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#include <functional>
#include<map>
#include<string>
 
class Calc {
 
private:
 
    void Naming() {}
    std::map<std::string, std::function <void(Calc*)>> Command_Name;
 
public:
 
    Calc() {
        Command_Name["var"] = &Calc::Naming;
    }
 
    void operator()() {
        Command_Name["var"]();
    }
};
 
int main() {
    Calc calculater;
    calculater();
    return 0;
}

prog.cpp: In member function ‘void Calc::operator()()’:
prog.cpp:19:29: error: no match for call to ‘(std::map<std::__cxx11::basic_string<char>, std::function<void(Calc*)> >::mapped_type {aka std::function<void(Calc*)>}) ()’
         Command_Name["var"]();
                             ^
In file included from /usr/include/c++/8/functional:59,
                 from prog.cpp:1:
/usr/include/c++/8/bits/std_function.h:682:5: note: candidate: ‘_Res std::function<_Res(_ArgTypes ...)>::operator()(_ArgTypes ...) const [with _Res = void; _ArgTypes = {Calc*}]’
     function<_Res(_ArgTypes...)>::
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8/bits/std_function.h:682:5: note:   candidate expects 1 argument, 0 provided

Standard output is empty