how could i declare & define function in header file

[thinktwice]

i have just made a test project win32 console)

//file : func.h
#ifndef _FUNC_H_
#define _FUNC_H_

void func1()
{
return;
};

void func2()
{
return;
};

#endif

//file : use1.cpp
#include "func.h"

void user1()
{
func1();
func2();
}

//file : use2.cpp
#include "func.h"
void user1()
{
func1();
func2();
}

//file : main.cpp
int main(int argc, char* argv[])
{
return 0;
};

but it failed when linking
Linking...
use2.obj : error LNK2005: "void __cdecl func1(void)" (?func1@@YAXXZ)
already defined in use1.obj
use2.obj : error LNK2005: "void __cdecl func2(void)" (?func2@@YAXXZ)
already defined in use1.obj
Debug/HelloWorld.exe : fatal error LNK1169: one or more multiply
defined symbols found
Error executing link.exe.

but it works if the function is template function. why?

 

[Alf P. Steinbach]

Use the 'inline' keyword, that's what it's for.

 

[thinktwice]

thanks , it works

 

[roberts.noah]

Use the 'inline' keyword, that's what it's for.

Well, the inline keyword wasn't made so you could define functions in a
header instead of a source file...they were made so that you could give
the compiler the hint to inline that function sort of like a safer
macro. You of course know this, but the OP might not. You don't
inline everything so that you can write the code in a header instead of
source file...you inline things that are short and you want to be
fast...no cost of function call. Normally you declare in a header and
define in source file. You don't do this with inline functions because
the compiler needs to know about the body in order to inline the code
and it needs to know this for each object file it compiles....the
linker is not involved.

 

[Alf P. Steinbach]

* (e-mail address removed):

Well, the inline keyword wasn't made so you could define functions in a
header instead of a source file...they were made so that you could give
the compiler the hint to inline that function sort of like a safer
macro. You of course know this, but the OP might not. You don't
inline everything so that you can write the code in a header instead of
source file...you inline things that are short and you want to be
fast...no cost of function call. Normally you declare in a header and
define in source file. You don't do this with inline functions because
the compiler needs to know about the body in order to inline the code
and it needs to know this for each object file it compiles....the
linker is not involved.

I'm sorry, but that's incorrect in almost all respects except the
historical.

However, it's a very very common misconception, and so it's nothing to be
ashamed of not knowing.

First, the inline keyword does not, in practice, give any reliable
optimization hint: it is primarily in support of the one-definition rule,
and not for optimization.

Second, with a modern optimizing compiler there's no point in inlining
things that are short, because the compiler does that if you instruct it
optimize for speed and such inlining will increase speed (the compiler is
generally much better at determining that than the programmer).

Third, to do reliable machine code level inlining you'll have to use
compiler-specific means instead of the 'inline' keyword -- however, as
explained in the previous paragraph, that may not always be wise.

Fourth, with a modern compiler the linker is indeed involved in inlining
(look up "whole program optimization"), which then has nothing to do with
the keyword 'inline'.

Fifth, the compiler (+ linker) need not then have access to the textual
function definition to do machine code inlining.

And so on.

The upshot is to use 'inline' for the case of function definitions in header
files, and possibly also for self-documenting code (telling the _programmer_
that "I expect this to be inlined, use freely without thinking of cost") and
not for misguided optimization where it generally doesn't work, and where if
it works it may have the opposite effect of what you want, _preventing_ the
old compiler from doing a reasonable optimization.

Hope this helps,

- Alf