Trying to get to the bottom of a myclass::size_type

This is a discussion on Trying to get to the bottom of a myclass::size_type within the C++ Programming forums, part of the General Programming Boards category; I have a class PDCstr that stores a null-terminated array of chtypes. A chtype is a 32bit formated character that ...

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    (?<!re)tired Mario F.'s Avatar
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    Trying to get to the bottom of a myclass::size_type

    I have a class PDCstr that stores a null-terminated array of chtypes. A chtype is a 32bit formated character that can be represented in the screen. I use this class as a helper class for PDCurses (a portable curses implementation) daily usage.

    PDCstr, among other things, defines two subscript operator overloads and an observer function:
    Code:
    chtype& operator[] (const size_type idx) { return chtstr_[idx]; }
    const chtype& operator[] (const size_type idx) const { return chtstr_[idx]; }
    size_type size() const { return str_.size() );
    chtstr_ and str_ are defined as:
    Code:
    private:
        std::string str_;
        boost::shared_array<chtype> chtstr_;
    size_type above is currently being defined as:
    Code:
    public:
        typedef size_t size_type;
    I'm not sure if my typedef is the correct one anymore. I'm divided between what I have now and these two:
    Code:
    typedef std::string::size_type size_type;
    
    typedef std::ptrdiff_t size_type //boost way (and then assert as boost does)
    As a final note, I'm using the std::string (which is initialized from a constructor parameter) to build the null-terminated array of 32bit characters. That is, the array is a character-by-character 32bit representation of the 8bit std::string.
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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.

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    Frequently Quite Prolix dwks's Avatar
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    I wouldn't use ptrdiff_t. I've heard there are some problems with it, not the least that it's usually a 16-bit data type which cannot represent all possible values. ptrdiff_t is the type generated when you subtract two pointers (in C at least), but even there there are some problems, because the difference between two pointers can be greater than 2^16. I'd avoid ptrdiff_t if at all possible. But if Boost uses it, then, well . . .

    I would probably go with
    Code:
    typedef std::string::size_type size_type;
    if you know that std::strings will be used, otherwise just std::size_t.
    dwk

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by dwks View Post
    I wouldn't use ptrdiff_t. I've heard there are some problems with it, not the least that it's usually a 16-bit data type which cannot represent all possible values. ptrdiff_t is the type generated when you subtract two pointers (in C at least), but even there there are some problems, because the difference between two pointers can be greater than 2^16. I'd avoid ptrdiff_t if at all possible. But if Boost uses it, then, well . . .
    It's funny how many supposed "expert" sites out there tell people to use ptrdiff_t because "an int isn't guaranteed to be able to hold the difference between any two ARBITRARY pointers." Funny, because a ptrdiff_t isn't guaranteed to be able to do that either.

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    and the hat of wrongness Salem's Avatar
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    How did you figure out that ptrdiff_t isn't good enough?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    (?<!re)tired Mario F.'s Avatar
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    Interesting reading, robatino.

    I'll change to string::size_type, thanks dwks.

    > I'd avoid ptrdiff_t if at all possible. But if Boost uses it, then, well . . .

    I think boost uses it just so that it can implement an underflow assertion (< 0). I won't really need that. Contrary to boost shared_array, I will also be coding an at() function where proper range checking can then be done.

    Thanks again. string::size_type it is.
    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.

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