Array size referencing struct member

This is a discussion on Array size referencing struct member within the C++ Programming forums, part of the General Programming Boards category; Is it possible to do the following: Code: 136 struct s 137 { 138 int a; 139 char b[a]; 140 ...

  1. #1
    Registered User Russell's Avatar
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    Array size referencing struct member

    Is it possible to do the following:
    Code:
    136 struct s
    137 {
    138     int a;
    139     char b[a];
    140 };
    I get the following error:
    Code:
    file.h:138: error: invalid use of non-static data member `s::a'
    file.h:139: error: from this location

  2. #2
    Supermassive black hole cboard_member's Avatar
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    The bad way to do it would be to make the variable a global. So don't do that. Instead you could use a either number:

    Code:
    struct s 
    {
    	char b[120];
    };
    Or - the way I would do it - use a string.

    Code:
    struct s 
    {
    	string b;
    };
    You could use a vector as well.

    I may be completely wrong i'm just tapping from the top of my head so experiment
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  3. #3
    Registered User Russell's Avatar
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    The reason I don't want to use a string is because the data is binary and may have null characters.

  4. #4
    Supermassive black hole cboard_member's Avatar
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    Try

    Code:
    struct s
    {
    	vector<char> b;
    };
    May be what you're looking for
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  5. #5
    Registered User Russell's Avatar
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    It looks like std::string will work. Thanks for your input, ahluka.

  6. #6
    Supermassive black hole cboard_member's Avatar
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    Anytime
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  7. #7
    *this
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    Quote Originally Posted by OP
    Code:
    struct s
    137 {
    138     int a;
    139     char b[a];
    140 };
    The reason why your compiler is complaining is because a is not initialized to anything, so b will become a very large array from a garbage value. Also when you change a, it will change the size of b and you cannot change sizes of indexing arrays.

  8. #8
    *this
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    Code:
    The reason I don't want to use a string is because the data is binary and may have null characters.
    Strings can handle null characters.

  9. #9
    ATH0 quzah's Avatar
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    Um no it can't. Not more than one it can't. A string by definition ends in a null. You can't use the string class for plain old binary data that may potentially contain more than one null. The null terminates the string. It can't have more than one. Other wise all of the string length functions and such will vomit when they hit the first null, considering the string as ended.

    Someone feel free to show otherwise, but I'm fairly sure the string class considers a null character to be "the" end.


    Quzah.
    Hope is the first step on the road to disappointment.

  10. #10
    aoeuhtns
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    I don't know exactly what the C++ standard says about the string class. But generally speaking, implementations of the string class do not use a null-terminator to mark the end. For example:

    Code:
    #include <iostream>
    #include <string>
    using std::string; using std::endl; using std::cout;
    
    int main()
    {
    
    	string x("abcdefghijklmnop");
    
    	x[5] = 0;
    
    	cout << x << endl;
    
    }
    This will output
    Code:
    abcde ghijklmnop
    on some terminals. Other terminals might behave differently with the zero character; cygwin Xterm is displaying
    Code:
    abcdeghijklmnop
    If I reroute stdout to a file, then NUL is clearly and obviously written to the file. C++ string classes generally do not use strlen when you call .size().

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