Question

ISO C++ says that the inline definition of member function in C++ is the same as declaring it with inline. This means that the function will be defined in every compilation unit the member function is used. However, if the function call cannot be inlined for whatever reason, the function is to be instantiated "as usual". (http://msdn.microsoft.com/en-us/library/z8y1yy88%28VS.71%29.aspx)

The problem I have with this definition is that it does not tell in which translation unit it would be instantiated. The problem I encountered is that when facing two object files in a single static library, both of which have the reference to some inline member function which cannot be inlined, the linker might "pick" an arbitrary object file as a source for the definition. This particular choice might introduce unneeded dependencies. (among other things)

For instance: In a static library

A.h:   class A{   public:     virtual bool foo() { return true; } }; 

U1.cpp:

A a1; 

U2.cpp:

A a2; 

and lots of dependencies

In another project main.cpp:

#include "A.h"  int main(){   A a;   a.foo();   return 0; } 

The second project refers the first. How do I know which definition the compiler will use, and, consequently which object files with their dependencies will be linked in? Is there anything the standard says on that matter? (Tried, but failed to find that)

Thanks

Edit: since I’ve seen some people misunderstand what the question is, I’d like to emphasize: If the compiler decided to create a symbol for that function (and in this case, it will, because of ‘virtualness’, there will be several (externally-seen) instantiations in different object file, which definition (from which object file?) will the linker choose?)

Answer 1

When you have an inline method that is forced to be non-inlined by the compiler, it will really instantiate the method in every compiled unit that uses it. Today most compilers are smart enough to instantiate a method only if needed (if used) so merely including the header file will not force instantiation. The linker, as you said, will pick one of the instantiations to include in the executable file – but keep in mind that the record inside the object module is of a special kind (for instance, a COMDEF) in order to give the linker enough information to know how to discard duplicated instances. These records will not, therefore, result in unwanted dependencies between modules, because the linker will use them with less priority than ‘regular’ records to resolve dependencies.

In the example you gave, you really don’t know, but it doesn’t matter. The linker won’t resolve dependencies based on non-inlined instances alone ever. The result (in terms of modules included by the linker) will be as good as if the inline method didn’t exist.