C++ How should functions and templates be declared together

Question

code:

#include <stdio.h>

T foo(T x,T y);
int main(void)
{
    
}

template <typename T>
T func(T x,T y) {return x + y;}

error:

        3 | T foo(T x,T y);
      | ^
main.c:3:7: error: unknown type name ‘T’
    3 | T foo(T x,T y);
      |       ^
main.c:3:11: error: unknown type name ‘T’
    3 | T foo(T x,T y);
      |           ^
main.c:9:10: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘<’ token
    9 | template <typename T>
      |          ^

I thought I should declare the template, so I tried to declare the template before declaring the function:

#include <stdio.h>

template <typename T>
T foo(T x,T y);
int main(void)
{
    
}


T func(T x,T y) {return x + y;}

But this is still wrong, so wondering how should I declare the template

Answer 1

foo() and func() are not the same function. And in the 1st example, foo() is not a template, and in the 2nd example, func() is not a template, so that is why the compiler doesn't know what T is in them.

Like any other function, if you are going to separate a template function's declaration from its definition (which is doable, though do be aware of this issue), they need to match either other in name and signature, including the template<>, eg:

template <typename T>
T foo(T x,T y);

int main()
{
    // use foo() as needed...    
}

template <typename T>
T foo(T x,T y) {return x + y;}

Though, if you move foo()'s definition above main(), you can declare and define it in one operation:

template <typename T>
T foo(T x,T y) {return x + y;}

int main()
{
    // use foo() as needed...
}