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saeedm

casting a void ** to object **

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Hi, the TA from my class is taking forever to respond, so I thought I might give the forums a shot. I think the question is a simple one: I have a void ** which I would like to point to either an array of Vertex pointers or Edge pointers. So I tried this, but no go. earlier in code: void ** array; if(type=='v') (Vertex **)array = new Vertex * [size]; As a note, this worked if I instead pointed 'array' to an array of Vertex's (not Vertex pointers): void * array; if(type=='v') (Vertex *)array = new Vertex [size]; But that would instantiate a Vertex() 'size' times, which I don't want to do. However, I'm still not sure what is more suitable for my situation, I'm still fairly new to c++.

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Quote:
Original post by saeedm
I have a void ** which I would like to point to either an array of Vertex pointers or Edge pointers.

You probably don't, actually. Legitimate uses for void*s in C++ are few and far between. Why are you trying to do this?

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I'm looking for a way of organizing a set of Vertex's and a set of Edges (in heap memory, since # of inputs will be huge). I need to constantly add edges and vertex's as they are read from input and as my algorithm progresses. I created a DynArray class that tries to do that, by storing a void * to either a Vertex array or an Edge array. I will create several DynArray objects, some containing Edges, some Vertex's. However, just after writing this post I realized that I might be better off using polymorphism. I'm still unsure about whether I'm better off having an array of objects, or an array of object pointers. How do I decide what works better for my situation (or is one usually prefered?)

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It sounds like your "DynArray" class is just a reimplementation of std::vector. Why are you preferring it over that?

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I knew someone was going to mention that :)
I was told to avoid using Vector since it would slow down my algorithm time with its resize operations. But then I remembered there is a feature to set a Vector's start size, which would eliminate this problem. The idea was still a little grey to me so I decided why not just do my own quick implementation of exactly what I need, and I came up with DynArray. I think I will end up using std::Vector since I'm pressed for time. BTW that question about arrays of objects vs arrays of pointers to objects still stands! (I guess it would now be Vector of objects vs Vector of pointers to objects).

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It depends on your usage patterns and size of the objects. The only way to know for sure is to code it both ways and see which one performs better.

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Quote:
Original post by saeedm
I knew someone was going to mention that :)
I was told to avoid using Vector since it would slow down my algorithm time with its resize operations.
A little bit of knowledge is a dangerous thing. Someone who understands the idea of vector resizing, but who does not understand amortized complexity analysis, is apt to get entirely the wrong opinion about std::vector.
Quote:
But then I remembered there is a feature to set a Vector's start size, which would eliminate this problem. The idea was still a little grey to me so I decided why not just do my own quick implementation of exactly what I need, and I came up with DynArray.

Lack of understanding is a terrible basis for decisions.
Quote:
BTW that question about arrays of objects vs arrays of pointers to objects still stands! (I guess it would now be Vector of objects vs Vector of pointers to objects).
The two are different things. An array of objects is a contiguous area of storage, with each slice of the storage holding an object. An array of pointers to objects is a contiguous area of storage, with each slice of storage large enough only to hold a pointer. The actual objects are stored elsewhere. You'll do the latter thing if your objects are stored elsewhere and you only want to reference them in the array. You'll also need it if your objects are polymorphic, since in that case each one will be of a different size.

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Q1:Wouldn't an array of pointers to objects always perform better, since the actual object would remain in the same place in memory and the only data moved around when operations are performed on the array would be the actual 4byte pointers?

Q2:I need to dynamically allocate my Vertex's and Edges, is the following the only way of doing it, while storing them in a Vector? It seems like data is being copied around unnecessarily...

myVector.push_back(*(new Edge(2,3,10)));

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Quote:
I was told to avoid using Vector since it would slow down my algorithm time with its resize operations.
'Slow down' in comparison to what?

If you know in advance how large the array needs to be (such as when loading vertex, color, and index data from a model file), then there's only one allocation involved, and there's no problem.

If this is a situation where you need to add things to the array as the application proceeds, you can use reserve(), use a custom allocator, use a different container type, or - wait for it - just not worry about it (the cost of resizing is unlikely to be significant unless you're doing something unusual and/or ill-advised, such as building a local vertex buffer from scratch using push_back() for every model that's rendered in a given frame).

How does your DynArray class handle reallocations? How do you know it handles them more efficiently than vector?

It seems like you've already figured this out, but the advice you got was not very good (unless there was more to it than was presented in your post). std::vector is a perfectly suitable solution for storing mesh/model data.

I'd probably need to read your original post again to give you a good answer to your second question, but a good general rule is, store objects in your containers rather than pointers to objects unless you have a good reason to do otherwise.

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A1) Dereferencing pointers isn't free. Storing pointers to objects takes up additional space. You lose spatial locality of access.

A2) myVector.push_back(Edge(2, 3, 10));

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