std::string += && =+ compare
C++ 中，string 的拼接运算，常用的方法有 先拼接再赋值=+ 和 直接使用累加赋值运算符 +=
在 Windows10 下，使用 mingw64 8.1.0 ，测试对比了 字符串使用着两种方法的效率差异。
实验结果显示，在该操作重复次数比较大时，累加赋值运算符 += 的效率明显高于先拼接再赋值=+。顺便测试了 int 型这两种累加方式的效率，在测试的累计额范围内，两者没有差别。这里的测试累计额次数为 200000，每个测试结果取 10 的平均值。当累计额次数更多的时候，字符串的两种操作的结果效率差异会更加明显。
test code：
#include<iostream>
#include<time.h>
int main(){
std::string str1 = "a";
std::string str2 = "a";
double total_time = 0;
int test_num = 10;
// += for string
for (int cnt = 0; cnt < test_num; cnt++) {
clock_t start1 = clock();
for (int i = 0; i < 200000; i++) {
str1 += "a";
}
clock_t end1 = clock();
total_time += (double)(end1 - start1) / CLOCKS_PER_SEC;
}
std::cout << "time of += for string: " << total_time / test_num << " s" << std::endl;
// =+ for string
total_time = 0;
for (int cnt = 0; cnt < test_num; cnt++) {
clock_t start2 = clock();
for (int i = 0; i < 200000; i++) {
str2 = str2 + "a";
}
clock_t end2 = clock();
total_time += (double)(end2 - start2) / CLOCKS_PER_SEC;
}
std::cout << "time of =+ for string: " << total_time / test_num << " s" << std::endl;
int a = 0;
int b = 0;
// += for int
total_time = 0;
for (int cnt = 0; cnt < test_num; cnt++) {
clock_t start3 = clock();
for (int i = 0; i < 200000; i++) {
a += 1;
}
clock_t end3 = clock();
total_time += (double)(end3 - start3) / CLOCKS_PER_SEC;
}
std::cout << "time of += for int: " << total_time / test_num << " s" << std::endl;
// =+ for int
total_time = 0;
for (int cnt = 0; cnt < test_num; cnt++) {
clock_t start4 = clock();
for (int i = 0; i < 200000; i++) {
b = b + 1;
}
clock_t end4 = clock();
total_time += (double)(end4 - start4) / CLOCKS_PER_SEC;
}
std::cout << "time of =+ for int: " << total_time / test_num << " s" << std::endl;
return 0;
}
result：
