which is faster?

This is a discussion on which is faster? within the C++ Programming forums, part of the General Programming Boards category; Suppose I have some kind of structure: Code: typedef struct MyStruct { int a; } will there be any difference ...

  1. #1
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    which is faster?

    Suppose I have some kind of structure:
    Code:
    typedef struct MyStruct {
       int a;
    }
    will there be any difference in the execution time of this code:
    Code:
    MyStruct* msp;
    msp = new MyStruct;
    and this code:
    Code:
    MyStruct ms;
    and if there is, how significant will it be?
    And a related question: which (if either) of these will be faster - this:
    Code:
    msp->a = 5;
    or this:
    Code:
    MyStruct& msr = &ms;
    ms.a = 5;
    Thanks.

  2. #2
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    It depends on the implementation but the second one should be a tiny bit faster because the memory is allocated at execution rather than runtime. Also your third would be faster than the fourth or the same depending on whether the optimiser picks it up.

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    Brian, thanks!

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    Which is faster should be irrelevant to you. Which suits what you are tryign to accomplish better? You should be more concerned with how to clearly solve your problem and work on speed later.

  5. #5
    Toaster Zach L.'s Avatar
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    Echo orbitz.

    And the speed difference is going to be imperceptible. Go with whichever is more appropriate to the particular situation.
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  6. #6
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    And in general, you create variables on the stack unless you have a reason not to. Reasons to use new to allocate on the heap include a need to maintain the object lifetime beyond the current scope, or if the object is large and requires a lot of memory (e.g. a huge array).

  7. #7
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    It's generally more helpful, in my opinion, to think about how to create objects not by the stack or heap (since tehse arn't even C++ terms) but their lifetime. Allocate an object locally when it should die when exiting the current scope. Allocate it on the freestore when it needs to live to an undetermined point.

  8. #8
    and the hat of wrongness Salem's Avatar
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    orbitz is on the money here, premature optimisation is a killer.
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