Multiple definitions of a function template

Question

Suppose a header file defines a function template. Now suppose two implementation files #include this header, and each of them has a call to the function template. In both implementation files the function template is instantiated with the same type.

// header.hh
template <typename T>
void f(const T& o)
{
    // ...
}

// impl1.cc
#include "header.hh"

void fimpl1()
{
    f(42);
}

// impl2.cc
#include "header.hh"

void fimpl2()
{
    f(24);
}

One may expect the linker would complain about multiple definitions of f(). Specifically, if f() wouldn't be a template then that would indeed be the case.

• How come the linker doesn't complain about multiple definitions of f()?
• Is it specified in the standard that the linker must handle this situation gracefully? In other words, can I always count on programs similar to the above to compile and link?
• If the linker can be clever enough to disambiguate a set of function template instantiations, why can't it do the same for regular functions, given they are identical as is the case for instantiated function templates?

Answer 1

In order to support C++, the linker is smart enough to recognize that they are all the same function and throws out all but one.

EDIT: clarification: The linker doesn't compare function contents and determine that they are the same. Templated functions are marked as such and the linker recognizes that they have the same signatures.