lst-0085-book.cpp
// https://godbolt.org/z/8ejPs6zr6
#include <vector>
struct Number {
int value_ = 0;
Number() {} // Default constructor
explicit Number(int v) : value_{v} {}
};
int main() {
std::vector<Number> numbers{}; // okay: Number meets the requirements
numbers.push_back( Number{2} );
}