Push_back in vector of objects from aggregate class

Question

I'm having a little issue with vectors. From what I understand, push_back() will copy the argument and insert the copy in the vector.

struct BridgeData
{
    string name;
    string type;
};

For a simple aggregate class such as mine, the copy of an object should be made by copying both fields.

for ( int i = 0; i<len; i++)
{
    BridgeData data = {grp.get(1+i).asString().c_str()};
    v.push_back(data);
    cout << v[i].name << endl;
}

with vector<BridgeData> &v.

When I print data.name, I get the value I used in the braced list but when I print v[i].name, the field appears to be empty... Is the default copy "constructor" for such aggregate classes default initializing all fields?

EDIT:

Here's more code if that was not enough. I've got a class which contains as a data member vector<BridgeData> yarpGroups. I then pass it as a reference in the body of a method from the same class : readBridgeDataVector(bGeneral,"yarpgroups",yarpGroups,numberOfYarpGroups);. Please ignore the other arguments as they are irrelevant (I am sure of it). The earlier snipped is from this function :

void readBridgeDataVector(Bottle &rf, string name, vector<BridgeData> &v, int len)
{
    v.resize(len);
    if(rf.check(name.c_str()))
    {
        Bottle &grp = rf.findGroup(name.c_str());
        for ( int i = 0; i<len; i++)
        {
            BridgeData data = {grp.get(1+i).asString().c_str(),"float"};
            v.push_back(data);
            cout << v[0].name << endl;
        }
    }
    else
    {
        cout << "Could not find parameters for " << name << ". "
            << "Setting everything to null by default" << endl;
    }
}

I see that you have ref to vector. Perhaps the index does not match your push_back call? (Not empty on entry?) — Photon, May 23 '13 at 08:42
No, I'm pretty sure what you are trying to do should work. You'll have to post some more code / a minimal repro. — Mike Vine, May 23 '13 at 08:43
Did you provide a size for your vector at its construction ? — JBL, May 23 '13 at 08:43
Indeed, the vector is passed from another function. This snipped is from a function whose goal is to `push_back` elements in it. — wrousseau, May 23 '13 at 08:43
If you already have data in the vector your new item will go at pos *N* but you'll print out the value at pos 0 - is this your problem? Try printing out v.back().name. — Mike Vine, May 23 '13 at 08:46
@Mike Vine, you seem to be right, it prints the correct values. I am however puzzled as for where data was previously inserted in the vector. I have added some code up there as it was requested but I'm not sure it will help much... Maybe the "resize" fills `len` elements with empty `DataBridge` ? — wrousseau, May 23 '13 at 08:50

score 1 · Accepted Answer · answered May 23 '13 at 08:50

You've resized the vector to size len.

This crates len objects in the vector using their default constructor.

So when you pushback() another object it's in place len+1.

The object in cell 0 is actually one of the default constructed objects.

What I think you wanted to do is use reserve() just to have enough room for the objects.

vector::resize()

If n is greater than the current container size, the content is expanded by inserting at the end as many elements as needed to reach a size of n. If val is specified, the new elements are initialized as copies of val, otherwise, they are value-initialized.

vector::reserve()

If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater).

score 0 · Answer 2 · answered May 23 '13 at 08:51

0

You are resizing your vector to len empty elements

You are then adding a new item (via push_back) at position len

You then print item at position 0 which is one of your original empty elements

Did you mean to reserve and not resize?

(Note: reserve() is generally not recommended though. Just leave your vector empty and push_back() the new elements you need). Also if you need to get the last element just use v.back() rather than your assumed index for ease of use.

answered May 23 '13 at 08:51

Mike Vine

9,468
25
44

That was exactly the issue, now fixed. Thanks a lot, I thought that by providing no secondary argument to `resize`, it would have the effect of `reserve` (I did not know about that function though). – wrousseau May 23 '13 at 08:53

Push_back in vector of objects from aggregate class

2 Answers2