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;
}
Related
I was just wondering where is it necessary/right to include a specific header file according to the example below. Let's assume I have a definition of an exception class:
//exc.hpp
#ifndef EXC_H
#define EXC_H
class MyException : public exception {
};
#endif /* EXC_H */
Then I have another class definition throwing such exception:
//a.cpp
void SomeClass::someMethod(void) {
throw MyException(...);
}
And having another file handling that exception, e.g.:
//main.cpp
#include "a.hpp"
int main() {
...
catch(MyException & e) { ... }
}
So my question is, where should I place #include "exc.hpp"? Just to a.hpp, or both a.hpp and main.cpp?
And when it comes to makefile... How should be the targets specified within such file?
Every translation unit that throws or catches exceptions of that type will need to be able to see its definition.
Loosely speaking, that means every .cpp file containing a throw or catch relating to that exception must include your .hpp file.
That's even if you only catch by reference and never inspect the exception object, which is not the case in other areas of C++ (where a forward declaration will do).
Makefiles are unrelated.
Both the file with implementation (a.cpp) and all files the use the class (main.cpp) should have the #include.
a.cpp without the include must not compile at all.
You need to include the .hpp file in both main.cpp and a.cpp. The purpose of the #ifndef sequence is to prevent accidental multiple inclusion (through indirect #includes). There is also a #pragma once directive in MS compilers that does the same thing.
The compiler figures out what .h/.hpp files to read based on the #includes in the .cpp files; the make file is not involved.
Remember that the compiler processes each source file independently, and doesn't remember anything from the source file once it's done processing. Even if you list several source files on a single compiler command-line.
You have a header-file that defines a type. Naturally, you must #include the header file in every source file where you need that type to be defined. (The compiler will not remember having seen the types when processing earlier source files.)
It might be tempting to #include headers within other headers, just so you don't have to #include so many things within the .c or .cpp files, but this should be avoided to the degree possible. It produces what is known as "header coupling", and it makes code that is hard to re-use later on other projects.
There's also a fine point hiding in what I said above: "where you need that type to be defined". There are two very specific concepts in C and C++ related to variables:
declaration -- when you make the compiler aware that a type exists, and
definition -- where you tell the compiler the details of the type.
You need to #include your header wherever you need the definition. I.e., when you intend to instantiate an object of the type, define members of the type in another struct or class, or call one of its methods (assuming C++). If instead, you only want to store a reference to objects of the type without creating or using them, a declaration will suffice, and you can just forward-declare the class. I.e.,
class MyException;
void setFileNotFoundExceptionObject(const MyException *exc) { ... }
Many APIs are designed specifically around only using pointers or references to objects so that the API headers only need forward delcarations of the types, and not the complete definitions (This keeps the internal members of the objects hidden to prevent developers from abusing them.)
Thinking Time - Why do you want to split your file anyway?
As the title suggests, the end problem I have is multiple definition linker errors. I have actually fixed the problem, but I haven't fixed the problem in the correct way. Before starting I want to discuss the reasons for splitting a class file into multiple files. I have tried to put all the possible scenarios here - if I missed any, please remind me and I can make changes. Hopefully the following are correct:
Reason 1 To save space:
You have a file containing the declaration of a class with all class members. You place #include guards around this file (or #pragma once) to ensure no conflicts arise if you #include the file in two different header files which are then included in a source file. You compile a separate source file with the implementation of any methods declared in this class, as it offloads many lines of code from your source file, which cleans things up a bit and introduces some order to your program.
Example: As you can see, the below example could be improved by splitting the implementation of the class methods into a different file. (A .cpp file)
// my_class.hpp
#pragma once
class my_class
{
public:
void my_function()
{
// LOTS OF CODE
// CONFUSING TO DEBUG
// LOTS OF CODE
// DISORGANIZED AND DISTRACTING
// LOTS OF CODE
// LOOKS HORRIBLE
// LOTS OF CODE
// VERY MESSY
// LOTS OF CODE
}
// MANY OTHER METHODS
// MEANS VERY LARGE FILE WITH LOTS OF LINES OF CODE
}
Reason 2 To prevent multiple definition linker errors:
Perhaps this is the main reason why you would split implementation from declaration. In the above example, you could move the method body to outside the class. This would make it look much cleaner and structured. However, according to this question, the above example has implicit inline specifiers. Moving the implementation from within the class to outside the class, as in the example below, will cause you linker errors, and so you would either inline everything, or move the function definitions to a .cpp file.
Example: _The example below will cause "multiple definition linker errors" if you do not move the function definition to a .cpp file or specify the function as inline.
// my_class.hpp
void my_class::my_function()
{
// ERROR! MULTIPLE DEFINITION OF my_class::my_function
// This error only occurs if you #include the file containing this code
// in two or more separate source (compiled, .cpp) files.
}
To fix the problem:
//my_class.cpp
void my_class::my_function()
{
// Now in a .cpp file, so no multiple definition error
}
Or:
// my_class.hpp
inline void my_class::my_function()
{
// Specified function as inline, so okay - note: back in header file!
// The very first example has an implicit `inline` specifier
}
Reason 3 You want to save space, again, but this time you are working with a template class:
If we are working with template classes, then we cannot move the implementation to a source file (.cpp file). That's not currently allowed by (I assume) either the standard or by current compilers. Unlike the first example of Reason 2, above, we are allowed to place the implementation in the header file. According to this question the reason is that template class methods also have implied inline specifiers. Is that correct? (It seems to make sense.) But nobody seemed to know on the question I have just referenced!
So, are the two examples below identical?
// some_header_file.hpp
#pragma once
// template class declaration goes here
class some_class
{
// Some code
};
// Example 1: NO INLINE SPECIFIER
template<typename T>
void some_class::class_method()
{
// Some code
}
// Example 2: INLINE specifier used
template<typename T>
inline void some_class::class_method()
{
// Some code
}
If you have a template class header file, which is becoming huge due to all the functions you have, then I believe you are allowed to move the function definitions to another header file (usually a .tpp file?) and then #include file.tpp at the end of your header file containing the class declaration. You must NOT include this file anywhere else, however, hence the .tpp rather than .hpp.
I assume you could also do this with the inline methods of a regular class? Is that allowed also?
Question Time
So I have made some statements above, most of which relate to the structuring of source files. I think everything I said was correct, because I did some basic research and "found out some stuff", but this is a question and so I don't know for sure.
What this boils down to, is how you would organize code within files. I think I have figured out a structure which will always work.
Here is what I have come up with. (This is my class code file organization/structure standard, if you like. Don't know if it will be very useful yet, that's the point of asking.)
1: Declare the class (template or otherwise) in a .hpp file, including all methods, friend functions and data.
2: At the bottom of the .hpp file, #include a .tpp file containing the implementation of any inline methods. Create the .tpp file and ensure all methods are specified to be inline.
3: All other members (non-inline functions, friend functions and static data) should be defined in a .cpp file, which #includes the .hpp file at the top to prevent errors like "class ABC has not been declared". Since everything in this file will have external linkage, the program will link correctly.
Do standards like this exist in industry? Will the standard I came up with work in all cases?
Your three points sound about right. That's the standard way to do things (although I've not seen .tpp extension before, usually it's .inl), although personally I just put inline functions at the bottom of header files rather than in a separate file.
Here is how I arrange my files. I omit the forward declare file for simple classes.
myclass-fwd.h
#pragma once
namespace NS
{
class MyClass;
}
myclass.h
#pragma once
#include "headers-needed-by-header"
#include "myclass-fwd.h"
namespace NS
{
class MyClass
{
..
};
}
myclass.cpp
#include "headers-needed-by-source"
#include "myclass.h"
namespace
{
void LocalFunc();
}
NS::MyClass::...
Replace pragma with header guards according to preference..
The reason for this approach is to reduce header dependencies, which slow down compile times in large projects. If you didn't know, you can forward declare a class to use as a pointer or reference. The full declaration is only needed when you construct, create or use members of the class.
This means another class which uses the class (takes parameters by pointer/reference) only has to include the fwd header in its own header. The full header is then included in the second class's source file. This greatly reduces the amount of unneeded rubbish you get when pulling in a big header, which pulls in another big header, which pulls in another...
The next tip is the unnamed namespace (sometimes called anonymous namespace). This can only appear in a source file and it is like a hidden namespace only visible to that file. You can place local functions, classes etc here which are only used by the the source file. This prevents name clashes if you create something with the same name in two different files. (Two local function F for example, may give linker errors).
The main reason to separate interface from implementation is so that you don't have to recompile all of your code when something in the implementation changes; you only have to recompile the source files that changed.
As for "Declare the class (template or otherwise)", a template is not a class. A template is a pattern for creating classes. More important, though, you define a class or a template in a header. The class definition includes declarations of its member functions, and non-inine member functions are defined in one or more source files. Inline member functions and all template functions should be defined in the header, by whatever combination of direct definitions and #include directives you prefer.
Do standards like this exist in industry?
Yes. Then again, coding standards that are rather different from the ones you expressed can also be found in industry. You are talking about coding standards, after all, and coding standards range from good to bad to ugly.
Will the standard I came up with work in all cases?
Absolutely not. For example,
template <typename T> class Foo {
public:
void some_method (T& arg);
...
};
Here, the definition of class template Foo doesn't know a thing about that template parameter T. What if, for some class template, the definitions of the methods vary depending on the template parameters? Your rule #2 just doesn't work here.
Another example: What if the corresponding source file is huge, a thousand lines long or longer? At times it makes sense to provide the implementation in multiple source files. Some standards go to the extreme of dictating one function per file (personal opinion: Yech!).
At the other extreme of a thousand-plus line long source file is a class that has no source files. The entire implementation is in the header. There's a lot to be said for header-only implementations. If nothing else, it simplifies, sometimes significantly, the linking problem.
Some have the habit of adding header file imports/includes to the header file. Some on the other hand, write a forward declaration in the header file and write the actual #include or #import lines in the implementation file.
Is there a standard practice for this? Which is better and why?
Given X.h and X.c, if you #include everything from X.h then clients of "X" that #include <X.h> will also include all those headers, even though some may only be needed in X.c.
X.h should include only what's needed to parse X.h. It should assume no other headers will have been included by the translation unit to ensure that reordering inclusions won't break a client. X.c should include any extras needed for implementation.
This minimises recompilation dependencies. You don't want a change only to the implementation to affect the header and hence trigger a client recompilation. You should simply include directly from X.c.
Including forward references instead of headers is necessary when classes have shallow dependencies. For example:
A.h
#include "B.h"
class A {
B* pointer;
};
B.h
#include "A.h"
class B {
A* pointer;
};
Will break at compilation.
A.h
class B;
class A {
B* pointer;
};
B.h
class A;
class B {
A* pointer;
};
Will work as each class only needs to know the other class exists at the declaration.
I write my imports in header files, so that every implementation file has only one inclusion directive. This also has the advantage of hiding dependencies from the user of the module's code.
However, that same hiding has a disadvantage: your module's user may be importing all kinds of other headers included in your header that he may not need at all. From that point of view, it's better to have the inclusion directives in the implementation file, even if it means manually resolving dependencies, because it leads to lighter code.
I don't think there's a single answer. Considering the reasons I gave, I prefer the first approach, I think it leads to cleaner code (albeit heavier and possibly with unnecessary imports).
I don't remember who I'm quoting (and thus the phrase is not exact), but I always remember reading: "programs are written for human beings to read, and ocasionally for computers to execute". I don't particularly care if there are a few kilobytes of code the user of my module won't need, as long as he can cleanly, easily import it and use it with a single directive.
Again, I think it's a matter of taste, unless there's something I failed to consider. Comments are more than welcome in that case!
Cheers.
Instead of doing
#include "MyClass.cpp"
I would like to do
#include "MyClass.h"
I've read online that not doing so is considered bad practice.
Separate compilation in a nutshell
First, let's get some quick examples out there:
struct ClassDeclaration; // 'class' / 'struct' mean almost the same thing here
struct ClassDefinition {}; // the only difference is default accessibility
// of bases and members
void function_declaration();
void function_definition() {}
extern int global_object_declaration;
int global_object_definition;
template<class T> // cannot replace this 'class' with 'struct'
struct ClassTemplateDeclaration;
template<class T>
struct ClassTemplateDefinition {};
template<class T>
void function_template_declaration();
template<class T>
void function_template_definition() {}
Translation Unit
A translation unit (TU) is a single source file (should be a **.cpp* file) and all the files it includes, and they include, etc. In other words: the result of preprocessing a single file.
Headers
Include guards are a hack to work around lack of a real module system, making headers into a kind of limited module; to this end, including the same header more than once must not have an adverse affect.
Include guards work by making subsequent #includes no-ops, with the definitions available from the first include. Because of their limited nature, macros which control header options should be consistent throughout a project (oddball headers like <assert.h> cause problems) and all #includes of public headers should be outside of any namespace, class, etc., usually at the top of any file.
See my include guard naming advice, including a short program to generate include guards.
Declarations
Classes, functions, objects, and templates may be declared almost anywhere, may be declared any number of times, and must be declared before referring to them in any way. In a few weird cases, you can declare classes as you use them; won't cover that here.
Definitions
Classes may be defined at most once[1] per TU; this typically happens when you include a header for a particular class. Functions and objects must be defined once in exactly one TU; this typically happens when you implement them in a **.cpp* file. However, inline functions, including implicitly inline functions inside class definitions, may be defined in multiple TUs, but the definitions must be identical.
For practical purposes[2], templates (both class templates and function templates) are defined only in headers, and if you want to use a separate file, then use another header[3].
[1] Because of the at-most-once restriction, headers use include guards to prevent multiple inclusion and thus multiple definition errors.
[2] I won't cover the other possibilities here.
[3] Name it blahblah_detail.hpp, blahblah_private.hpp, or similar if you want to document that it's non-public.
Guidelines
So, while I'm sure everything above is all a big ball of mud so far, it's less than a page on what should take up a few chapters, so use it as a brief reference. Understanding the concepts above, however, is important. Using those, here's a short list of guidelines (but not absolute rules):
Always name headers consistently in a single project, such as **.h* for C and **.hpp* for C++.
Never include a file which is not a header.
Always name implementation files (which are going to be directly compiled) consistently, such as **.c* and **.cpp*.
Use a build system which can compile your source files automatically. make is the canonical example, but there are many alternatives. Keep it simple in simple cases. For example, make can be used its built-in rules and even without a makefile.
Use a build system which can generate header dependencies. Some compilers can generate this with command-line switches, such as -M, so you can make a surprisingly useful system easily.
Build Process
(Here's the tiny bit that answers your question, but you need most of the above in order to get here.)
When you build, the build system will then go through several steps, of which the important ones for this discussion are:
compile each implementation file as a TU, producing an object file (**.o*, **.obj*)
each is compiled independently of the others, which is why each TU needs declarations and definitions
link those files, along with libraries specified, into a single executable
I recommend you learn the rudiments of make, as it is popular, well-understood, and easy to get started with. However, it's an old system with several problems, and you'll want to switch to something else at some point.
Choosing a build system is almost a religious experience, like choosing an editor, except you'll have to work with more people (everyone working on the same project) and will likely be much more constrained by precedent and convention. You can use an IDE which handles the same details for you, but this has no real benefit from using a comprehensive build system instead, and you really should still know what it's doing under the hood.
File Templates
example.hpp
#ifndef EXAMPLE_INCLUDE_GUARD_60497EBE580B4F5292059C8705848F75
#define EXAMPLE_INCLUDE_GUARD_60497EBE580B4F5292059C8705848F75
// all project-specific macros for this project are prefixed "EXAMPLE_"
#include <ostream> // required headers/"modules"/libraries from the
#include <string> // stdlib, this project, and elsewhere
#include <vector>
namespace example { // main namespace for this project
template<class T>
struct TemplateExample { // for practical purposes, just put entire
void f() {} // definition of class and all methods in header
T data;
};
struct FooBar {
FooBar(); // declared
int size() const { return v.size(); } // defined (& implicitly inline)
private:
std::vector<TemplateExample<int> > v;
};
int main(std::vector<std::string> args); // declared
} // example::
#endif
example.cpp
#include "example.hpp" // include the headers "specific to" this implementation
// file first, helps make sure the header includes anything it needs (is
// independent)
#include <algorithm> // anything additional not included by the header
#include <iostream>
namespace example {
FooBar::FooBar() : v(42) {} // define ctor
int main(std::vector<std::string> args) { // define function
using namespace std; // use inside function scope, if desired, is always okay
// but using outside function scope can be problematic
cout << "doing real work now...\n"; // no std:: needed here
return 42;
}
} // example::
main.cpp
#include <iostream>
#include "example.hpp"
int main(int argc, char const** argv) try {
// do any global initialization before real main
return example::main(std::vector<std::string>(argv, argv + argc));
}
catch (std::exception& e) {
std::cerr << "[uncaught exception: " << e.what() << "]\n";
return 1; // or EXIT_FAILURE, etc.
}
catch (...) {
std::cerr << "[unknown uncaught exception]\n";
return 1; // or EXIT_FAILURE, etc.
}
This is called separate compilation model. You include class declarations into each module where they are needed, but define them only once.
In addition to hiding implementation details in cpp files (check other replies), you can additionally hide structure details by class forward declaration.
class FooPrivate;
class Foo
{
public:
// public stuff goes here
private:
FooPrivate *foo_private;
};
The expression class FooPrivate says that FooPrivate is completely defined somewhere else (preferably in the same file where Foo's implementation resides, before Foo's stuff comes. This way you make sure that implementation details of Foo(Private) aren't exposed via the header file.
You needn't include .c or .cpp files - the compiler will compile them regardless whether they're #included in other files or not. However, the code in the .c/.cpp files is useless if the other files are unaware of the classes/methods/functions/global vars/whatever that's contained in them. And that's where headers come into play. In the headers, you only put declarations, such as this one:
//myfile.hpp
class MyClass {
public:
MyClass (void);
void myMethod (void);
static int myStaticVar;
private:
int myPrivateVar;
};
Now, all .c/.cpp files that will #include "myfile.hpp" will be able to create instances of MyClass, operate on myStaticVar and call MyClass::myMethod(), even though there's no actual implementation here! See?
The implementation (the actual code) goes into myfile.cpp, where you tell the compiler what all your stuff does:
//myfile.cpp
int MyClass::myStaticVar = 0;
MyClass::MyClass (void) {
myPrivateVar = 0;
}
void MyClass::myMethod (void) {
myPrivateVar++;
}
You never include this file anywhere, it's absolutely not necessary.
A tip: create a main.hpp (or main.h, if you prefer - makes no difference) file and put all the #includes there. Each .c/.cpp file will then only need to have have this line: #include "main.hpp". This is enough to have access to all classes, methods etc. you declared in your entire project :).
You should not include a source file (.c or .cpp). Instead you should include the corresponding header file(.h) containing the declarations. The source files needs to be compiled separately and linked together to get the final executable.
Cpp files should be defined in your compiler script to be compiled as object files.
What ide are you using?
I am going to assume you are compiling with gcc, so here is the command to compile two .cpp files into one executable
gcc -o myclasses.out myclass.cpp myotherclass.cpp
You should only use #include to include class definitions, not the implentation
One thing you will want to watch out for when including you class declarations from a .h/.hpp is make sure it only ever gets included once. If you don't do this you will get some possibly cryptic compiler errors that will drive you up the wall.
To do this you need to tell the compiler, using a #define, to include the file only if the #define does not already exist.
For example (MyClass.h):
#ifndef MYCLASS_H
#define MYCLASS_H
class MyClass
{
// Memebers and methods
}
#endif
// End of file
This will guarantee your class declaration only gets included once even if you have it included in many different .cpp files.
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)