看起来使用struct
是更快的方法:
11:01:56 ~/try
> g++ -std=c++11 main.cpp ; ./a.out
in 2265 ms, functor as a struct result = 1.5708e+16
in 31855 ms, functor through bind result = 1.5708e+16
11:02:33 ~/try
> clang++ -std=c++11 main.cpp ; ./a.out
in 3484 ms, functor as a struct result = 1.5708e+16
in 21081 ms, functor through bind result = 1.5708e+16
这段代码:
#include <iostream>
#include <functional>
#include <chrono>
using namespace std;
using namespace std::placeholders;
using namespace std::chrono;
struct fs {
double s;
fs(double state) : s(state) {}
double operator()(double x) {
return x*s;
}
};
double fb(double x, double state) {
return x*state;
}
int main(int argc, char const *argv[]) {
double state=3.1415926;
const auto stp1 = system_clock::now();
fs fstruct(state);
double sresult;
for(double x=0.0; x< 1.0e8; ++x) {
sresult += fstruct(x);
}
const auto stp2 = high_resolution_clock::now();
const auto sd = duration_cast<milliseconds>(stp2 - stp1);
cout << "in " << sd.count() << " ms, ";
cout << "functor as a struct result = " << sresult << endl;
const auto btp1 = system_clock::now();
auto fbind = bind(fb, _1, state);
double bresult;
for(double x=0.0; x< 1.0e8; ++x) {
bresult += fbind(x);
}
const auto btp2 = high_resolution_clock::now();
const auto bd = duration_cast<milliseconds>(btp2 - btp1);
cout << "in " << bd.count() << " ms, ";
cout << "functor through bind result = " << bresult << endl;
return 0;
}
更新(1)
一个函数也可以是一个函数对象:
struct fbs {
double operator()(double x, double state) const {
return x*state;
}
};
在 main.cpp 中:
const auto bstp1 = system_clock::now();
auto fbindstruct = bind(fbs(), _1, state);
double bsresult;
for(double x=0.0; x< 1.0e8; ++x) {
bsresult += fbindstruct(x);
}
const auto bstp2 = high_resolution_clock::now();
const auto bsd = duration_cast<milliseconds>(bstp2 - bstp1);
cout << "in " << bsd.count() << " ms, ";
cout << "functor through bind-struct result = " << bsresult << endl;
速度没有改变:
> g++ -std=c++11 main.cpp ; ./a.out
hi
in 2178 ms, functor as a struct result = 1.5708e+16
in 31972 ms, functor through bind result = 1.5708e+16
in 32083 ms, functor through bind-struct result = 1.5708e+16
12:15:27 ~/try
> clang++ -std=c++11 main.cpp ; ./a.out
hi
in 3758 ms, functor as a struct result = 1.5708e+16
in 23503 ms, functor through bind result = 1.5708e+16
in 23508 ms, functor through bind-struct result = 1.5708e+16
更新(2)
添加优化后,时间相似:
> g++ -std=c++11 -O2 main.cpp ; ./a.out
hi
in 536 ms, functor as a struct result = 1.5708e+16
in 510 ms, functor through bind result = 1.5708e+16
in 472 ms, functor through bind-struct result = 1.5708e+16
12:31:33 ~/try
> clang++ -std=c++11 -O2 main.cpp ; ./a.out
hi
in 388 ms, functor as a struct result = 1.5708e+16
in 419 ms, functor through bind result = 1.5708e+16
in 456 ms, functor through bind-struct result = 3.14159e+16
GCC 4.8.1和Clang 3.3
注意 Clang 3.3在“bind-struct”情况下会给出错误的结果
更新(3)
在有没有基于宏的适配器可以从类中制作一个函数对象?中进行了更多性能测试。
std::bind(f, _1, state)
中的f
是什么? - kennytmstd::bind
的作用基本上就像在您的第二个案例中创建一个对象。因此实际上并没有太大的区别。第二种情况在书籍等方面更为常见,因为std::bind
和 C++11 仍然是相当新的。 - Some programmer dude