Function Templates

The copy-paste problem

Say you want a function that returns the bigger of two values. You'd need one for int, one for double, one for string... all with the same code. A template lets you write it once for any type:

#include <iostream>
#include <string>
using namespace std;

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

int main() {
    cout << biggest(3, 9) << "\n";
    cout << biggest(2.5, 1.5) << "\n";
    cout << biggest(string("apple"), string("pear")) << "\n";
    cout << biggest<char>('x', 'b') << "\n";
    return 0;
}

Output

9
2.5
pear
x

• template <typename T> says: “T stands for some type, to be decided later”.
• When you call biggest(3, 9), C++ sees two ints and creates an int version for you.
• You can also say the type yourself: biggest<char>(...).

Think of it like this: A template is like a cookie cutter. The shape is fixed, but you can press it into any dough: chocolate, vanilla, gingerbread. The type is the dough.

Templates that work on containers

#include <iostream>
#include <string>
#include <vector>
using namespace std;

template <typename T>
void printAll(const vector<T>& items) {
    for (const T& item : items) {
        cout << "[" << item << "] ";
    }
    cout << "\n";
}

int main() {
    printAll(vector<int>{1, 2, 3});
    printAll(vector<string>{"red", "green"});
    return 0;
}

Output

[1] [2] [3] 
[red] [green] 

Remember: You've been using templates all along: vector<int>, map<string, int> and unique_ptr<Robot> are all templates. That's where the T in STL comes from.