Function scalar_parser

example/cacheHierarchy/fkYAML/node.hpp:6134–6134

std::string's copy constructor/assignment operator may throw a exception.

    // std::string's copy constructor/assignment operator may throw a exception.
    scalar_parser(const scalar_parser&) = default;
    scalar_parser& operator=(const scalar_parser&) = default;

    scalar_parser(scalar_parser&&) noexcept = default;