#include <vector>

class intvec {
public:
    // This one allows the implicit conversion
    // intvec(std::vector<int>::size_type length)
    // : vec_(length, 0);

    // This one disallows it.
    explicit intvec(std::vector<int>::size_type length)
        : vec_(length, 0)
    {
    }

private:
    std::vector<int> vec_;
};

auto main() -> int
{
    int const size = 20;
    // Explictly calling the constructor.
    intvec container1 { size }; // Construction
    intvec container2 = intvec { size }; // Assignment

    // Implicit conversion.
    // Probably not what we want.
    // intvec container3 = size;
}