A Default Type Argument Fills the Gap

1#include <iostream>
2 
3template <typename T, typename U = T>
4class Range {
5    T m_low;
6    U m_high;
7public:
8    Range(T low, U high) : m_low(low), m_high(high) {}
9    U high() const { return m_high; }
10    void print() const {
11        std::cout << m_low << ".." << m_high << "\n";
12    }
13};
14 
15int main() {
16    Range<int> a(1, 9);
17    Range<int, double> b(1, 9.5);
18    Range<int> c(1, 9.5);
19    Range<double> d(0.5, 2.5);
20    a.print();
21    b.print();
22    c.print();
23    d.print();
24    std::cout << b.high() * 2 << " " << c.high() * 2 << "\n";
25    return 0;
26}