lazy_prvalue.hpp

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#pragma once
 
/// @file userver/utils/lazy_prvalue.hpp
/// @brief @copybrief utils::LazyPrvalue
 
#include <type_traits>
#include <utility>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// @ingroup userver_universal
///
/// @brief Can be used with various emplace functions to allow in-place
/// constructing a non-movable value using a callable.
///
/// @snippet universal/src/utils/lazy_prvalue_test.cpp  LazyPrvalue sample
template <typename Func>
class LazyPrvalue final {
    static_assert(!std::is_reference_v<Func>);
 
public:
    constexpr explicit LazyPrvalue(const Func& func)
        : func_(func)
    {}
 
    constexpr explicit LazyPrvalue(Func&& func)
        : func_(std::move(func))
    {}
 
    LazyPrvalue(LazyPrvalue&&) = delete;
    LazyPrvalue& operator=(LazyPrvalue&&) = delete;
 
    constexpr /* implicit */ operator std::invoke_result_t<Func&&>() && { return std::move(func_)(); }
 
private:
    [[no_unique_address]] Func func_;
};
 
}  // namespace utils
 
USERVER_NAMESPACE_END