C++ Templates

What are Templates?

Templates allow writing generic code that works with any data type. The compiler generates the actual type-specific code at compile time.

Function Templates

#include <iostream>
using namespace std;

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

template <typename T>
void printArray(T arr[], int size) {
    for (int i = 0; i < size; i++) cout << arr[i] << " ";
    cout << endl;
}

int main() {
    cout << maximum(3, 7)        << endl;    // int version: 7
    cout << maximum(3.14, 2.71)  << endl;    // double: 3.14
    cout << maximum('a', 'z')    << endl;    // char: z

    int    ints[]   = {1, 2, 3, 4, 5};
    double doubles[] = {1.1, 2.2, 3.3};
    printArray(ints,    5);
    printArray(doubles, 3);
    return 0;
}

Output:

7
3.14
z
1 2 3 4 5
1.1 2.2 3.3

Class Templates

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

Stack<int>    intStack;
Stack<string> strStack;

intStack.push(10); intStack.push(20);
cout << intStack.top() << endl;   // 20

strStack.push("hello");
cout << strStack.top() << endl;   // hello

Multiple Type Parameters

template <typename K, typename V>
class Pair {
  public:
    K key; V value;
    Pair(K k, V v) : key(k), value(v) { }
    void print() { cout << key << " -> " << value << endl; }
};

Pair<string, int>    p1("age", 25);
Pair<int,    double> p2(1, 3.14);
p1.print();   // age -> 25
p2.print();   // 1 -> 3.14

#include <iostream>
using namespace std;

template <typename T>
void swapValues(T& a, T& b) {
    T temp = a; a = b; b = temp;
}

int main() {
    int    x = 5,   y = 10;   swapValues(x, y); cout << x << " " << y << endl;
    double a = 1.5, b = 2.5;  swapValues(a, b); cout << a << " " << b << endl;
    string s1 = "hi", s2 = "bye"; swapValues(s1, s2); cout << s1 << " " << s2 << endl;
    return 0;
}

Summary

• Templates enable generic, type-independent code.
• Function templates work with any type: template <typename T> T func(T a).
• Class templates create generic data structures.
• The compiler instantiates the correct version at compile time.