
#ifndef Include_Guard
#define Include_Guard
class MyClass {
public:
void DoSomething() {
/* ... code goes here ... */
}
};
#endif
Then you're not risking breaking the ODR. If you rewrite this as
#ifndef Include_Guard
#define Include_Guard
class MyClass {
public:
void DoSomething();
};
void MyClass::DoSomething() {
/* ... code goes here ... */
}
#endif
then you would be breaking the ODR, since the member function isn't marked inline and if multiple clients #include this file there will be multiple definitions of MyClass::DoSomething.
So to summarize - you should probably split up your classes into a .h/.cpp pair to avoid breaking the ODR. However, if you're writing a class template, you don't need the .cpp file (and probably shouldn't have one at all), and if you're okay marking every single member function of your class inline you can also avoid the .cpp file.
The drawback of putting definition in header files is as follows:-
Header file A - contains definition of metahodA()
Header file B - includes header file A.
Now let us say you change the definition of methodA. You would need to compile file A as well as B because of the inclusion of header file A in B.
The biggest difference is that every function is declared as an inline function. Generally your compiler will be smart enough that this won't be a problem, but worst case scenario it will cause page faults on a regular basis and make your code embarrassingly slow. Usually the code is separated for design reasons, and not for performance.
In general, it is a good practice to seperate implementation from headers. However, there are exceptions in cases like templates where the implementation goes in the header itself.
Two particular problems with putting everything in the header:
Compile times will be increased, sometimes greatly. C++ compile times are long enough that that's not something you want.
If you have circular dependencies in the implementation, keeping everything in headers is difficult to impossible. eg:
header1.h
struct C1
{
void f();
void g();
};
header2.h
struct C2
{
void f();
void g();
};
impl1.cpp
#include "header1.h"
#include "header2.h"
void C1::f()
{
C2 c2;
c2.f();
}
impl2.cpp
#include "header2.h"
#include "header1.h"
void C2::g()
{
C1 c1;
c1.g();
}