Classes and OOP · Lesson 17 of 21

Templates

Write a function or class once and use it with any type.

10 minutes2 games

Instead of writing largest for int, double and string, write it once as a template. The compiler generates a version for each type you use, at compile time, so it's as fast as hand-written code. That's exactly how std::vector<T> and std::map<K, V> are built.

C++

#include <iostream>
#include <string>
#include <vector>

template <typename T>
T largest(const std::vector<T>& items) {
    T best = items.at(0);
    for (const T& x : items) {
        if (x > best) best = x;
    }
    return best;
}

int main() {
    std::cout << largest(std::vector<int>{3, 9, 4}) << "\n";
    std::cout << largest(std::vector<double>{2.5, 1.5}) << "\n";
    std::cout << largest(std::vector<std::string>{"pear", "apple", "zebra"}) << "\n";
    return 0;
}

template <typename T> T largest(...)

largest(vector<int>) → compiler generates an int version

largest(vector<string>) → a string version

A template isn't running code; it's a recipe the compiler cooks from.

Class templates

C++

#include <iostream>
#include <string>

template <typename K, typename V>
struct Pair {
    K key;
    V value;
    void print() const { std::cout << key << " => " << value << "\n"; }
};

template <typename T, int N>
struct FixedStack {
    T items[N];
    int count = 0;
    bool push(const T& x) {
        if (count == N) return false;
        items[count++] = x;
        return true;
    }
};

int main() {
    Pair<std::string, int> p{"xp", 320};
    p.print();

    FixedStack<char, 2> s;
    std::cout << s.push('a') << s.push('b') << s.push('c') << "\n";
    return 0;
}

Complete the code

Complete the template that swaps two values of any type.

#include <iostream>
 <typename T>
void swapAny(T& a, T& b) {
     temp = a;
    a = b;
    b = temp;
}
int main() {
    int x = 1, y = 2;
    swapAny(x, y);
    std::cout << x << y << "\n";
}

Exercise

Write a template T sumAll(const std::vector<T>& v) returning the sum, and use it with {1, 2, 3} and {0.5, 0.25}. Two lines of output: 6 and 0.75.

Expected output:

6
0.75

C++

#include <iostream>
#include <vector>


int main() {
    std::cout << sumAll(std::vector<int>{1, 2, 3}) << "\n";
    std::cout << sumAll(std::vector<double>{0.5, 0.25}) << "\n";
    return 0;
}

Learn more from

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#include <iostream> #include <string> #include <vector> template <typename T> T largest(const std::vector<T>& items) { T best = items.at(0); for (const T& x : items) { if (x > best) best = x; } return best; } int main() { std::cout << largest(std::vector<int>{3, 9, 4}) << "\n"; std::cout << largest(std::vector<double>{2.5, 1.5}) << "\n"; std::cout << largest(std::vector<std::string>{"pear", "apple", "zebra"}) << "\n"; return 0; }