Question

according to this site

http://www.cplusplus.com/reference/std/functional/unary_function/

this code should work

#include <iostream>
#include <functional>
using namespace std;
struct isdigit : public unary_function<char,bool>{
    bool operator() (char a){ return (a>='0' && a<='9');}



};
int main(){


    isdigit myobject;
    isdigit::argument_type input;
    isdigit::result_type result;
    cout<<"please enter  char";
    cin>>input;
    result=myobject(input);
    cout<<"char"<<input<<"is "<<(result?"digit":"word")<<"\n";

     return 0;
}

but it shows that somewhere bracket is missed but where?

Configuration: Debug Win32 ------
1>  function_in_c++.cpp
1>c:\users\david\documents\visual studio 2010\projects\functions_in_c++\function_in_c++.cpp(12): error C2146: syntax error : missing ';' before identifier 'object'
1>c:\users\david\documents\visual studio 2010\projects\functions_in_c++\function_in_c++.cpp(12): warning C4551: function call missing argument list
1>c:\users\david\documents\visual studio 2010\projects\functions_in_c++\function_in_c++.cpp(12): error C2065: 'object' : undeclared identifier
1>c:\users\david\documents\visual studio 2010\projects\functions_in_c++\function_in_c++.cpp(17): error C3861: 'object': identifier not found
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

isdigit is an already existing function.
Try renaming your class IsDigit for example, or put it in a namespace, and it should work.