select.hpp

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#pragma once
 
/// @file userver/compiler/select.hpp
/// @brief Utilities for selection of platform specific values
/// @ingroup userver_universal
 
#include <cstddef>
 
USERVER_NAMESPACE_BEGIN
 
namespace compiler {
 
namespace impl {
[[noreturn]] void Abort() noexcept;
}
 
/// @brief Selects the proper value for the current compiler and standard
/// library.
///
/// If proper value for the standard library was set, prefers it over the
/// generic x32/x64 values.
///
/// ## Example usage:
/// @snippet utils/widget_fast_pimpl_test.hpp  FastPimpl - header
template <typename T>
class SelectValue final {
public:
    constexpr SelectValue() = default;
 
    constexpr SelectValue& ForLibCpp64(T value) noexcept { return Set(Bits::k64, StdLibs::kCpp, value); }
 
    constexpr SelectValue& ForLibStdCpp64(T value) noexcept { return Set(Bits::k64, StdLibs::kStdCpp, value); }
 
    constexpr SelectValue& ForLibCpp32(T value) noexcept { return Set(Bits::k32, StdLibs::kCpp, value); }
 
    constexpr SelectValue& ForLibStdCpp32(T value) noexcept { return Set(Bits::k32, StdLibs::kStdCpp, value); }
 
    constexpr SelectValue& For64Bit(T value) noexcept { return Set(Bits::k64, StdLibs::kAny, value); }
 
    constexpr SelectValue& For32Bit(T value) noexcept { return Set(Bits::k32, StdLibs::kAny, value); }
 
    constexpr operator T() const noexcept {
        if (has_stdlib_value_) {
            return stdlib_value_;
        } else if (has_bits_value_) {
            return bits_value_;
        } else {
            compiler::impl::Abort();
        }
    }
 
private:
    enum class Bits {
        k32,
        k64,
    };
 
    enum class StdLibs {
        kAny,
        kStdCpp,
        kCpp,
    };
 
    constexpr SelectValue& Set(Bits bits, StdLibs lib, T value) noexcept {
        constexpr auto kBits = (sizeof(void*) == 8 ? Bits::k64 : Bits::k32);
        constexpr auto kLib =
#if defined(_LIBCPP_VERSION)
            StdLibs::kCpp
#else
            StdLibs::kStdCpp
#endif
            ;
 
        if (bits == kBits) {
            if (lib == kLib) {
                has_stdlib_value_ = true;
                stdlib_value_ = value;
            } else if (lib == StdLibs::kAny) {
                has_bits_value_ = true;
                bits_value_ = value;
            }
        }
 
        return *this;
    }
 
    T stdlib_value_ = {};
    T bits_value_ = {};
    bool has_stdlib_value_ = false;
    bool has_bits_value_ = false;
};
 
/// @brief Alias for std::size_t values selection for the current compiler and
/// standard library.
using SelectSize = SelectValue<std::size_t>;
 
}  // namespace compiler
 
USERVER_NAMESPACE_END