Inline function linkage

Question

I can't make sense of the following behavior: one header with some basic types, and another header in which I use these types in several functions. Afterward I started constructing classes based on my defined types and functions. In the function header if I leave the following signature:

void whateverFunction(parameters) 

The linker points out that there are multiple definitions of whateverFunction. Now if change it to:

inline void whateverFunction(parameters) 

the linkage problem is gone and all compiles and links well. What I know concerning inline is that it replaces every function call with it's code other than that it's a pretty dark, so my question is:

How does the linker treats inline functions in C++?

Answer 1

When the function in the header is not inline, then multiple definitions of this function (e.g. in multiple translation units) is a violation of ODR rules.

Inline functions by default have external linkage. Hence, as a consequence of ODR rules (given below), such multiple definitions (e.g. in multiple translation units) are Okay:

$3.2/5- "There can be more than one definition of a class type (Clause 9), enumeration type (7.2), inline function with external linkage (7.1.2), class template (Clause 14), non-static function template (14.5.6), static data member of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for which some template parameters are not specified (14.7, 14.5.5) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. 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 [...]

How the linker treats inline functions is a pretty much implementation level detail. Suffice it to know that the implementation accepts such mulitple defintions within the limitations of ODR rules

Note that if the function declaration in header is changed to 'static inline....', then the inline function explicitly has internal linkage and each translation unit has it's own copy of the static inline function.