非类型模板参数vs函数参数

Question

I wrote a trivial example, which is somehow compiles.

#include <iostream>
using namespace std;

template <int A>
void func()
{
    cout << 1 + A << endl;
    return;
}

int main()
{
    // I can not even use this strange func()
    int a = 1; func(a); // this does not compile
    func(1);            // this does not compile as well 
    return 0;
}

This example frustrates me a lot:

First, I gave non-type template parameter to template, but did not provide any parameters (in parentheses) to function itself. Looks like template parameter becomes the function parameter, but why?

Second, even though it compiles, I can not find a way to use this template, see my comments in main .

And third, what is the reason for existence of template functions with non-type template parameter of integral type? How is it any different from having a regular function?

Answer 1

int A is not a function parameter, it's a template parameter. func takes no arguments and you instantiate/call it like this:

func<1>(); // compile-time constant needed

Please review C++ function templates. You can't use template parameters in the way you want.

On the other hand, having a type template parameter and one function parameter:

template <typename A>
void func(A a)
{
    cout << 1 + a << endl;
}

will make your program valid. Maybe that's what you wanted.

Edit:

To your request, here's a usage of such non-type function template parameter:

template <size_t S>
void func(const int (&array)[S])
{
    cout << "size of the array is: " << S << endl;
}

or std::array version:

template <size_t S>
void func(std::array<int, S> const& array)
{
    cout << "size of the array is: " << S << endl;
}

S here is deduced to the size of the passed array.