错误：“ operator ==”不匹配，模板参数推导/替换失败：

Question

Just a little frustrated at the moment. Having fun as a noob with C++. Using g++ compiler. In the search function, I come up with several errors on line 54. Below are the first two, since I know this all has to do with one or two errors that I cannot see at the moment.

ghp1.cpp: In function 'int search(std::string*, int)':
ghp1.cpp:54:22: error: no match for 'operator==' (operand types are 'std::string {aka std::basic_string}' and 'int')

 if (casino[area] == area) ^ 

ghp1.cpp:54:22: note: candidates are:
In file included from /usr/local/lib/gcc48/include/c++/iosfwd:40:0,
from /usr/local/lib/gcc48/include/c++/ios:38,
from /usr/local/lib/gcc48/include/c++/ostream:38,
from /usr/local/lib/gcc48/include/c++/iostream:39,
from ghp1.cpp:7:
/usr/local/lib/gcc48/include/c++/bits/postypes.h:216:5: note: template bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&)

 operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs) ^ 

/usr/local/lib/gcc48/include/c++/bits/postypes.h:216:5: note: template argument deduction/substitution failed:
ghp1.cpp:54:25: note: 'std::string {aka std::basic_string}' is not derived from 'const std::fpos<_StateT>'

 if (casino[area] == area) ^ 

Below is the code for my program which is supposed to make a search and display the result in a function (outside of main() ).

 1  #include <iostream>
 2  #include <cstring>
 3
 4  using namespace std;
 5
 6  int search (string casino[ ], int area);        //Prototype
 7
 8  int main(void)
 9  {
10  int i=0;
11  string casino[11];                              //Creates array
12
13  casino[1] = "Alpha";                            //Fills array
14  casino[2] = "Bravo";
15  casino[3] = "Charlie";
16  casino[4] = "Delta";
17  casino[5] = "Echo";
18  casino[6] = "Foxtrot";
19  casino[7] = "Golf";
20  casino[8] = "Hotel";
21  casino[9] = "India";
22  casino[10] = "Juno";
23
24  /*Query user for search value*/
25
26  cout<<"     This program will help you find the first ten gaming zones of ";
27  cout<<"a casino."<<endl;
28  cout<<"Pick a number between 1 and 10: "<<endl;
29  cin>>i;
30
31  cout<<"     This program will help you find the first ten gaming zones of ";
32  cout<<"a casino."<<endl;
33  cout<<"Pick a number between 1 and 10: "<<endl;
34  cin>>i;
35
36  /*Call Search Function & Display of Result*/
37
38  search(casino, i);
39
40  cout<<"    *** Good Bye! ***"<<endl;
41
42  return 0;
43  }
44
45
46  int search (string casino[ ], int area)
47  {
48  string result;
49  bool answer;
50  answer=false;
51 
52  while ((area <= 10) && (!answer))
53     {                               //Check if number is in search area.
54     if (casino[area] == area)       //***BAD LINE***//
55         answer=true;
56     else
57         area++;
58     }
59  if (answer)
60     cout<<" That zone is named: "<<casino[area]<<endl;
61  else                                 //Display of incorrect input.
62     cout<<" Nice try smartie pants!"<<endl;
63     cout<<" I said, between 1 and 10."<<endl;
64 
65  return 0;
66  }

Answer 1

Your comparison is between a string and an integer. There is no defined operator overload for this, so the compiler gives an error.

Perhaps you want this

#include <iostream>
#include <cstring>

using namespace std;

void search (string casino[ ], int casinolen, unsigned int area);        //Prototype

int main(int, char**)
{
    unsigned int i=0;
    string casino[10];                              //Creates array

    casino[0] = "Alpha";                            //Fills array
    casino[1] = "Bravo";
    casino[2] = "Charlie";
    casino[3] = "Delta";
    casino[4] = "Echo";
    casino[5] = "Foxtrot";
    casino[6] = "Golf";
    casino[7] = "Hotel";
    casino[8] = "India";
    casino[9] = "Juno";

    /*Query user for search value*/

    cout<<"     This program will help you find the first ten gaming zones of ";
    cout<<"a casino."<< endl;
    cout<<"Pick a number between 1 and 10: "<<endl;
    cin>>i;

    /*Call Search Function & Display of Result*/

    search(casino, 10, i);

    cout << "    *** Good Bye! ***" << endl;

    return 0;
}

void search (string casino[ ], int casinolen, unsigned int area)
{
    if (area > 0 && area <= casinolen)
    {
        cout<<" That zone is named: "<<casino[area-1]<<endl;
    }
    else
    {//Display of incorrect input.
        cout<<" Nice try smartie pants!"<<endl;
        cout<<" I said, between 1 and " << casinolen << "."<<endl;
    }
}

You can input a number between 1 and 10 and it will print the name of the relevant casino.

It passes the length of the array (hardcoded here but could be obtained using sizeof ) to the search function to know if the number you have entered is within the bounds of the array. Its shifted by 1 as you want to input 1-10, whereas the array indices are actually 0-9.

Passing the length is an approach commonly used in C. For C++ you would do better use a vector , which know its own length using its size method.

Answer 2

The problem is pretty simple actually. The compiler is complaining about not "know" about any operator==(...) overload for comparing string and int :

casino[area] == area  // bad string

casino is an array of strings, so casino[area] is a string while area is an integer. You function search should be expecting a string as second parameter:

int search (string casino[ ], string area)