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std::match_results<BidirIt,Alloc>:: ready

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std::match_results
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("big-red-cat");
    std::smatch sm;
    std::cout << "Default constructed smatch is "
              << (sm.ready() ? "ready.\n" : "not ready.\n");
    std::regex re1(".*-red-.*");
    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.