The short story is I am trying to have function templates and the main function in different compilation units like this (I am using g++ (GCC) 14.2.1 if it matters):

// a.cpp
template<typename T> T func(T a, T b, T c) {
    return a + b + c;
}

template int func<int>(int, int, int);
template float func<float>(float, float, float);
template double func<double>(double, double, double);

// b.cpp
#include <iostream>
using namespace std;

template<typename T> T func(T a, T b, T c);

int main() {
    cout << func<int>(1,2,3) << "n";
    cout << func<float>(1,2,3) << "n";
    cout << func<double>(1,2,3) << "n";
}

But in practice, my func has a really long argument list. Here I have 3 arguments a, b, c just as an example, but in my actual code there are dozens, whose names are also very long. So I would really like to do a simpler instantiation:

// a2.cpp
template<typename T> T func(T a, T b, T c) {
    return a + b + c;
}

template func<int>;
template func<float>;
template func<double>;

Surely this won’t compile, but in principle, there is only one template argument T. So I should have given enough information for the compile to know what function I want to instantiate.

Interestingly, I find using the following method involving function pointers works:

//a3.cpp
template<typename T> T func(T a, T b, T c) {
    return a + b + c;
}

auto func_int = func<int>;
auto func_float = func<float>;
auto func_double = func<double>;

Questions:

1. Is the trick in a3.cpp legit, as a simpler way to instantiate function templates without writing down all parameters?
2. If not, what could be potential problems? And is there another way to instantiate the templates without writing down parameters a, b, c?

Update 1: I don’t think the dup link answers my question. That post talks about the usage of extern template which is to avoid repeated instantiation in different compilation units. But I am asking about a simpler syntax to do instantiation.

Update 2: Error messages for my previous code:

For a2.cpp above, I get

a2.cpp:6:10: error: ISO C++ forbids declaration of ‘func’ with no type [-fpermissive]
    6 | template func<int>;
      |          ^~~~~~~~~
a2.cpp:6:10: error: template-id ‘func<int>’ used as a declarator
a2.cpp:6:10: error: ‘func(T, T, T)’ is not a variable template
a2.cpp:7:10: error: ISO C++ forbids declaration of ‘func’ with no type [-fpermissive]
    7 | template func<float>;
      |          ^~~~~~~~~~~
a2.cpp:7:10: error: template-id ‘func<float>’ used as a declarator
a2.cpp:7:10: error: ‘func(T, T, T)’ is not a variable template
a2.cpp:8:10: error: ISO C++ forbids declaration of ‘func’ with no type [-fpermissive]
    8 | template func<double>;
      |          ^~~~~~~~~~~~
a2.cpp:8:10: error: template-id ‘func<double>’ used as a declarator
a2.cpp:8:10: error: ‘func(T, T, T)’ is not a variable template

If I remove all the template keywords in a2.cpp, I get

a2.cpp:6:1: error: ‘func<int>’ does not name a type
    6 | func<int>;
      | ^~~~~~~~~
a2.cpp:7:1: error: ‘func<float>’ does not name a type
    7 | func<float>;
      | ^~~~~~~~~~~
a2.cpp:8:1: error: ‘func<double>’ does not name a type
    8 | func<double>;
      | ^~~~~~~~~~~~

Moreover, I get the same errors even if I add extern to b.cpp