dolphin/Source/Core/Common/Lazy.h

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// Copyright 2017 Dolphin Emulator Project
// Licensed under GPLv2+
// Refer to the license.txt file included.
#pragma once
#include <functional>
#include <utility>
#include <variant>
namespace Common
{
// A Lazy object holds a value. If a Lazy object is constructed using
// a function as an argument, that function will be called to compute
// the value the first time any code tries to access the value.
template <typename T>
class Lazy
{
public:
Lazy() : m_value(T()) {}
Lazy(const std::variant<T, std::function<T()>>& value) : m_value(value) {}
Lazy(std::variant<T, std::function<T()>>&& value) : m_value(std::move(value)) {}
const Lazy<T>& operator=(const std::variant<T, std::function<T()>>& value)
{
m_value = value;
return *this;
}
const Lazy<T>& operator=(std::variant<T, std::function<T()>>&& value)
{
m_value = std::move(value);
return *this;
}
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const T& operator*() const { return *ComputeValue(); }
const T* operator->() const { return ComputeValue(); }
T& operator*() { return *ComputeValue(); }
T* operator->() { return ComputeValue(); }
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private:
T* ComputeValue() const
{
if (!std::holds_alternative<T>(m_value))
m_value = std::get<std::function<T()>>(m_value)();
return &std::get<T>(m_value);
}
mutable std::variant<T, std::function<T()>> m_value;
};
} // namespace Common