Class basic_hashed_string

Synopsis

#include <src/entt/core/hashed_string.hpp>

template<typename Char>
class basic_hashed_string

Description

Zero overhead unique identifier.

A hashed string is a compile-time tool that allows users to use human-readable identifers in the codebase while using their numeric counterparts at runtime.
Because of that, a hashed string can also be used in constant expressions if required.

Template Parameters:

Char - Character type.

Mentioned in

• Core Functionalities / Wide characters

Methods

	basic_hashed_string	Constructs an empty hashed string.
	data	Returns the human-readable representation of a hashed string.
	operator const value_type *	Returns the human-readable representation of a hashed string.
	operator hash_type	Returns the numeric representation of a hashed string.
	operator==	Compares two hashed strings.
	value	Returns directly the numeric representation of a string.

Source

Lines 63-201 in src/entt/core/hashed_string.hpp.

template<typename Char>
class basic_hashed_string {
    using traits_type = internal::fnv1a_traits<id_type>;

    struct const_wrapper {
        // non-explicit constructor on purpose
        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
        const Char *str;
    };

    // Fowler–Noll–Vo hash function v. 1a - the good
    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
        auto value = traits_type::offset;

        while(*curr != 0) {
            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
        }

        return value;
    }

public:
    /*! @brief Character type. */
    using value_type = Char;
    /*! @brief Unsigned integer type. */
    using hash_type = id_type;

    /**
     * @brief Returns directly the numeric representation of a string.
     *
     * Forcing template resolution avoids implicit conversions. An
     * human-readable identifier can be anything but a plain, old bunch of
     * characters.<br/>
     * Example of use:
     * @code{.cpp}
     * const auto value = basic_hashed_string<char>::to_value("my.png");
     * @endcode
     *
     * @tparam N Number of characters of the identifier.
     * @param str Human-readable identifer.
     * @return The numeric representation of the string.
     */
    template<std::size_t N>
    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
        return helper(str);
    }

    /**
     * @brief Returns directly the numeric representation of a string.
     * @param wrapper Helps achieving the purpose by relying on overloading.
     * @return The numeric representation of the string.
     */
    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
        return helper(wrapper.str);
    }

    /**
     * @brief Returns directly the numeric representation of a string view.
     * @param str Human-readable identifer.
     * @param size Length of the string to hash.
     * @return The numeric representation of the string.
     */
    [[nodiscard]] static hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
        id_type partial{traits_type::offset};
        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
        return partial;
    }

    /*! @brief Constructs an empty hashed string. */
    constexpr basic_hashed_string() ENTT_NOEXCEPT
        : str{nullptr}, hash{}
    {}

    /**
     * @brief Constructs a hashed string from an array of const characters.
     *
     * Forcing template resolution avoids implicit conversions. An
     * human-readable identifier can be anything but a plain, old bunch of
     * characters.<br/>
     * Example of use:
     * @code{.cpp}
     * basic_hashed_string<char> hs{"my.png"};
     * @endcode
     *
     * @tparam N Number of characters of the identifier.
     * @param curr Human-readable identifer.
     */
    template<std::size_t N>
    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
        : str{curr}, hash{helper(curr)}
    {}

    /**
     * @brief Explicit constructor on purpose to avoid constructing a hashed
     * string directly from a `const value_type *`.
     * @param wrapper Helps achieving the purpose by relying on overloading.
     */
    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
        : str{wrapper.str}, hash{helper(wrapper.str)}
    {}

    /**
     * @brief Returns the human-readable representation of a hashed string.
     * @return The string used to initialize the instance.
     */
    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
        return str;
    }

    /**
     * @brief Returns the numeric representation of a hashed string.
     * @return The numeric representation of the instance.
     */
    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
        return hash;
    }

    /*! @copydoc data */
    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }

    /**
     * @brief Returns the numeric representation of a hashed string.
     * @return The numeric representation of the instance.
     */
    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }

    /**
     * @brief Compares two hashed strings.
     * @param other Hashed string with which to compare.
     * @return True if the two hashed strings are identical, false otherwise.
     */
    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
        return hash == other.hash;
    }

private:
    const value_type *str;
    hash_type hash;
};

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