Class Templates

Classes can be templates too. Here's a tiny box that can hold one value of any type:

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

template <typename T>
class Box {
    T value;
public:
    Box(T v) : value(v) {}
    T get() const { return value; }
    void set(T v) { value = v; }
};

int main() {
    Box<int> a(42);
    Box<string> b("hello");
    a.set(a.get() + 1);
    cout << a.get() << " " << b.get() << "\n";
    return 0;
}

Output

43 hello

A simple stack of anything

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

template <typename T>
class Stack {
    vector<T> items;
public:
    void push(const T& x) { items.push_back(x); }
    void pop() { items.pop_back(); }
    T top() const { return items.back(); }
    bool empty() const { return items.empty(); }
    int size() const { return items.size(); }
};

int main() {
    Stack<string> history;
    history.push("home");
    history.push("news");
    history.push("sports");
    cout << "Pages: " << history.size() << "\n";
    history.pop();
    cout << "Back to: " << history.top() << "\n";
    return 0;
}

Output

Pages: 3
Back to: news

Templates with two types

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

template <typename K, typename V>
struct Entry {
    K key;
    V value;
};

int main() {
    Entry<string, double> e = {"pi", 3.14};
    Entry<int, char> f = {7, 'z'};
    cout << e.key << "=" << e.value << ", " << f.key << "=" << f.value << "\n";
    return 0;
}

Output

pi=3.14, 7=z

Tip: You won't write class templates every day, but understanding them helps you read error messages from the STL – and use it with confidence.