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Lord Maz

[C++] Boost's scoped_array

11 posts in this topic

Hey, first of all I must say there's a distinct lack of examples of boost code right now.. been googlin' for a couple of hours and not much luck in helping me with my problem. Alright; I'm trying to do this:
//header
scoped_array<HWND> pFeatureButtons;

//source
u32 count = 3;
pFeatureButtons = //how do I allocate an array with 3 elements?

I'm currently a complete boost noob, so have patience with me ;)
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Try this:


uint32 count=3;
scoped_array<HWND> pFeatureButtons( new HWND[count] );




or


uint32 count=3;
scoped_array<HWND> pFeatureButtons;
pFeatureButtons.reset( new HWND[count] );


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The first one won't do, pointer must be created at object scope. However the second one might work, I'll try it.

By the way, I was thinking about using get to fetch the pointer and allocate the memory myself; is this a legal way to do it?
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All get() does is to return a pointer to the memory you previously allocated. You can't change the stored pointer with it. When using reset(), you are in fact "allocating it yourself". reset() or the constructor is the only "legal" way to do this.
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std::vector is slower than boost::scoped_array because it does dynamic resizing of the array.
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mm, but actually I'm just trying out boost here, from a performance standpoint it doesn't matter at all if I go with std::vectors, boost::scoped_ptr or normal arrays; there won't be more than a dozen elements in the array anyway.
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Quote:
Original post by Konfusius
std::vector is slower than boost::scoped_array because it does dynamic resizing of the array.
You are kofused. How is pFeatureButtons.reset( new HWND[count] ); not "dynamically resizing the array"? Why do you feel it should be faster than vec.resize(count); which basically does the same?
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Quote:
Original post by Anonymous Poster
You are kofused. How is pFeatureButtons.reset( new HWND[count] ); not "dynamically resizing the array"? Why do you feel it should be faster than vec.resize(count); which basically does the same?

scoped_array does not resize the array, ever. It can't, because it doesn't know how big the array is in the first place. It is just a wrapper over a pointer that ensures things get deleted properly. It serves an entirely different job than vector.

CM
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Quote:
Original post by Conner McCloud
Quote:
Original post by Anonymous Poster
You are kofused. How is pFeatureButtons.reset( new HWND[count] ); not "dynamically resizing the array"? Why do you feel it should be faster than vec.resize(count); which basically does the same?

scoped_array does not resize the array, ever. It can't, because it doesn't know how big the array is in the first place. It is just a wrapper over a pointer that ensures things get deleted properly. It serves an entirely different job than vector.

CM


There is no std::auto_array because it was felt that std::vector allready filled that role. They're going to preform nearly identically:

std::vector< int > pie_v ( 31415 );
boost::scoped_array< int > pie_sa ( new int [31415] );

Advantages of the former include:
1) much better preformance during a resize (which for a scoped array would involve allways reallocating)
2) as std::vector tracks how much it allocates, there's the potential for a more optimized deallocation of the associated data on destruction.

In instances where you're not using a custom allocator and will never resize your array, I'm guessing that boost::scoped_array and std::vector are going to behave nearly the same, in which case I'd arge it boils down to intent. std::vector hints that you may be reallocating, boost::scoped_array hints that you probably are not.

What's the best way to find out? You guessed it... profile!!!
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Quote:
Original post by MaulingMonkey
std::vector hints that you may be reallocating, boost::scoped_array hints that you probably are not.

This is the big thing I was trying to get at. scoped_array is a static array, while vector is a dynamic one. In terms of performance, I don't see there being a difference. *edit: Except in the realm of custom allocators, which I know nothing about.

CM
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Quote:
Original post by Conner McCloud
scoped_array does not resize the array, ever.
What I meant was this:
scoped_array pFeatureButtons; //"array size=0" (points to 0)
pFeatureButtons.reset( new HWND[count] ); //array size=count

std::vector would do basically the same allocations here, but the new call is just hidden inside the resize call.
Quote:
It serves an entirely different job than vector.
It seemed that in this case pFeatureButtons was simply mimicing a dynamic array. The question now was only if std::vector was supposed to have this performance penalty over scoped_array or not.
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