How to print an integral template argument at compile time in C++

Question

Say I have implemented a template class like this:

template <size_t N> class C 
{
     void f()
     {
        // print out N here?
     }
};

I wish that while the compiler compiles a clause like

C<20> c;

it would print out a message

"class C is templated with N = 20"

I've tried with #pragma and static_assert in vain.

The problem is that

1. with #pragma and static_assert, I could not embed an integral(20 here) into a message;
2. with preprocessors, it's too early that N is not substituted with 20 yet.

Is there any way or no way?

Thanks.

Answer 1

You could add a post-build step that finds all instantiations within the output binary after all compilations of the template(s) are complete. For instance, using the GNU toolchain you could do this:

make
nm foo | c++filt | grep 'C<[^>]\+>::f'

Where foo is the name of the output binary.

The regular expression obviously needs to be altered to find the template instantiations you are looking for, but this example works for your example class C.

You could even use a very broad regex to find all template instantiations, of any kind:

grep '<[^>]\+>::'

This is incidentally a great way to illustrate how use of the STL or iostream libraries bloats even seemingly tiny programs. The number of template instantiations can be truly astounding!