If you have a set of header files with include guards, is it really necessary to have forward declarations?
I've been doing this out of habit in my code, and I recommended a colleague add forward declarations to their code, but when they asked whether it was really necessary because of the include guards already present, I was at a bit of a loss. It seems like it's good practice, but wouldn't include guards solve all of the problems that forward declarations do?
Here's an example. Suppose I have a header file like so:
//This is the header file of class B
class MyClassA; //This is forward declaration
class B{
private:
MyClassA *myClass; //pointer to MyClassA
public:
B(){}
~B(){};
void print(MyClassA* myClass); //Passing by reference. Not calling any methods of MyClassA.
}
If I know that there is an include guard in MyClassA.h, would the above be better practice than the following? And, if so, why?
#include “MyClassA.h”
class B{
private:
MyClassA *myClass; //pointer to MyClassA
public:
B(){}
~B(){};
void print(MyClassA* myClass); //Passing by reference. Not calling any methods of MyClassA.
}
Would the second example also prevent a circular declaration/multiple declaration?
You are mixing two unrelated concepts.
You use forward declaration when you need something by name only.
You use include guards to prevent processing of the same code multiple times in a single translation unit, i.e. .cpp file. Whether you use include guards or not is based on the intended use of the .h file. Most of the time, you don't want the contents of the file to be processed multiple times in a .cpp file but there are use cases where allowing that is essential. For the later category of uses, you must not use include guards.
You can combine them anyway you want.
It's ok to:
Use forward declarations AND use include guards.
Use forward declarations AND not use include guards.
Not use forward declarations AND use include guards.
Not use forward declarations AND not use include guards.
Related
When I started learning C++ I learned that header files should typically be #included in other header files. Just now I had someone tell me that I should include a specific header file in my .cpp file to avoid header include creep.
Could someone tell me what exactly that is, why it is a problem and maybe point me to some documentation on when I would want to include a file in another header and when I should include one in a .cpp file?
The "creep" refers to the inclusion of one header including many others. This has some unwanted consequences:
a tendency towards every source file indirectly including every header, so that a change to any header requires everything to be recompiled;
more likelihood of circular dependencies causing heartache.
You can often avoid including one header from another by just declaring the classes you need, rather than including the header that gives the complete definition. Such an incomplete type can be used in various ways:
// Don't need this
//#include "thingy.h"
// just this
class thingy;
class whatsit {
// You can declare pointers and references
thingy * p;
thingy & r;
// You can declare static member variables (and external globals)
// They will need the header in the source file that defines them
static thingy s;
// You can declare (but not define) functions with incomplete
// parameter or return types
thingy f(thingy);
};
Some things do require the complete definition:
// Do need this
#include "thingy.h"
// Needed for inheritance
class whatsit : thingy {
// Needed for non-static member variables
thingy t;
// Needed to do many things like copying, accessing members, etc
thingy f() {return t;}
};
Could someone tell me what exactly [include creep] is
It's not a programming term, but interpreting it in an Engish-language context would imply that it's the introduction of #include statements that are not necessary.
why it is a problem
Because compiling code takes time. So compiling code that's not necessary takes unnecessary time.
and maybe point me to some documentation on when I would want to include a file in another header and when I should include one in a .cpp file?
If your header requires the definition of a type, you will need to include the header that defines that type.
#include "type.h"
struct TypeHolder
{
Type t;
// ^^^^ this value type requires the definition to know its size.
}
If your header only requires the declaration of a type, the definition is unnecessary, and so is the include.
class Type;
struct TypeHolder
{
Type * t;
// ^^^^^^ this pointer type has the already-known size of a pointer,
// so the definition is not required.
}
As an anecdote that supports the value of this practice, I was once put on a project whose codebase required ONE HOUR to fully compile. And changing a header often incurred most or all of that hour for the next compilation.
Adding forward-declarations to the headers where applicable instead of includes immediately reduced full compilation time to 16 minutes, and changing a header often didn't require a full rebuild.
Well they were probably referring to a couple of things ("include creep" is not a term I've ever heard before). If you, as an extreme rule, only include headers from headers (aside from one matching header per source file):
Compilation time increases as many headers must be compiled for all source files.
Unnecessary dependencies increase, e.g. with a dependency based build system, modifying one header may cause unnecessary recompilation of many other files, increasing compile time.
Increased possibility of namespace pollution.
Circular dependencies become an issue (two headers that need to include each other).
Other more advanced dependency-related topics (for example, this defeats the purpose of opaque pointers, which causes many design issues in certain types of applications).
As a general rule of thumb, include the bare minimum number of things from a header required to compile only the code in that header (that is, enough to allow the header to compile if it is included by itself, but no more than that). This will combat all of the above issues.
For example, if you have a source file for a class that uses std::list in the code, but the class itself has no members or function parameters that use std::list, there's no reason to #include <list> from the class's header, as nothing in the class's header uses std::list. Do it from the source file, where you actually need it, instead of in the header, where everything that uses the class also has to compile <list> unnecessarily.
Another common technique is forward declaring pointer types. This is the only real way to combat circular dependency issues, and has some of the compilation time benefits listed above as well. For example, if two classes refer to each other but only via pointers:
// A.h
class B; // forward declaration instead of #include "B.h"
class A {
B *b;
}
// B.h
class A; // forward declaration instead of #include "A.h"
class B {
A *a;
}
Then include the headers for each in the source files, where the definitions are actually needed.
Your header files should include all headers they need to compile when included alone (or first) in a source file. You may in many cases allow the header to compile by using forward declarations, but you should never rely on someone including a specific header before they include yours.
theres compie time to think of, but there's also dependancies. if A.h includes B.h and viceversa, code won't compile. You can get around this by forward referancing your class, then including the header in the cpp
A.h
class B;
class A
{
public:
void CreateNewB():
private:
B* m_b;
};
A.cpp
#include "B.h"
A::CreateNewB()
{
m_b = new B;
}
I'm pretty clear on when I can/can't use forward declaration but I'm still not sure about one thing.
Let's say I know that I have to include a header sooner or later to de-reference an object of class A.
I'm not clear on whether it's more efficient to do something like..
class A;
class B
{
A* a;
void DoSomethingWithA();
};
and then in the cpp have something like..
#include "A.hpp"
void B::DoSomethingWithA()
{
a->FunctionOfA();
}
Or might I as well just include A's header in B's header file in the first place?
If the former is more efficient then I'd appreciate it if someone clearly explained why as I suspect it has something to do with the compilation process which I could always do with learning more about.
Use forward declarations (as in your example) whenever possible. This reduces compile times, but more importantly minimizes header and library dependencies for code that doesn't need to know and doesn't care for implementation details. In general, no code other than the actual implementation should care about implementation details.
Here is Google's rationale on this: Header File Dependencies
When you use forward declaration, you explicitly say with it "class B doesn't need to know anything about internal implementation of class A, it only needs to know that class named A exists". If you can avoid including that header, then avoid it. - it's good practice to use forward declaration instead because you eliminate redundant dependencies by using it.
Also note, that when you change the header file, it causes all files that include it to be recompiled.
These questions will also help you:
What are the drawbacks of forward declaration?
What is the purpose of forward declaration?
Don't try to make your compilation efficient. There be dragons. Just include A.hpp in B.hpp.
The standard practice for C and C++ header files is to wrap all of the header file in an #ifndef to make sure it is compiled only once:
#ifndef _A_HPP_
#define _A_HPP_
// all your definitions
#endif
That way, if you #include "A.hpp" in B.hpp, you can have a program that includes both, and it won't break because it won't try to define anything twice.
I am sure this problem is asked a lot but I can't seem to find anything relevant. I have multiple source files in a C++ project. In typical fashion, there are multiple header files with class and function declarations and associated source files with their definitions. The problem is that when I try to use one of my classes defined in another file as a member for a class in a different file, I get compile errors even when uses the #include directive. What fixes the problem is by prototyping (is that the right word?) the class first before declaring it a member. So if ClassA is in one file and ClassB is in another and I want to use a ClassA member in ClassB I must write:
// ClassA.h
class ClassA {
public: ClassA (void); };
// ClassB.h
class ClassA; // prototype
class ClassB {
public: ClassA* ca; };
Is this normal? It doesn't matter if I use pointers or instances, I still must prototype them. I have found that I also must prototype structs and enums if they are in separate files. I can't seem to use constants declared using #define, or const across multiple files I get errors that they are undefined so I am not sure how to give them more than file scope. The same goes for typedefs. I am sure there is some easy fix to this that I am not remembering... any help is appreciated!
The word you’re looking for is “declare”, not “prototype”.
In any case, that’s normal. Usually you’d just include the relevant header files:
// ClassB.h
#include "ClassA.h"
class ClassB {
public: ClassA* ca; };
This will cause problems with circular references, though (but in all other cases it’s fine).
Another thing: Don’t forget your include guards, to protect against multiple inclusion of a file. That is, always write header files in the following way:
#ifndef UNIQUE_IDENTIFIER_HERE
#define UNIQUE_IDENTIFIER_HERE
// Rest of header file here.
#endif // ndef UNIQUE_IDENTIFIER_HERE
The UNIQUE_IDENTIFIER_HERE is usually an all-uppercase variant of the header file name, e.g. ‹PROJECTNAME›_‹PATH_TO_HEADER›_‹HEADERNAME›_H. For example, I’m currently working in a project (called “SeqAn”) that has a header file in the path parallel/taskdata.h. The unique identifier I use is thus SEQAN_PARALLEL_TASKDATA_H.
Yes it is obvious way.
This approach is most suitable when you're trying to avoid mutual dependencies among headers. If you're sure that no mutual dependency appears between two headers it is normal that you #include one in another.
To expand on Konrad's answer:
Yes, that's fine and dandy when you don't have to deal with circular references. If you do have to worry about circular inheritance, the diamond problem, or anything of the sort, here's a solution I found in someone else's question:
// ClassA.h
class ClassB;
class ClassA {
public:
ClassB* cb;
};
// ClassA.cpp
#include "ClassA.h"
#include "ClassB.h"
// code
// ClassB.h
#include "ClassA.h"
class ClassB {
public:
ClassA* ca;
};
// ClassB.cpp
#include "ClassB.h"
// code
As long as ClassA only tries to access members of ClassB inside member functions, and the member functions are all in the source file, you can just use a class declaration in the header, and leave the #include for the source file, where ClassA.h's include guard will solve the problem for you.
I'm using #pragma once, not #include guards on all my h files. What do I do if a.h needs to #include b.h and b.h needs to #include a.h?
I'm getting all sorts if errors because by doing this, the pragma once takes effect and one of them is missing each other. How should I do this.
Thanks
You need to forward declare the definitions you need. So if A uses B as a parameter value, you need to forward declare B, and vice versa.
It could be that just forward declaring the class names:
class A;
class B;
solves your problems.
The accepted answer to this question provides some additional guidance.
One possibility is to refactor some portion of the files a.h and b.h into a third file say c.h, and include it from both a.h and b.h. This way, the latter two would no longer need to mutually include each other.
Another possibility is to merge the separate header files into one.
A third possibility is the situation when two classes legitimately need to refer to each other. In such cases you have to use pointers. Moreover, you can forward declare the class instead of including its header file. [Mentioned also by jdv] For example,
// file a.h
struct B;
struct A { B * b_ };
// file b.h
struct A;
struct B { A * a_; };
However, without knowing your particular situation it is difficult to provide specific suggestion.
It depends on what is needed from each other's header file. IF it's a class definition, but it only is using a pointer to the class, then instead of including the head file just put in a forward declaration like:
class MyClassA;
The solution for this issue is 'forward declaration'.
If you have a class or a function that needs to be used in 2 headers one of the headers needs to forward declare the used class or type.
Or you need to consider to restructure your headers.
This is a common beginner issue that circular dependencies are causing such issues. If you google on 'forward declaration' will find tons of results.
Since your question was too unspecific I can't give you an exact answer, sorry for this.
You can not use incomplete types, but you can just forward declare them. You just tell the compiler:"Don't get syntax errors, I know what i am doing". Which means that the linker will go and find complete types from libraries whatsoever.
C++ headers
If I have A.cpp and A.h as well as b.h, c.h, d.h
Should I do:
in A.h:
#include "b.h"
#include "c.h"
#include "d.h"
in A.cpp:
#include "A.h"
or
in A.cpp:
#include "A.h"
#include "b.h"
#include "c.h"
#include "d.h"
Are there performance issues? Obvious benefits? Is something bad about this?
You should only include what is necessary to compile; adding unnecessary includes will hurt your compilation times, especially in large projects.
Each header file should be able to compile cleanly on its own -- that is, if you have a source file that includes only that header, it should compile without errors. The header file should include no more than is necessary for that.
Try to use forward declarations as much as possible. If you're using a class, but the header file only deals with pointers/references to objects of that class, then there's no need to include the definition of the class -- just use a forward declaration:
class SomeClass;
// Can now use pointers/references to SomeClass
// without needing the full definition
The key practice here is having around each foo.h file a guard such as:
#ifndef _FOO_H
#define _FOO_H
...rest of the .h file...
#endif
This prevents multiple-inclusions, with loops and all such attendant horrors. Once you do ensure every include file is thus guarded, the specifics are less important.
I like one guiding principle Adam expresses: make sure that, if a source file just includes a.h, it won't inevitably get errors due to a.h assuming other files have been included before it -- e.g. if a.h requires b.h to be included before, it can and should just include b.h itself (the guards will make that a noop if b.h was already included previously)
Vice versa, a source file should include the headers from which it is requiring something (macros, declarations, etc), not assume that other headers just come in magically because it has included some.
If you're using classes by value, alas, you need all the gory details of the class in some .h you include. But for some uses via references or pointers, just a bare class sic; will suffice. E.g., all other things being equal, if class a can get away with a pointer to an instance of class b (i.e. a member class b *my_bp; rather than a member class b *my_b;) the coupling between the include files can be made weaker (reducing much recompilation) -- e.g. b.h could have little more than class b; while all the gory details are in b_impl.h which is included only by the headers that really need it...
What Adam Rosenfield told you is spot on. An example of when you can use a forward declaration is:
#ifndef A_H_
#define A_H_
#include "D.h"
class B; //forward declaration
class C; //forward declaration
class A
{
B *m_pb; //you can forward declare this one bacause it's a pointer and the compilier doesn't need to know the size of object B at this point in the code. include B.h in the cpp file.
C &m_rc; //you can also forware declare this one for the same reason, except it's a reference.
D m_d; //you cannot forward declare this one because the complier need to calc the size of object D.
};
#endif
Answer: Let A.h include b.h, c.h and d.h only if it's needed to make the build succeed. The rule of thumb is: only include as much code in a header file as necessary. Anything which is not immediately needed in A.h should be included by A.cpp.
Reasoning: The less code is included into header files, the less likely you will need to recompile the code which uses the header file after doing some change somewhere. If there are lots of #include references between different header files, changing any of them will require rebuilding all other files which include the changed header file - recursivel. So if you decide to touch some toplevel header file, this might rebuild huge parts of your code.
By using forward declarations wherever possible in your header files, you reduce the coupling of the source files and thus make the build faster. Such forward declarations can be used in many more situations than you might think. As a rule of thumb, you only need the header file t.h (which defines a type T) if you
Declare a member variable of type T (note, this does not include declaring a pointer-to-T).
Write some inline functions which access members of an object of type T.
You do not need to include the declaration of T if your header file just
Declares constructors/functions which take references or pointers to a T object.
Declares functions which return a T object by pointer, reference or value.
Declares member variables which are references or pointers to a T object.
Consider this class declaration; which of the include files for A, B and C do you really need to include?:
class MyClass
{
public:
MyClass( const A &a );
void set( const B &b );
void set( const B *b );
B getB();
C getC();
private:
B *m_b;
C m_c;
};
You just need the include file for the type C, because of the member variable m_c. You can often remove this requirement as well by not declaring your member variables directly but using an opaque pointer to hide all the member variables in a private structure, so that they don't show up in the header file anymore.
Prefer not including headers in other headers - it slows down compilation and leas to circular reference
There would be no performance issues, it's all done at compile time, and your headers should be set up so they can't be included more than once.
I don't know if there's a standard way to do it, but I prefer to include all the headers I need in source files, and include them in the headers if something in the header itself needs it (for example, a typedef from a different header)