Vector with explicit resizing

Question

I am using a vector but for a number of reasons, referring to individual elements using pointers. The problem is that when I add elements, the vector resizes and may move the elements to another larger memory address. This invalidates my pointers.

To avoid the reallocation, I call vector<T>::reserve before using the vector. However there are instances where I have not reserved enough space. In this instance I would like the vector to assert or throw an exception rather than trying to silently resize.

Is there any way to do this using vectors or another data structure? Can I use C99 variable length arrays and if so, how do I initialized one to the correct length in my constructor? Or do I need to roll out my own explicitly resizable vector?

Not quite a duplicate, but a similar question I remember, which might have valuable suggestions: stackoverflow.com/questions/12674749/… (and btw, VLAs are C99). — BoBTFish
– BoBTFish, Commented Apr 11, 2013 at 10:04

Mats Petersson · Accepted Answer · 2013-04-11 10:16:03Z

2

If you check much space you have reserved, you could simply solve this "In this instance I would like the vector to assert or throw an exception rather than trying to silently resize." with

if (v.size() == v.capacity()) throw ...

You could write a function like this:

template <typename T>
void safe_vector_push_back(std::vector<T>& v, const T &e)
{
   if (v.size() == v.capacity()) throw ... 
   v.push_back(e);
}

Another possible method would of course be to NOT store pointers to content in your vector at all. Just store the index into the vector. That way, the vector can resize as much as it likes, and nothing will go wrong.

It is generally a bad idea to store pointers to data inside another object (in this case, vector is an object).

edited Apr 11, 2013 at 10:16

answered Apr 11, 2013 at 10:07

Mats Petersson

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6 Comments

BoBTFish Over a year ago

You don't even have to remember it, you can use std::vector::capacity.

doron Over a year ago

Except, I need to run this test every time I call a push_back, I would like this to be defined I one place only.

juanchopanza Over a year ago

@doron you could create your own vector-like container, that wraps an std::vector and applies these checks where required.

Mats Petersson Over a year ago

@doron: I have updated the answer with an example of code that does it in one place. You just end up with a function call instead of a method call on the vector.

BoBTFish Over a year ago

Grr, I just wrote pretty much the same code and came back to post it.

|

Peter R · Accepted Answer · 2013-04-11 13:50:40Z

1

Inheritance is your friend:

template<typename T, typename A = std::allocator<T>>
class myvector : public std::vector<T, A>
{
    typedef std::vector<T,A> base_t;
public:
    myvector()
    {
    }

    myvector(size_t size) : base_t(size)
    {
    }
    void push_back(T val)
    {
        if(capacity() == size())
            throw "no more room";
        base_t::push_back(val);
    }
};

answered Apr 11, 2013 at 13:50

Peter R

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