C++前后自增效率分析

1、前自增/后自增操作符示例

class Integer
{
public:
	// ++i  first +1,then return new value
	Integer &operator++()
	{
		value_ += 1;
		return *this;
	}

	// i++  first save old value,then +1,last return old value
	Integer operator++(int)
	{
		Integer old = *this;
		value_ += 1;
		return old;
	}

private:
	int value_;
};

2、分别基于内置数据类型和自定义数据类型做测试

#include <iostream>
#include <vector>
#include <windows.h>

int main()
{
	const int sizeInt = 0x00fffffe;
	const int sizeVec = 0x000ffffe;

	LARGE_INTEGER frequency;
	QueryPerformanceFrequency(&frequency);

	{
		int* testValue = new int[sizeInt];

		LARGE_INTEGER start;
		LARGE_INTEGER stop;
		QueryPerformanceCounter(&start);
		for (int i = 0; i < sizeInt; ++i)
		{
			testValue[i]++;
		}
		QueryPerformanceCounter(&stop);		

		const auto interval = static_cast<double>(stop.QuadPart - start.QuadPart);
		const auto timeSpan = interval / frequency.QuadPart * 1000.0; //ms
		std::cout << "i++ " << sizeInt << " times takes " << timeSpan << "ms." << std::endl;

		delete[] testValue;
	}
	{
		int* testValue = new int[sizeInt];

		LARGE_INTEGER start;
		LARGE_INTEGER stop;
		QueryPerformanceCounter(&start);
		for (int i = 0; i < sizeInt; ++i)
		{
			++testValue[i];
		}
		QueryPerformanceCounter(&stop);		

		const auto interval = static_cast<double>(stop.QuadPart - start.QuadPart);
		const auto timeSpan = interval / frequency.QuadPart * 1000.0; //ms
		std::cout << "++i " << sizeInt << " times takes " << timeSpan << "ms." << std::endl;

		delete[] testValue;
	}

	{
		const std::vector<int> testVec(sizeVec);
		LARGE_INTEGER start;
		LARGE_INTEGER stop;
		QueryPerformanceCounter(&start);
		for (auto iter = testVec.cbegin(); iter != testVec.cend(); iter++)
		{
		}
		QueryPerformanceCounter(&stop);

		const auto interval = static_cast<double>(stop.QuadPart - start.QuadPart);
		const auto timeSpan = interval / frequency.QuadPart * 1000.0; //ms
		std::cout << "iterator++ " << sizeVec << " times takes " << timeSpan << "ms." << std::endl;
	}
	{
		const std::vector<int> testVec(sizeVec);
		LARGE_INTEGER start;
		LARGE_INTEGER stop;
		QueryPerformanceCounter(&start);
		for (auto iter = testVec.cbegin(); iter != testVec.cend(); ++iter)
		{
		}
		QueryPerformanceCounter(&stop);

		const auto interval = static_cast<double>(stop.QuadPart - start.QuadPart);
		const auto timeSpan = interval / frequency.QuadPart * 1000.0; //ms
		std::cout << "++iterator " << sizeVec << " times takes " << timeSpan << "ms." << std::endl;
	}

	return 0;
}

3、五次执行结果

4、结果分析及结论

从上面的执行结果可以看出来，对int类型的测试中前自增和后自增耗费时间基本不变；而对std::vector中iterator的测试显示，前自增所耗费的时间几乎是后自增的一半。这是因为，在后自增的操作中，会首先生成原始对象的一个副本，然后将副本中的值加1后返回给调用者，这样一来每执行一次后自增操作，就会增加一个对象副本，效率自然降低了。

因此可以得出结论：对于C++内置类型的自增而言，前自增、后自增的效率相差不大；对于自定义类型（类、结构体）的自增操作而言，前自增的效率几乎比后自增大一倍。

5、注意事项

上述试验的循环代码如果在Release模式下会被C++编译器优化掉，因此需要在Debug模式下才能获得预期效果，但在实际项目中大概率是不会被编译器优化的。