c++ 前自增/后自增操作符效率分析

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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  #include  #include    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 (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 (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 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(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 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(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。