Casting

This is a discussion on Casting within the C++ Programming forums, part of the General Programming Boards category; Hi. Does static casting really gve any overhead, when no real "conversion" is performed? Consider the following: Code: int i1 ...

  1. #1
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    Casting

    Hi.

    Does static casting really gve any overhead, when no real "conversion" is performed?

    Consider the following:
    Code:
    int i1 = 1;
    char *pc = (char*)&i1;
    int i2 = *(int*)pc;
    I could have used c++ style casts but i don't think it matters. The point is, does this kind of casting really take any time; or is it solely to ensure type safety?

    Thanks!

  2. #2
    The larch
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    These are not static_casts, but reinterpret_casts. I would guess that it won't take any time, and it does the opposite of type safety.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  3. #3
    C++まいる!Cをこわせ!
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    Casting can cause overhead, yes. Consider when you have a 32-bit integer variable and suddenly want to convert it to a 64-bit variable. Unless you're on a 64-bit system, it will generate a move overhead for transferring the data over to a temporary.
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

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  4. #4
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    Quote Originally Posted by tra86 View Post

    Consider the following:
    Code:
    int i1 = 1;
    char *pc = (char*)&i1;
    int i2 = *(int*)pc;
    This example has overhead, yes.
    As soon as you take the address of a variable it must be stored in memory. So if i1 was in a register, it's not anymore.
    Then the final line must reload from the address of pc. This is because the contents of that memory address could have changed by another process/thead since we wrote to it.

    You're also declaring new pointer variables which need to be stored somewhere, and that'll have register usage or stack cost.

    More generally:

    Code:
    int i = 1;
    char c = 2;
    i = (int)c;   // overhead -- value of c must be sign extended
    c = (char)i; // overhead -- value of i must have top bits masked
    I think for this example it is possible for the compiler to optimise and assign the constant 2 to both variables.

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