Could I provide same function definition in different TUs

Question

I was reading about internal and external linkage, and I found that by default a function has an external linkage.

So I was thinking if is it possible to declare a function in a header filer and provide multiple definitions of it in different Translation units.

So far I did declare a funtion in a header file

void fct();

and provide 2 definitions in two files each of which is contained into an anonymous namespace:

namespace
{
    void fct()
    {    
    }    
}

But I didn't see how this could be a good examle of using multiple definitions of a function in different TU.

could someone show me a simple example of that (even using inline) thank you

Answer 1

Could I provide same function definition in different TUs

If the function is not declared inline, then no; that would violate the one definition rule.

A inline function with external linkage can be defined in multiple TUs - with additional requirement that the definition must be the same. In fact, the inline declaration would make it mandatory to provide the definition in all TUs that odr-use the function.

and provide 2 definitions in two files each of which is contained into an anonymous namespace:

That doesn't violate standard rules. Those two functions are not the same, nor are they the same as the global ::fct.

But I didn't see how this could be a good examle of using multiple definitions of a function in different TU.

could someone show me a simple example of that (even using inline) thank you

Here you go:

// header.hpp
inline void foo() {}

// a.cpp
#include "header.hpp"

// b.cpp
#include "header.hpp"

Here there are two TU's, each of which contain the definition for the function ::foo, included from the same header. This is allowed because the function is declared inline.

---

Relevant standard quotes (current draft, irrelevant details redacted by me):

One-definition rule [basic.def.odr]

A ... function, ... shall not be defined where a prior definition is necessarily reachable ([module.reach]); no diagnostic is required if the prior declaration is in another translation unit.

...

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required. ... A definition of an inline function or variable shall be reachable in every translation unit in which it is odr-used outside of a discarded statement.

...

There can be more than one definition of a ... inline function with external linkage ([dcl.inline]) ... in a program provided that no prior definition is necessarily reachable ([module.reach]) at the point where a definition appears, and provided the definitions satisfy the following requirements. ... no diagnostic is required unless a prior definition is reachable at a point where a later definition appears. Given such an entity named D defined in more than one translation unit, then

• Each definition of D shall consist of the same sequence of tokens; and
• ~ lenghy list of other limitations ~