Cannot add pointer to array of class pointers vector

Question

I am trying to add a pointer to an object in a pointer object vector. The 'message_list' vector lists pointers to an abstract class Message which is either adding a new Topic or Reply, two subclasses which inherit the superclass Message. My problem is when I try to add either a new Topic or a Reply to the vector, I get an error at compile time

error: no matching function for call to 'std::vector<Message*, std::allocator<Message*> >::push_back(Topic*&) const'

/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_vector.h:602: note: candidates are: void std::vector<_Tp, _Alloc>::push_back(const _Tp&)
[with _Tp = Message* , _Alloc = std::allocator<Message*> ] <near match>

The error is on the line with the messag_list.push_back(msg) :

Message* msg = new Topic( current_user->get_username(), subject, body, (message_list.size()+1) );
message_list.push_back(msg);

Why can't I add this pointer to my pointer vector? Thank you for the help!

EDIT: Here is the full function:

void Bboard::add_topic() const
{
    string subject;
    cout << "Enter Subject: ";
    cin >> subject;

    string body;
    cout << "Enter Body: ";
    cin >> body;

    Message* msg = new Topic( current_user->get_username(), subject, body, (message_list.size()+1) );

    message_list.push_back(msg);

    cout << endl;
    cout << "Message Recorded!" << endl;
    cout << endl;
}

Answer 1

void Bboard::add_topic() const

Its a const member function which means this function promises not to modify the object, but the fact is that you want to modify the object, as message_list is a member of the class and you're adding item to it. So const is inappropriate here. Just remove it and make it as:

void Bboard::add_topic();

Problem solved!

A little more explanation :

In a const member function, every member of the class becomes const, unless its declared with the keyword mutable , so in your const member function, message_list is const object, so when you want to call push_back on this object, the compiler generates error, because push_back function can be called only on non-const object.