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;
}

How is message_list declared? also you are trying to do this from a const member function? — Naveen
– Naveen, Commented May 14, 2011 at 6:26
The compiler seems to be looking for a const function to call Is message_list possibly a parameter passed by const reference? — Bo Persson
– Bo Persson, Commented May 14, 2011 at 6:27

Sarfaraz Nawaz · Accepted Answer · 2011-05-14 06:51:03Z

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.

edited May 14, 2011 at 6:51

answered May 14, 2011 at 6:24

Sarfaraz Nawaz

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4 Comments

Jimmathy Over a year ago

Yes, Topic is derived from Message.

Jimmathy Over a year ago

Topic::Topic(const string& athr, const string& sbjct, const string& body, unsigned id) :Message(athr, sbjct, body, id) {}

Jimmathy Over a year ago

Message::Message(const string& athr, const string& sbjct, const string& body, unsigned id) :author(athr), subject(sbjct), body(body), id(id) {}

Sarfaraz Nawaz Over a year ago

@MikeTrottman: Post in your question, not here. I can't read it here. I'm not a compiler. Also, these code doesn't help. Post MORE code, not just the constructor!

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