c++ - Is there a C++11 strncmp alternative that performs as well? -

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i want prefix in string. have been using this:

if (s.substr (0, 7) == "prefix_")   ...

while works, relatively slower strncmp, shown test:

#include <iostream> #include <chrono> #include <string> #include <string.h>  int main () {   std::string s = "prefix_this passing test match";    auto t0 = std::chrono::high_resolution_clock::now ();   (int = 0; < 1000000; i++)     if (s.substr (0, 7) == "prefix_")       ;    auto t1 = std::chrono::high_resolution_clock::now ();   (int = 0; < 1000000; i++)     if (! s.compare (0, 7, "prefix_", 0, 7))       ;    auto t2 = std::chrono::high_resolution_clock::now ();   (int = 0; < 1000000; i++)     if (! strncmp (s.c_str (), "prefix_", 7))       ;    auto t3 = std::chrono::high_resolution_clock::now ();    std::cout << "[1] s.substr (0, 7) == \"prefix_\"          "             << std::chrono::duration_cast<std::chrono::microseconds>(t1 - t0).count()             << " μs\n"             << "[2] ! strncmp (s.c_str (), \"prefix_\", 7)  "             << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count()             << " μs\n"             << "[3] ! s.compare (0, 7, \"prefix_\", 0, 7)   "             << std::chrono::duration_cast<std::chrono::microseconds>(t3 - t2).count()             << " μs\n";   return 0; }

i seeing performance, surprised me:

[1] s.substr (0, 7) == "prefix_"          33022 μs [2] ! strncmp (s.c_str (), "prefix_", 7)  32547 μs [3] ! s.compare (0, 7, "prefix_", 0, 7)   12953 μs

i prefer use c++11, without dropping libc, results not good.

yes.

you may avoid string construction following, actual analogue strncmp:

if (s.compare(0, 7, "prefix_") == 0) {}

in reality, of course, i'd recommend not hardcoding n value.

as always, spending 5 minutes perusing standard library reference goes long way…


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