C++: no match for call to '(concat) (std::string&)' -

i have been trying write c++ program concatenate strings taken input user using operator overloading on operator(+=). , have defined constructor initialize objects take string argument. still shows error

error: no match call '(concat) (std::string&)'|

here's code 3 files=> main.cpp

#include <iostream> #include "concat.h" #include<string> using namespace std;  int main() { concat ob[10]; concat result; int i; string ip; cout<<"enter strings upto 10 in different lines.. \n"; for(i=0;i<(sizeof(ob)/sizeof(ob[0]));i++){     cin>>ip;     ob[i](ip); }  while(i!=0){     result += ob[i++]; } cout<<result.input; } 

concat.h

#ifndef concat_h #define concat_h #include<string> using namespace std;  class concat { public:     string input;     concat();     concat(string ip);     concat operator+=(concat );  };  #endif // concat_h 

concat.cpp

#include <iostream> #include "concat.h" #include<string> using namespace std;  concat::concat(){ } concat::concat(string ip) :input(ip) { } concat concat::operator+=(concat ipobj){     this->input += ipobj.input;     return *this; } 

that how concat class prototype should look:

class concat { public:     string input;     concat() = default;     concat& operator+=(const concat&);      // assignment operator     concat& operator=(const string& str) {           input = str;         return *this;     } }; 

and use ob[i] = ip; instead of ob[i](ip).

also, program crashes because wrote result += ob[i++];
instead of result += ob[--i];