Invalid operands to binary expressions - recursion

Question

I was solving N queens problem and I wrote isvalid function, but the function gives "invalid operands to binary expressions" error. The error occurs at board[X][Y] == 'Q':
bool isvalid(vector<vector<string>>& board , int &row , int &col , int &n){
int x = row ;
int y = col ;
while(y >= 0){
if(board[x][y] == 'Q'){// THE ERROR IS OCCURING HERE AT BOARD[X][Y]== 'Q'
return false;
}
y-- ;
}
int x = row ;
int y = col ;
while(x >=0 and y>=0){
if(board[x][y] == 'Q')return false;
x-- ;
y-- ;
}
int x = row ;
int y = col ;
while(x < n and y>=0){
if(board[x][y] == 'Q')return false;
x++ ;
y-- ;
}
return true
}

Answer 1

The problem is that there's no corresponding operator== that could be called for the comparison. board[x][y] is a std::string, whereas 'Q' is a char. As you can see on cppreference, second parameter of operator== for comparison with std::string (which is std::basic_string<char>) can either be another std::string (taken by const reference) or const char* (C-style string). Neither of these is char or could be obtained through implicit conversion from char. In particular, see that there's no constructor of std::string taking a single character.
So, the easiest solution is to compare with a string literal "Q" instead. This is an array object of type const char[2] (second character is a null terminator '\0'), which decays into const char* pointer through array-to-pointer conversion, so appropriate overload of operator can be used:
board[x][y] == "Q"
Also, note that you missed a ; after the return statement.