Template instance in different translation units [duplicate]

Question

As far as I know, each template have different instances on each translation unit, and for my understanding a translation unit is roughly a cpp file.

So, if I have a file named test.hpp with the following contents:

// test.hpp
template <typename T> void test()
{
    static T t = T(0);
    return t++;
}

For each translation unit I should have a different instance of test even if the template parameter T is the same in each of them. I've decided to test it so I've created the following files (include guards are omitted for the sake of brevity):

// a.hpp
namespace A { void f(); }

// a.cpp
#include <iostream>
#include "a.hpp"
#include "test.hpp"
namespace A
{
void f() { std::cout << test<int>(); }
}

// b.hpp
namespace B { void f(); }

// b.cpp
#include <iostream>
#include "b.hpp"
#include "test.hpp"
namespace B
{
void f() { std::cout << test<int>(); }
}

As we can see, both a.cpp and b.cpp uses the int instance of the test() template but in different translation units, so executing the following program:

// main.cpp
#include "a.hpp"
#include "b.hpp"

int main()
{
    A::f();
    B::f();
    return 0;
}

I was expecting an output of 00 but i get 01 instead. The IDE I'm using to test this code is MSVC2010 V10.0.4 SP1.

So what's the question?

• Is my understanding of the templates and translation units wrong? or...
• I did something wrong with this test code?

Heliotrope

Answer 1

Is my understanding of the templates and translation units wrong?

Yes. It's wrong.
A copy of template function is created per type and not per translation unit.

In your case, for test<int> only 1 copy is created and same is used across all the TUs.

Crabstick

Would you mind to mention where in the standard is this characteristic mentioned? – Heliotrope

@Heliotrope See the question suggested as duplicate. – Dwarfism

@Dwarfism thanks: Standard 3.2/5 One Definition Rule. It can be readed here. – Heliotrope

A copy of template function is created per type and not per translation unit It seems that it also happens with C++14 variable templates (see inWandbox), isn't it? – Heliotrope

@PaperBirdMaster, yes that's correct. Since it's templated, anyway it will have only 1 copy. However if you specialize it, then again it will result in multiple definition error. e.g. template<typename T> T a = 1; template<> int a = 0;, if you put this statement in a common header file then the 2nd statement will result in linker error. – Crabstick