std::match_results::ready(3)

NAME
std::match_results::ready - std::match_results::ready

Synopsis
   bool ready() const; (since C++11)
   Indicates if the match results are ready (valid) or not.
   A default-constructed match result has no result state (is not ready), and can only
   be made ready by one of the regex algorithms. The ready state implies that all match
   results have been fully established.
   The result of calling most member functions of the match_results object that is not
   ready is undefined.

Return value
   true if the match results are ready, false otherwise.

Example
// Run this code
 #include <iostream>
 #include <regex>
 #include <string>
 int main()
 {
     std::string target("pattern");
     std::smatch sm;
     std::cout << "default constructed smatch is "
               << (sm.ready() ? "ready\n" : "not ready\n");
     std::regex re1("tte");
     std::regex_search(target, sm, re1);
     std::cout << "after search, smatch is "
               << (sm.ready() ? "ready\n" : "not ready\n");
 }

Output:
 default constructed smatch is not ready
 after search, smatch is ready