How can a C++ header file include implementation?

Question

Ok, not a C/C++ expert by any means, but I thought the point of a header file was to declare the functions, then the C/CPP file was to define the implementation.

How

Answer 1

It is implicitly declared inline by virtue of being a member function defined within the class declaration. This does not mean the compiler has to inline it, but it means you won't break the one definition rule. It is completely unrelated to const^*. It is also unrelated to the length and complexity of the function.

If it were a non-member function, then you would have to explicitly declare it as inline:

inline void foo() { std::cout << "foo!\n"; }

^* See here for more on const at the end of a member function.

Answer 2

It is perfectly valid to have an implementation of a function in a header file. The only issue with this is breaking the one-definition-rule. That is, if you include the header from multiple other files, you will get a compiler error.

However, there is one exception. If you declare a function to be inline, it is exempt from the one-definition-rule. This is what is happening here, since member functions defined inside a class definition are implicitly inline.

Inline itself is a hint to the compiler that a function may be a good candidate for inlining. That is, expanding any call to it into the definition of the function, rather than a simple function call. This is an optimization which trades the size of the generated file for faster code. In modern compilers, providing this inlining hint for a function is mostly ignored, except for the effects it has on the one-definition-rule. Also, a compiler is always free to inline any function it sees fit, even if it has not been declared inline (explicitly or implicitly).

In your example, the use of const after the argument list signals that the member function does not modify the object on which it is called. In practice, this means that the object pointed to by this, and by extension all class members, will be considered const. That is, trying to modify them will generate a compile-time error.

Answer 3

As far as I know, there are two kinds of methods, which can be safely implemented inside the header file.

• Inline methods - their implementation is copied to places, where they are used, so there is no problem with double-definition linker errors;
• Template methods - they are actually compiled at the moment of template instantiation (eg. when someone inputs a type in place of template), so again there is no possibility of double-definition problem.

I believe, your example fits the first case.

Answer 4

C++ standard quotes

The C++17 N4659 standard draft 10.1.6 "The inline specifier" says that methods are implicitly inline:

4 A function defined within a class definition is an inline function.

and then further down we see that inline methods not only can, but must be defined on all translation units:

6 An inline function or variable shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case (6.2).

This is also explicitly mentioned in a note at 12.2.1 "Member functions":

1 A member function may be defined (11.4) in its class definition, in which case it is an inline member function (10.1.6) [...]

3 [ Note: There can be at most one definition of a non-inline member function in a program. There may be more than one inline member function definition in a program. See 6.2 and 10.1.6. — end note ]

GCC 8.3 implementation

main.cpp

struct MyClass {
    void myMethod() {}
};

int main() {
    MyClass().myMethod();
}

Compile and view symbols:

g++ -c main.cpp
nm -C main.o

output:

                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 W MyClass::myMethod()
                 U __stack_chk_fail
0000000000000000 T main

then we see from man nm that the MyClass::myMethod symbol is marked as weak on the ELF object files, which implies that it can appear on multiple object files:

"W" "w" The symbol is a weak symbol that has not been specifically tagged as a weak object symbol. When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the symbol is determined in a system-specific manner without error. On some systems, uppercase indicates that a default value has been specified.

Answer 5

Even in plain C, it is possible to put code in a header file. If you do it, you usually need to declare it static or else multiple .c files including the same header will cause a "multiply defined function" error.

The preprocessor textually includes an include file, so the code in an include file becomes part of the source file (at least from the compiler's point of view).

The designers of C++ wanted to enable object-oriented programming with good data hiding, so they expected to see lots of getter and setter functions. They didn't want an unreasonable performance penalty. So, they designed C++ so that the getters and setters could not only be declared in the header but actually implemented, so they would inline. That function you showed is a getter, and when that C++ code is compiled, there won't be any function call; code to fetch out that value will just be compiled in place.

It is possible to make a computer language that doesn't have the header file/source file distinction, but just has actual "modules" that the compiler understands. (C++ didn't do that; they just built on top of the successful C model of source files and textually included header files.) If source files are modules, it would be possible for a compiler to pull code out of the module and then inline that code. But the way C++ did it is simpler to implement.

Answer 6

Keeping the implementation in the class header file works, as I'm sure you know if you compiled your code. The const keyword ensures you don't change any members, it keeps the instance immutable for the duration of the method call.