如何在 C++ 中专门化一个可变参数模板函数？

Question

Let the function foo be given with the following "signature":

template<typename ...Ts>
void foo(Ts ...args)

This is a bit overdone since I need foo to proccess double s only. How can I modify foo so it accepts double s only?

The original code has been Godbolted:

#include <tuple>
#include <iostream>

template<typename ...Ts>
void foo(Ts ...args)
{
    std::tuple<Ts...> tu(args...);
    std::apply([](Ts const&... tupleArgs)
        {
            ((std::cout << tupleArgs << " "), ...);
        }, tu);
}

int main()
{
    foo(-2.44, -5.66, 78.99);
}

The output reads:

-2.44 -5.66 78.99

Answer 1

You can constrain your template like this:

template<typename ...Ts>
requires std::conjunction_v<std::is_same<double, Ts>...>
void foo(Ts ...args)
{
  // ...
}

Here's a demo

As pointed out by HolyBlackCat, you can write the template much more conveniently like this:

void foo(std::same_as<double> auto ...args)
{
    (std::cout << ... << args);
}

Note also that you can use a fold expression to print the arguments.

Here's a demo .

Answer 2

Those of us who are still stuck with C++17 will have to do with a slightly uglier syntax :(

template<typename ...Ts>
std::enable_if_t<std::conjunction_v<std::is_same<Ts, double>...>> foo(Ts ...args)
{
    (std::cout << ... << args);
}