On Sun, 30 Dec 2018 at 17:05, Ireneusz Szcześniak via Boost-users <boost-users@lists.boost.org> wrote:

I'm writing to ask a C++ question, not specifically Boost-related, but
this list is the best place I know.

A better place would be https://www.reddit.com/r/cpp_questions/ [relatively informal] or https://stackoverflow.com/ [more formal, requirement for good formulation of your problem].

How can I write a function template, which has a parameter of the
universal reference to a specific type?

I can write a function template like this:

template <typename T>
void
foo(T &&t)
{
// The primary template implementation.
cout << "Primary: " << __PRETTY_FUNCTION__ << endl;
}

You have to read up on universal references [https://en.cppreference.com/w/cpp/language/reference] as in the above a UR seems in-appropriate, as printing in general should [normally] not involve "consuming" [i.e. move] the value your printing. It's saves typing but "using namespace std;" will, once you get bigger projects, give you [potentially] grief, it's advisable to just type std:: everywhere. If there are really long [and several nested namespaces] you can just write a short-cut, like so

namespace fs = std::filesystem;

And so I can call this function with an expression of any value
category, and any type, cv-qualified or not, like this:

int
main()
{
foo(1);
int i = 1;
foo(i);
const int ci = 1;
foo(ci);
}

In my code I need to provide different implementations of foo for
different types (and these types are templated). How can I accomplish
that?

You can simply overload the <<-operator for the types you need.

Like [just copied some code as an example]:

template<typename Stream>
[[ maybe_unused ]] Stream & operator << ( Stream & out_, const point2f & p_ ) noexcept {
out_ << '<' << p_.x << ' ' << p_.y << '>';
return out_;
}

after which you can just write:

point2f p { 1.2f, 5.6f };

std::cout << p << '\n';

The nice thing [of the above] is that it [your overload] will also work with other stream-types [https://en.cppreference.com/w/cpp/io].

degski

“If something cannot go on forever, it will stop" - Herbert Stein