To understand why explicit constructors are necessary and how they work, we will first look at converting constructors. The following class has three constructors: a default constructor (without parameters), a constructor that takes an int, and a constructor that takes two parameters, an int and a double. They don't do anything, except printing a message. As of C++11, these are all considered converting constructors. The class also has a conversion operator that converts the type to a bool:
struct foo { foo() { std::cout << "foo" << std::endl; } foo(int const a) { std::cout << "foo(a)" << std::endl; } foo(int const a, double const b) { std::cout << "foo(a, b)" << std::endl; } operator bool() const { return true; } }; ...
