Explicitly defaulted default constructor

Implicit default constructor is only generated when class does not have any constructor. There are cases when the class has constructors and we want to have implicit default constructor. We can achieve this by using = default syntax. For example,

#include <iostream>
 
class Point
{
    int m_x{};
    int m_y{};
 
public:
    Point() = default;
 
    Point(int x, int y)
        : m_x{x}, m_y{y}
    {
        std::cout << "Constructing using Point(" << x << ", " << y << ")\n";
    }
    void print() const
    {
        std::cout << "{" << m_x << ", " << m_y << "}\n";
    }
};
 
int main(int argc, char const *argv[])
{
    Point{}.print();
    Point{1, 2}.print();
 
    return 0;
}

How is this different from user defined default constructors?

Default constructors either implicitly generated or explicitly using = default do does zero initialization first and then default initialization, when value-initialized. User defined default constructors do default initialization. For example, there are three classes:

1. Class A that has user define default constructor and one data member as int value;. When class is value initialized A{}, value will be default initialized to some garbage value.
2. Class B and C which have implicit and explicit default constructors respectively. If they have one data member defined as int value; and when value initializing the class object, value would be zero initialized.

References

1. https://www.learncpp.com/cpp-tutorial/default-constructors-and-default-arguments/