Re: Multiple headers, but one function body

From:

Juha Nieminen <nospam@thanks.invalid>

Newsgroups:

comp.lang.c++

Date:

10 Sep 2010 18:12:17 GMT

Message-ID:

<4c8a7501$0$12279$7b1e8fa0@news.nbl.fi>

PGK <graham.keir@gmail.com> wrote:

template <typename T> void foo(T x) { std::cout << x <<
std::endl; }

I must create:

template <> void foo(int x) { std::cout << x << std::endl; }

template <> void foo(float x) { std::cout << x << std::endl; }

template <> void foo(double x) { std::cout << x << std::endl; }

As there are often only a few different declarations, all with the
same function bodies, I'd like to avoid copying and pasting by hand.
Ideally I could do something like:

template <>
void foo(int x), void foo(float x), void foo(double x)
{ std::cout << x << std::endl; }

A function object, or proxy function will not work, as it too will
need similar multiple function bodies.

Perhaps I should define a cpreprocessor macro, but this will require
putting newline tokens at the end of each line of all (often long)
function bodies.

Are there any better methods?

  Yes: Google for "explicit template instantiation".

  In your case you would write:

// Template definition:
template <typename T> void foo(T x) { std::cout << x << std::endl; }

// Explicit template instantiations of foo():
template void foo(int);
template void foo(float);
template void foo(double);

  Explicit template instantiation is a special syntax offered by C++ to
do exactly what you want. (It can also be used to explicitly instantiate
template classes.)