在类定义中定义的成员函数是否与在C ++中其他地方定义的成员函数的编译方式不同？

Question

Are member functions defined in a class definition compiled differently than member functions defined elsewhere in C++? For example, consider the following foo.h

#pragma once
struct foo {
    void bar() {}
    void buz();
};

and foo.cpp

#include "foo.h"
void foo::buz() {};

If we look at the symbols for foo.o

$ g++ -c foo.cpp

$ nm -a foo.o
0000000000000000 b .bss
0000000000000000 n .comment
0000000000000000 d .data
0000000000000000 r .eh_frame
0000000000000000 a foo.cpp
0000000000000000 n .note.GNU-stack
0000000000000000 t .text
0000000000000000 T _ZN3foo3buzEv

$ c++filt _ZN3foo3buzEv
foo::buz()

we see that we only have a symbol for foo::buz . Now, say that we compile multiple files that all include foo.h and then create a library from the result. Are the member functions bar and buz treated differently?

Answer 1

Yes, there is a difference. If a member function is defined inside class definition then the compiler tries to make it an inline function. For your example, the function is simple enough to make it inline. So the compiler has made bar inline and you only see symbol of baz .

Whether it will be good or bad depends largely on the specific functions and your use case. Inline function do not need an actual function call, the there is a performance improvement there. But the downside is if you include the class header in many places then that will increase the binary size.

Also note that inline is a request to the compiler. The compiler is free to ignore the request and treat it a normal method.

Answer 2

As from 9.2.1/1 :

A member function may be defined in its class definition, in which case it is an inline member function

On the other side, from 9.2.1/2 :

An inline member function (whether static or non-static) may also be defined outside of its class definition provided either its declaration in the class definition or its definition outside of the class definition declares the function as inline orconstexpr.

The question was: Are member functions defined in a class definition compiled differently than member functions defined elsewhere in C++?

It mostly depends on how you define them, as you can deduce from the citations above.
In your example, they are actually different.