How to call function template from another function defined earlier? - c++

Question

I have two function templates A and B. I define A and then B in the same file. Now I would like to call B in A. How can I realize this? Normal function prototype doesn't work in this case. (Please assume you cannot change the order of A and B or split files.)
#include <iostream>
template <class Type>
Type A(Type x) {
return 2 * B(x);
}
template <class Type>
Type B(Type x) {
return 3 * x;
}
int main() {
int x = 3;
std::cout << A(x) << "\n"; //=> ERROR
}
ERROR from g++:
test.cpp: In instantiation of ‘Type A(Type) [with Type = int]’:
test.cpp:40:21: required from here
test.cpp:29:17: error: ‘B’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
return 2 * B(x);
~^~~
test.cpp:33:6: note: ‘template<class Type> Type B(Type)’ declared here, later in the translation unit
Type B(Type x) {
^

Answer 1

If by prototype you mean declaration, it certainly does work in this case!
You can declare a function template just fine:
#include <iostream>
// Non-defining declaration B
template <class Type>
Type B(Type x);
// Defining declaration A
template <class Type>
Type A(Type x) {
return 2 * B(x);
}
// Defining declaration B
template <class Type>
Type B(Type x) {
return 3 * x;
}
int main() {
int x = 3;
std::cout << A(x) << "\n"; //=> NO ERROR
}
(live demo)