How to do it...

You should prefer to use lambda expressions to pass callbacks to standard algorithms instead of functions or function objects:

• Define anonymous lambda expressions in the place of the call if you only need to use the lambda in a single place:

        auto numbers =  
          std::vector<int>{ 0, 2, -3, 5, -1, 6, 8, -4, 9 }; 
        auto positives = std::count_if( 
          std::begin(numbers), std::end(numbers),  
          [](int const n) {return n > 0; });

• Define a named lambda, that is, one assigned to a variable (usually with the auto specifier for the type), if you need to call the lambda in multiple places:

        auto ispositive = [](int const n) {return n > 0; }; 
        auto positives = std::count_if( 
          std::begin(numbers), std::end(numbers), ispositive);

• Use generic lambda expressions if you need lambdas that only differ in their argument types (available since C++14):

        auto positives = std::count_if( 
          std::begin(numbers), std::end(numbers),  
          [](auto const n) {return n > 0; });