C++ · Templates

Templates are how C++ writes generic code. template <typename T> before a function or class turns it into a blueprint. When you call maxOf(3, 7), the compiler generates a real maxOf<int>. Calling it with double generates another version.

This is the compile-time cousin of Python’s duck typing. If T doesn’t support an operation the template uses, it’s a compile error, not a runtime one. The whole STL (vector<T>, map<K, V>) is built from templates.

#include <iostream>
#include <string>

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

template <typename T>
class Box {
public:
    explicit Box(T v) : value_(v) {}
    T get() const { return value_; }
    template <typename F>
    auto map(F f) const { return Box<decltype(f(value_))>(f(value_)); }
private:
    T value_;
};

int main() {
    std::cout << maxOf(3, 7) << ' ' << maxOf(2.5, 1.5) << ' '
              << maxOf(std::string("numpy"), std::string("pandas")) << '\n';

    Box<int> b(21);
    auto doubled = b.map([](int x) { return x * 2; });
    auto text = doubled.map([](int x) { return "value=" + std::to_string(x); });
    std::cout << doubled.get() << ' ' << text.get() << '\n';
}

Output:

7 2.5 pandas
42 value=42