Why no inline function body in a C++ file?

[Sharon]

Usually it is common to write the class member function in the class H file,
but some people like to write the function body in the C++ file.

Can anybody tell me what are the cases where inline function should not be
written in the C++ file? Or what are the disadvantages of inline function
body in a C++ file?

--
Regards
Sharon G.

[Carl Daniel [VC++ MVP]]

Sharon wrote:

Usually it is common to write the class member function in the class
H file, but some people like to write the function body in the C++
file.

Can anybody tell me what are the cases where inline function should
not be written in the C++ file? Or what are the disadvantages of
inline function body in a C++ file?

I'll assume that by "C++ file" you mean ".CPP" file. The .H file is just as
much C++.

Inline function bodies have to be visible to the compiler in every
translation unit where they're used. The most common way to ensure that is
to simply put the inline definitions in the same header file where they're
declared.

If a function has been declared as inline but the definition is not visible
to the compiler, the compiler simply cannot continue: since the function is
inline, the compiler can't generate a call to an out of line funciton,
neither can it make something up (of course).

If a function is used in only one .CPP file, that function's definition can
of course be placed in that .CPP file. The disadvantage to doing that, of
course, is that if you subsequently want to use that function in another
..CPP file, you'll have to move the definition. The wrong solution is to put
duplicate definitions of the function into two files. That's just asking
for trouble down the line when someone modifies one copy of the function and
not another. It's also a violation of what's known as the "One Definition
Rule".

-cd

[Sharon]

Hi Carl,

I think it is possible to write the inline function implementation in the
CPP file, and it will usually be OK.

But in some cases, like templates, the inline will not work.

Personally I’m using the inline only at the H file, but some college of mine
need to convince, and I do not remember the convincing reasons.

So I need some specific example for cases where the inline implementation in
the CPP is causing trouble. And the more examples the more convincing I will
be.

------
Thanks
Sharon

[Carl Daniel [VC++ MVP]]

Sharon wrote:

Hi Carl,

I think it is possible to write the inline function implementation in
the CPP file, and it will usually be OK.
Only if the function is only called from within that file.
But in some cases, like templates, the inline will not work.
It will if the template is only used in that file - template are really no
different from ordinary inline functions in this case.
Personally I'm using the inline only at the H file, but some college
of mine need to convince, and I do not remember the convincing
reasons.

So I need some specific example for cases where the inline
implementation in the CPP is causing trouble. And the more examples
the more convincing I will be.

The cases where it doesn't work should be pretty convincing.

// A.h

inline void f();

// A.cpp

#include "A.h"

inline void f()
{
}

// B.cpp

#include "A.h"

void foo()
{
f(); // linker error
}

-cd

[Dirk]

What about link time optimization, Carl? I thought this way a function from
another object file can be inlined.

"Carl Daniel [VC++ MVP]" <cp*****************************@mvps.org.nospam >
wrote in message news:e1**************@tk2msftngp13.phx.gbl...

Sharon wrote:

Hi Carl,

I think it is possible to write the inline function implementation in
the CPP file, and it will usually be OK.

Only if the function is only called from within that file.

But in some cases, like templates, the inline will not work.

It will if the template is only used in that file - template are really no
different from ordinary inline functions in this case.

Personally I'm using the inline only at the H file, but some college
of mine need to convince, and I do not remember the convincing
reasons.

So I need some specific example for cases where the inline
implementation in the CPP is causing trouble. And the more examples
the more convincing I will be.

The cases where it doesn't work should be pretty convincing.

// A.h

inline void f();

// A.cpp

#include "A.h"

inline void f()
{
}

// B.cpp

#include "A.h"

void foo()
{
f(); // linker error
}

-cd

[Carl Daniel [VC++ MVP]]

Dirk wrote:

What about link time optimization, Carl? I thought this way a
function from another object file can be inlined.

It can. But that involves an optimization for functions that are not
declared inline, ironically enough! The basic link must first succeed
before cross module inlining is even considered.

-cd

[Ioannis Vranos]

Sharon wrote:

Usually it is common to write the class member function in the class H file,
but some people like to write the function body in the C++ file.

Can anybody tell me what are the cases where inline function should not be
written in the C++ file? Or what are the disadvantages of inline function
body in a C++ file?

A function, member of a class or not, should be written as inline when
it is small and called frequently and so we can avoid the time cost of
continuous function calls with minimal impact on the produced binary size.
Now VS 2003 unfortunately creates everything as inlined, basically
ignoring the optimisation abilities that C++ provides making it to be
like VC# 2003.
Under this view, any such large inlined function that is called more
than once, produces significant binary size impact in comparison to the
non-inlined definition, and thus should be made a regular definition
(that is non-inlined) by hand.

--
Ioannis Vranos