Re: why not use i++ instead of ++i in for()?

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From: Mark S (mark110.DONTSPAMME_at_mindspring.com)
Date: 02/23/04

  • Next message: r norman: "Re: why not use i++ instead of ++i in for()?" 
    Date: Mon, 23 Feb 2004 00:22:33 GMT

    "r norman" <rsn_@_comcast.net> wrote in message
    news:6due309in9sbtd7oa6hhn1lkp9bp3pairv@4ax.com...
    > On Sat, 21 Feb 2004 21:49:21 +0800, "snnn" <amethyst_sun@sohu.com>
    > wrote:
    > >yes,here,i++ and ++i has the same effect;
    > >but,if the type of i is an iterator of vector(or other else),++i is more
    > >fast than i++;
    > >why not write all ++i instead?
    > >
    >
    > "Usually" depends on who is writing the code. You also say that the
    > pre-increment is faster than the post-increment. What gives you that
    > idea? If neither the pre-incremented value nor the post-incremented
    > value is used, I believe that most compilers will simply produce the
    > same code.

    Scott Meyer's "More Effective C++", "Item 6: Distinguish between prefix and
    postfix forms of increment and decrement operators" goes into great detail
    about pre and postfix operators and what the differences are.

    When it comes to basic types like "int", I'm not sure if there is enough of
    a difference to worry about, but as the previous poster points out, if i is
    an iterator, or some other more complicated class, the pre-increment is more
    efficient.

    The reason for this is that with a pre-increment operator, the object is
    incremented, and then it returns a reference to itself. The post-increment
    operator would need to save a copy of it's old value, increment itself, and
    return that old copy by value. That results in extra temporary(s) and copy
    constructors being called.

    So I typically always use ++i, out of habit.

    Another thing I do when working with strings or other classes that give me a
    choice between + and +=, when I want to concatenate a handful of strings:

       s = a + b + c + d;

    is not as efficient as

       s = a;
       s += b;
       s += c;
       s += d;

    Again, think about the temporaries that are being used and returned.

    -mark

  • Next message: r norman: "Re: why not use i++ instead of ++i in for()?"

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