Class basic_hashed_string

Synopsis

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

template<typename Char>
class basic_hashed_string: internal::basic_hashed_string<Char>

Description

Zero overhead unique identifier.

A hashed string is a compile-time tool that allows users to use human-readable identifiers 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.

Warning: This class doesn't take ownership of user-supplied strings nor does it make a copy of them.
Template Parameters: Char - Character type.

Mentioned in

Core Functionalities / Wide characters

Inheritance

Ancestors: internal::basic_hashed_string< Char >

Methods

	basic_hashed_string overload	Constructs an empty hashed string.
	basic_hashed_string overload	Constructs a hashed string from a string view.
	basic_hashed_string overload	Constructs a hashed string from an array of const characters.
	basic_hashed_string overload	Explicit constructor on purpose to avoid constructing a hashed string directly from a `const value_type *`.
	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.
	size	Returns the size a hashed string.
	value overload	Returns directly the numeric representation of a string view.
	value overload	Returns directly the numeric representation of a string.
	value overload	Returns the numeric representation of a hashed string.

Source

Lines 68-208 in src/entt/core/hashed_string.hpp.

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

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

    // Fowler–Noll–Vo hash function v. 1a - the good
    [[nodiscard]] static constexpr auto helper(const Char *str) ENTT_NOEXCEPT {
        base_type base{str, 0u, hs_traits::offset};

        for(; str[base.length]; ++base.length) {
            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[base.length])) * hs_traits::prime;
        }

        return base;
    }

    // Fowler–Noll–Vo hash function v. 1a - the good
    [[nodiscard]] static constexpr auto helper(const Char *str, const std::size_t len) ENTT_NOEXCEPT {
        base_type base{str, len, hs_traits::offset};

        for(size_type pos{}; pos < len; ++pos) {
            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[pos])) * hs_traits::prime;
        }

        return base;
    }

public:
    /*! @brief Character type. */
    using value_type = typename base_type::value_type;
    /*! @brief Unsigned integer type. */
    using size_type = typename base_type::size_type;
    /*! @brief Unsigned integer type. */
    using hash_type = typename base_type::hash_type;

    /**
     * @brief Returns directly the numeric representation of a string view.
     * @param str Human-readable identifier.
     * @param len Length of the string to hash.
     * @return The numeric representation of the string.
     */
    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) ENTT_NOEXCEPT {
        return basic_hashed_string{str, len};
    }

    /**
     * @brief Returns directly the numeric representation of a string.
     * @tparam N Number of characters of the identifier.
     * @param str Human-readable identifier.
     * @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 basic_hashed_string{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 constexpr hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
        return basic_hashed_string{wrapper};
    }

    /*! @brief Constructs an empty hashed string. */
    constexpr basic_hashed_string() ENTT_NOEXCEPT
        : base_type{} {}

    /**
     * @brief Constructs a hashed string from a string view.
     * @param str Human-readable identifier.
     * @param len Length of the string to hash.
     */
    constexpr basic_hashed_string(const value_type *str, const size_type len) ENTT_NOEXCEPT
        : base_type{helper(str, len)} {}

    /**
     * @brief Constructs a hashed string from an array of const characters.
     * @tparam N Number of characters of the identifier.
     * @param str Human-readable identifier.
     */
    template<std::size_t N>
    constexpr basic_hashed_string(const value_type (&str)[N]) ENTT_NOEXCEPT
        : base_type{helper(str)} {}

    /**
     * @brief Explicit constructor on purpose to avoid constructing a hashed
     * string directly from a `const value_type *`.
     *
     * @warning
     * The lifetime of the string is not extended nor is it copied.
     *
     * @param wrapper Helps achieving the purpose by relying on overloading.
     */
    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
        : base_type{helper(wrapper.repr)} {}

    /**
     * @brief Returns the size a hashed string.
     * @return The size of the hashed string.
     */
    [[nodiscard]] constexpr size_type size() const ENTT_NOEXCEPT {
        return base_type::length;
    }

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

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