1、前自增/后自增操作符示例
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
|
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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
|
#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。
以上就是c++ 前自增/后自增操作符效率分析的详细内容，更多关于c++ 前自增/后自增操作符的资料请关注服务器之家其它相关文章！
原文链接：https://www.cnblogs.com/xhubobo/p/12778403.html
