FP之惰性求值

1、初始版

/*
 * lazy_val.h
 *
 *  Created on: 2020年7月26日
 *      Author: xunye
 */

#ifndef LAZY_VAL_H_
#define LAZY_VAL_H_

#include 

template
class lazy_val
{
private:
	F m_computation;
	mutable bool m_cache_initialized;
	mutable decltype(m_computation()) m_cache;
	mutable std::mutex m_cache_mutex;

public:
	lazy_val(F computation) : m_computation(computation)
							, m_cache_initialized(false)
	{

	}

	operator const decltype(m_computation()) & () const
	{
		std::unique_lock lock{m_cache_mutex};
		if (not m_cache_initialized)
		{
			m_cache = m_computation();
			m_cache_initialized = true;
		}

		return m_cache;
	}
};

template
inline lazy_val make_lazy_val(F&& computation)
{
	return lazy_val(std::forward(computation));
}


#endif /* LAZY_VAL_H_ */

int call_back()
{
	return 23 * 45;
}

int main(int argc, char **argv) {

	auto lazy = make_lazy_val([]() { return 200 * 2001; });

	cout << "lambda express value: " << lazy << endl;

	auto lazyFunc = make_lazy_val(call_back);
	cout << "call_back func value: " << lazyFunc << endl;

	return 0;
}

结果：

lambda express value: 400200
call_back func value: 1035

2、优化版

/*
 * lazy_val_2.h
 *
 *  Created on: 2020年7月26日
 *      Author: xunye
 */

#ifndef LAZY_VAL_2_H_
#define LAZY_VAL_2_H_

#include 

template
class lazy_val_2
{
private:
	F m_computation;
	mutable decltype(m_computation()) m_cache;
	mutable std::once_flag m_value_flag;

public:
	lazy_val_2(F computation) : m_computation(computation)
							  , m_value_flag()
	{

	}

	operator const decltype(m_computation()) & () const
	{
		std::call_once(m_value_flag, [this]{
			m_cache = m_computation();
		});

		return m_cache;
	}
};

template
inline lazy_val_2 make_lazy_val(F&& computation)
{
	return lazy_val_2(std::forward(computation));
}

#endif /* LAZY_VAL_2_H_ */

lambda express value: 400200
call_back func value: 1035