Explicit instantiation across multiple libraries

Question

If I have some header with a template implementation:

#ifndef FOOHEADER_HPP
#define FOOHEADER_HPP
template <typename T>
struct Foo
{
void FooFun(){}
};
#endif

And two .cpp-files which provides explicit instantiations of this declaration:

FloatFoo.cpp:

#include "FooHeader.hpp"
template struct Foo<float>;

IntFoo.cpp:

#include "FooHeader.hpp"
template struct Foo<int>;

I can compile the .cpp-files to libraries IntFoo.so and FloatFoo.so.

The implementations will be used in an executable based on main.cpp:

#include "FooHeader.hpp"
int main()
{
    Foo<float> foo;
    Foo<int> iFoo;
    iFoo.FooFun();
    foo.FooFun();
}

When compiling the executable:

clang++ main.cpp -L./ -lIntFoo -lFloatFoo

How do I know if the compiler actually used the instantiations in IntFoo,FloatFoo, or if it instantiated new code implicitly from the initialization in main.cpp?

https://stackoverflow.com/questions/4448094/can-we-see-the-templates-instantiated-by-the-c-compiler — NathanOliver, Jun 15 '23 at 12:47
You should move definitions into .cpp file so compiler won't be able to instantiate new code. — user7860670, Jun 15 '23 at 13:29
You can be pretty sure `main.cpp` instanciates the templates. Without adding `extern template struct Foo; extern template struct Foo;` to `FooHeader.hpp` the compiler just doesn't know that you're expecting the instanciation to be done in some other translation unit... — fabian, Jun 15 '23 at 17:19

score 0 · Answer 1 · answered Jun 21 '23 at 14:36

extern template struct Foo<> in main.cpp solves this issue, as suggested by @fabian. With the following main.cpp:

#include "FooHeader.hpp"
extern template struct Foo<float>;
extern template struct Foo<int>;


int main()
{
    Foo<float> foo;
    Foo<int> iFoo;
    iFoo.FooFun();
    foo.FooFun();
}

The compiler will throw error if libraries IntFoo and FloatFoo are not linked.

Explicit instantiation across multiple libraries

1 Answers1