C++ Primer Program in VsCode error: no match for call to ‘(std::__cxx11::string…)

In the original file, total. Isbn() = = trans. Isbn() will report an error in vscode, “unable to match the call error”
compile. CPP: 12:37: error: no match for call to ‘(STD:: _cxx11:: String
{aka STD:: _cxx11:: basic_string}) ()’
if (total. Isbn() = = trans. Isbn())
solution: remove the brackets after. Isbn() and change the primitive sentence to

if (total.isbn==trans.isbn)

The reason is that ISBN should be a parameter value rather than a method, or it may be caused by my environment. It is for reference only.

#include <iostream>

#include"Sales_item.h"
int main(){
   Sales_item total;
   if (std::cin>>total)
   {Sales_item trans;
   while (std::cin>>trans)
   {
       if (total.isbn()==trans.isbn())
            total+=trans;
        else
        {
       std::cout<<total<<std::endl;
       total=trans;
        }
   
     
   }
   
    std::cout<<total<<std::endl;
   }
   else{
       std::cerr<<"NO Data?!"<<std::endl;
       return -1;
   }
   return 0;
    
}