Default arguments in class template

A class template can give a default type to its type parameters, in the same way functions give default argument values. If the user does not supply a type while creating an object, the default is used.

Syntax:

template <class T1 = int, class T2 = float>
class Test { ... };

Rules:

• Defaults are written in the template header (only once, in the class declaration, not in out-of-class member definitions).
• Once a parameter has a default, the parameters after it must also have defaults.
• Empty angle brackets <> must still be written when all defaults are used.
• A non-type parameter can also have a default value, e.g. int size = 10.

Example

#include <iostream>
using namespace std;

template <class T = int, int size = 5>
class Array {
    T a[size];
public:
    void set(int i, T v) { a[i] = v; }
    T get(int i) { return a[i]; }
    int length() { return size; }
};

int main() {
    Array<> x;                 // int, size 5 (both defaults)
    Array<float> y;            // float, size 5
    Array<char, 20> z;         // char, size 20
    x.set(0, 7);
    cout << x.get(0) << " " << x.length() << endl;   // 7 5
    cout << y.length() << " " << z.length() << endl; // 5 20
    return 0;
}