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copied!<p>Looks like <code>struct</code> is the faster way to do it:</p> <pre><code>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 </code></pre> <p>the code:</p> <pre><code>#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; } </code></pre> <p><strong>Update (1)</strong></p> <p>one can also have function as a function object:</p> <pre><code>struct fbs { double operator()(double x, double state) const { return x*state; } }; </code></pre> <p>and in the main.cpp:</p> <pre><code> 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; </code></pre> <p>didn't change the speed:</p> <pre><code>> 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 </code></pre> <p><strong>Update (2)</strong></p> <p>Adding optimization results in similar times:</p> <pre><code>> 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 </code></pre> <p>GCC 4.8.1 and Clang 3.3</p> <p><strong>note</strong> Clang 3.3 gives a wrong result for "bind-struct" case</p> <p><strong>Update(3)</strong></p> <p>there are more performance testing at <a href="https://stackoverflow.com/questions/18175309/is-there-a-macro-based-adapter-to-make-a-functor-from-a-class">Is there a macro-based adapter to make a functor from a class?</a></p>

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