c++ 前自增/后自增操作符效率分析

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。

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