vector<Foo> how to force default constructor

[lchian]

Hi,

I have a vector of class Foo.

When I do a push_back(), I expect stl to call the default constructor
I wrote for Foo. But instead, stl makes up its own default that is
initialized with garbage.

Is there a way to force stl to use MY default constructor?

Thanks

[atgraham]

We need to see some code.

[lchian]

Sorry, I typed the whole thing then the network went down.

class Foo
{
public:
vector<FooBar> vec;
.......
}

class FooBar
{
public:
FooBar(0;
FooBar(const Foobar &f){*this=f; }
Foobar &operator=(cons t FooBar &f);
~FooBar();
.......
}

int main()
{
Foo me;

FooBar b;
......

me.vec.push_bac k(b);
}

[atgraham]

I don't see anything wrong here. The FooBar() constructor is called
automatically for "b" before push_back() is called. The FooBar copy
constructor is unnecessary; that's the default behavior.

Most likely the bug is somewhere else in your code.

[lchian]

It is related to push_back.

As I understand it, STL always insert a copy of the object. in this
case, a copy of "b".

First, it calls the (wrong) default constructor, then use the
assignment operator to assign the data inside "b" to the newly created
object, then put it into "vec".

[atgraham]

That's right; a copy of "b" will be inserted into the vector. However,
I can't see your implementation of the FooBar() constructor. The bug
could be there. Your implementation of the copy constructor uses the
assignment operator. But I can't see your implementation of the
assignment operator either, so the bug could be there as well.
Nonetheless, The fact that you've attempted to define your own copy and
assignment constructor is curious. If you want it to exhibit the
default behavior, then don't define them at all. Otherwise, the bug is
probably there.

[BobR]

lc****@yahoo.co m wrote in message

class FooBar{
public:
FooBar(0;
FooBar(); // This should help
FooBar(const Foobar &f){*this=f; }
Foobar &operator=(cons t FooBar &f);
~FooBar();
.......
}

--
Bob R
POVrookie

[lchian]

There was a typo: it should be FooBar(), not FooBar(0, fogot the shift
key.

Anyway, I put break point in the default constructor and found it was
never called.

[Michiel.Salters]

lc****@yahoo.co m wrote:

It is related to push_back.

As I understand it, STL always insert a copy of the object. in this
case, a copy of "b".
True, that is what logically happens.
First, it calls the (wrong) default constructor, then use the
assignment operator to assign the data inside "b" to the newly created
object, then put it into "vec".

That is apparently what your implementation does. However, part of the
vector<T> specification is that T must have proper object semantics.
Apparently, your FooBar doesn't, and thus vector<FooBar> doesn't work.

That said, I'm not 100% sure if push_back officially requires the
default ctor.
Some vector members do, some don't, and I'd have to look up the exact
rules. Personally, I just stick to "int semantics".

HTH,
Michiel Salters

[jollygoodfellow]

"Apparently , your FooBar doesn't, and thus vector<FooBar> doesn't
work."

Its a typo, like the man said.

Lets get on with the answers.