Lesson 156 +10 XP

Function Templates

A function template is a function definition that works with many types. You write the body once with a T placeholder, and the compiler builds a matching version for each type you actually call it with.

One function, many types

template <typename T>
T max(T a, T b) {
  return (a > b) ? a : b;
}

Pass int arguments and T becomes int; pass double arguments and T becomes double. The same body serves any type that supports the comparison.

Calling it

using namespace std;

int main() {
  cout << max(3, 8) << "\n";     // int version
  cout << max(2.5, 1.5) << "\n"; // double version
  return 0;
}

There are no hand-written int and double overloads anywhere. One template plus the types you use, and the compiler generates a matching function for each call.

The type must support the operations

The type you use must satisfy the operation in the body. max needs the > operator, so a type with > works and a type without it produces a compile error.

TL;DR

A function template declares one function for many types.
template <typename T> makes T a placeholder type.
The compiler creates a matching version for each type you call with.
The concrete type must support whatever the body does.

Function Templates