C ++默认副本构造函数不可行

Question

I have 2 classes, User , and BBoard (Bulletin Board).

I have a member function in BBoard , that checks reads data from a file and pushes it back to the vector userList . I also have a member function that checks if a User exists (if the User exists, I want to use the default copy constructor and assign it to currentUser )

These are the private variables I have in BBoard:

// private:
//   std::string title;
//   std::vector<User> userList;
//   User currentUser;
//   std::vector<Message> messageList;

This is the member function in BBoard that pushes back User s:

while (inFS >> dataName && inFS >> dataPass) {
  User x(dataName, dataPass);
  userList.push_back(x);
}

This is the member function in BBoard that checks if a User exists and attempts to use the default copy constructor to assign it to currentUser .

bool BBoard::userExists(const string& uName, const string& uPass) const {
  for (int i = 0; i < userList.size(); ++i) {
    if (userList.at(i).getUsername() == uName) {
      if (userList.at(i).check(uName, uPass)) {
        currentUser = userList.at(i);    //<-- HERE
        return true;
      }
    }
  }
  return false;
}

The error I am getting is this:

BBoard.cpp:153:21: error: no viable overloaded '='
        currentUser = userList.at(i);
        ~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
./User.h:8:7: note: candidate function (the implicit copy assignment operator) not viable:
      'this' argument has type 'const User', but method is not marked const
class User {

I did not declare a copy constructor in User , so I expected the implicitly created one to work. I did not push back a const User either.

According to https://en.cppreference.com/w/cpp/language/copy_constructor implicit copy constructors have this form: T::T(const T&) so I'm not sure where I went wrong.

I can provide any other portions of the program.

Answer 1

This has nothing to do with any copy constructor.

bool BBoard::userExists(const string& uName, const string& uPass) const

This defines a const class method. That's what the trailing const keyword, at the end of the declaration, means. A const class method cannot modify any member of its class, they're all effectively const to the method (unless explicitly declared mutable , which is not the case here).

currentUser = userList.at(i);

This attempts to modify a member of the class. Since it is effectively a const class member, it cannot be modified.