Default Initialization of Aggregates / Arrays

This is a discussion on Default Initialization of Aggregates / Arrays within the C++ Programming forums, part of the General Programming Boards category; In VC2005 I have something like this: Code: struct t { int ia[100]; struct s { int a, b, c; ...

  1. #1
    Registered User Tonto's Avatar
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    Default Initialization of Aggregates / Arrays

    In VC2005 I have something like this:

    Code:
    struct t
    {
            int ia[100];
            struct s { int a, b, c; } type;
            t() : ia(), type() { }
    };
    And I get the warning

    Code:
    warning C4351: new behavior: elements of array 'CWindow::_ia' will be default initialized
    What is that new behavior thing? Should I not depend on this behavior? Is there a better way to default initialize members of the array? Also, am I guaranteed that all members of type will be default initialized?

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  2. #2
    MFC killed my cat! manutd's Avatar
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    I think this means that all the elements will be set by default to equal zero. Other than that, I've never seen that warning before.
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  3. #3
    (?<!re)tired Mario F.'s Avatar
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    That's standard behavior.

    Arrays follow the same rules as other variables concerning default initialization.

    However, to default initialize an array, give it one value. All other dimensions will be zero-initialized if it is an array of a built-in type.

    Code:
    int ia[100] = {0};  // initializes first element to 0, and zero-initializes all other elements
    EDIT: Actually just payed attention to the warning. You compiler is bypassing the standard behavior by default initializing the array. That is non portable.
    Last edited by Mario F.; 11-19-2006 at 04:48 PM.
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    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  4. #4
    Cat without Hat CornedBee's Avatar
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    You can't have compound initialization in an initializer list, though.

    I wasn't aware the syntax in the first post was even legal.
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  5. #5
    Registered User Tonto's Avatar
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    Code:
    Arrays follow the same rules as other variables concerning default initialization.
    
    However, to default initialize an array, give it one value. All other dimensions will be zero-initialized if it is an array of a built-in type.
    
    
    Code:
    int ia[100] = {0};  // initializes first element to 0, and zero-initializes all other elements
    EDIT: Actually just payed attention to the warning. You compiler is bypassing the standard behavior by default initializing the array. That is non portable.
    Yeah, as CornedBee said, I can't do such a thing in an initializer list and certainly not the body of a constructor. Any way to default initialize the array portably, or should I just memset or something? Will that syntax ever be adopted into the standard?

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  6. #6
    (?<!re)tired Mario F.'s Avatar
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    Being a struct you can always zero-initialize it at instantiation:

    Code:
    t object={0};
    But that's not what you are asking. Other than memset of explicit assignment, I don't think you can.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  7. #7
    (?<!re)tired Mario F.'s Avatar
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    Curiously enough it also zero-initialized the array on MinGW 3.4.5

    Code:
    class a {
        public:
            int arr[10];
            a():arr() {}
    };
    It compiles with no errors with -ansi and -pedantic on.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  8. #8
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    Stroustrup isn't totally clear on whether the initializer list can be empty, but with gcc this works for me:

    Code:
    int ia[100] = {};
    Is this portable?

  9. #9
    Cat without Hat CornedBee's Avatar
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    It's irrelevant to the problem. But yes, it's portable.

    I also found the place where it kind of allows default-initializing arrays in class initializer lists.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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