C++: Can I cast a vector to a vector during a function call?

Question

I have a existed class and function which look like this:

Class base_class{
    ...
}

void Func(...,vector<base_class> &vec_b,...){
    // inside the function, the vector vec_b is being re-organized and re-sized
}

and I defined a derived class which looks like:

Class derived_class: public base_class{
    ...
}

Now, without changing the function Func, can I pass a vector<derived_class> into Func, ex:

void main(){
    vector <derived_class> d;
    Func(...,d,...);
}

such that the derived class d undergoes the same re-organizing and re-sizing? I know I can cast derived class to base class in a function call with no problem, but once with a vector into play, there seems to be difficulties? I can't find the answer online, so any suggestions or help is greatly appreciated. thanks.

Your vectors are holding objects by value, not by pointer, so you can't have polymorphic behavior anyway. I.e., what's the point? — ildjarn, Feb 20 '12 at 17:14
A somewhat related thing: [http://www2.research.att.com/~bs/bs_faq2.html#conversion](http://www2.research.att.com/~bs/bs_faq2.html#conversion) — Zyx 2000, Feb 20 '12 at 17:20
I found [jimc's answer in this link](http://www.cplusplus.com/forum/general/34099/#msg183879) to be very comprehensible for this complex question. — Daan, Sep 01 '15 at 21:17

score 4 · Answer 1 · edited Feb 20 '12 at 17:31

This only works if you're using pointers.

If you're not using pointers, then there is a chance for slicing to occur. Eg.

class Base {
  protected:
    int foo;
};

class Child : public Base {
    int bar;
};

The class Base contains only a single int called foo, and the class Child contains two ints, foo and bar.

Child f;
Base sliced = (Child)f;

This will slice the child, causing it to remove some of the data from the object. You will not be able to cast the parent back into the child class, not unless your using pointers.

So, if you changed your vector<> to have pointers instead of instances of the class then you can cast back and forth between parent/child.

Using something like:

vector<Base*> vec;
vec[0] = static_cast<Base*>(new Child());

...
Func(vec);
...

Would allow you to cast the members of your vector into their child classes.

score 3 · Answer 2 · answered Feb 20 '12 at 17:14

3

No. Even if an A is a B, a vector<A> is not a vector<B>.

To pass one in place of the other, you can pass a vector of pointers (or use a Boost ptr_vector). Alternatively, you may be able to use a template to allow passing a vector of anything that provides the right interface.

Either of these will require changing your function though -- there's essentially no way to avoid that (and still provide the functionality you want).

answered Feb 20 '12 at 17:14

Jerry Coffin

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Are you sure *you can pass a vector of pointers* is okay here? – Bartek Banachewicz Oct 19 '15 at 11:10
1

@BartekBanachewicz: A `vector` isn't convertible to a `vector`--but the pointers in a `vector` can point to either base or derived objects. If, however, you start with a `vector`, it's up to you to copy those pointers into a `vector` before passing it to the function. – Jerry Coffin Oct 19 '15 at 15:42
Oh, I see what you meant now. – Bartek Banachewicz Oct 19 '15 at 15:42

score 2 · Answer 3 · answered Feb 20 '12 at 17:14

2

No, you cannot convert between vectors of different types. However, you can achieve what you're trying to do by making Func a template. It probably won't require you to change any code in the actual function body, but it depends on what you're doing.

template<typename T>
void Func(..., vector<T> &vec_b, ...){
    // inside the function, the vector vec_b is being re-organized and re-sized
}

answered Feb 20 '12 at 17:14

Collin Dauphinee

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The OP said "without changing Func." – John Zwinck Feb 20 '12 at 17:15

score 2 · Answer 4 · edited Feb 20 '12 at 17:30

2

No.

You can let Func take vector <base_class*> and let the point elements in the vector <base_class*> point to derived_class instances.

edited Feb 20 '12 at 17:30

Robᵩ

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answered Feb 20 '12 at 17:16

Ade YU

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score 0 · Answer 5 · answered Feb 20 '12 at 17:13

0

Substituting a vector of a base class with a vector of a derived class won't work if the two classes have different sizes. This is because the vector stores contiguous instances of the type internally, so the vector of bases would think the second element was in a different location than it actually is. You could use vectors of pointers, however.

answered Feb 20 '12 at 17:13

John Zwinck

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Wouldn't this require changing func? – MikMik Feb 21 '12 at 07:47

C++: Can I cast a vector to a vector during a function call?

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