c++ - 标准字符串连接性能

Question

在性能方面，现代 C++ 编译器中的以下函数之间有什么区别吗？

std::string ConcatA(const std::string& a, const std::string& b, const std::string& c)
{
    return a + b + c;
}

std::string ConcatB(const std::string& a, const std::string& b, const std::string& c)
{
    std::string r = a;
    r += b;
    r += c;
    return r;
}

Answer 1

ConcatB 有 1 个临时字符串，而 ConcatA 有 2 个临时字符串，因此 ConcatB 快两倍。

$猫cata.cpp

#include <string>
#include <iostream>
std::string ConcatA(const std::string& a, const std::string& b, const std::string& c)
{
    return a + b + c;
}
int main(){
  std::string aa="aa";
  std::string bb="bb";
  std::string cc="cc";
  int count = 0;
  for(int ii = 0; ii < 10000000; ++ii) {
    count += ConcatA(aa, bb, cc).size();
  }
    std::cout<< count <<std::endl;
}

$猫catb.cpp

#include <string>
#include <iostream>
std::string ConcatB(const std::string& a, const std::string& b, const std::string& c)
{
    std::string r = a;
    r += b;
    r += c;
    return r;
}
int main(){
  std::string aa="aa";
  std::string bb="bb";
  std::string cc="cc";
  int count = 0;
  for(int ii = 0; ii < 10000000; ++ii) {
    count += ConcatB(aa, bb, cc).size();
  }
    std::cout<< count <<std::endl;
}

$ clang++ -v

Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix

$ clang++ cata.cpp
$ time ./a.out

60000000

real    0m1.122s
user    0m1.118s
sys 0m0.003s

$ clang++ catb.cpp
$ time ./a.out
60000000

real    0m0.599s
user    0m0.596s
sys 0m0.002s
$