what this line of code means?
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  1. #1
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    Question what this line of code means?

    hello,
    I am a noob in C++ and need help about the bellow line of code:

    typedef basic_string<char, char_traits<char>, allocator<char> > string;

    what the above line of code means ? what it does?
    Last edited by soroush_vs; March 13th, 2009 at 06:33 AM.

  2. #2
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    Re: what this line of code means?

    It declares a typedef of type basic_string<char, char_traits<char>, allocator<char> > named string. What specific aspect of it do you want to know about?
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  3. #3
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    Re: what this line of code means?

    It defines a type under the name string. That type is an instantiation of the basic_string class template parameterized by types char, chart_traits and allocator (the last two are also class templates parameterized by type char).

    Basically, you need knowledge of C++ templates and the typedef keyword in order to understand that. Check them in your favorite C++ book or on the internet.

  4. #4
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    Re: what this line of code means?

    small point but typedef does not define a type. classes and structs define types. A typedef creates an alias for a type so you can refer to a long-windedly-named-type by its alias.
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  5. #5
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    Re: what this line of code means?

    Quote Originally Posted by Mybowlcut View Post
    It declares a typedef of type basic_string<char, char_traits<char>, allocator<char> > named string. What specific aspect of it do you want to know about?
    Thanks All.
    I want to know exactly what happens when we define a variable with type "basic_string<char, char_traits<char>, allocator<char> >" or simply "string".
    I what to know this line :>>> basic_string<char, char_traits<char>, allocator<char> >
    Last edited by soroush_vs; March 13th, 2009 at 02:53 PM.

  6. #6
    Lindley is offline Elite Member Power Poster
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    Re: what this line of code means?

    That's exactly what you said before. You haven't elaborated any on your confusion.

    Quote Originally Posted by Russco View Post
    small point but typedef does not define a type. classes and structs define types. A typedef creates an alias for a type so you can refer to a long-windedly-named-type by its alias.
    Technically, it is a definition (as opposed to a declaration).

  7. #7
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    Re: what this line of code means?

    I want to know what this data type store data .How this string store information and how we can manipulate it?
    I want to know why we use this instead of a simple char or TCHAR or CString or other simplest forms.
    I what to know why this defines at this such a form : basic_string<char, char_traits<char>, allocator<char> >
    I what to know this line of code means that string is really a char or char_traits or allocator or something other?
    Code:
    typedef basic_string<char, char_traits<char>, allocator<char> > string;
    How compiler deal with it?When compiler goes into the source code and saw a "string" then change it into basic_string<char, char_traits<char>, allocator<char> > then it means that this is a data type? like this?
    Code:
    string str1;
    basic_string<char, char_traits<char>, allocator<char> > str2;
    what are these str1 and str2 exactly ?

  8. #8
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    Re: what this line of code means?

    Quote Originally Posted by soroush_vs View Post
    I want to know what this data type store data .How this string store information and how we can manipulate it?
    Get a C++ book and read about std::string.
    I want to know why we use this instead of a simple char or TCHAR or CString or other simplest forms.
    A char is a single character. A TCHAR is also a single character and it only works for Windows. A CString only works for MFC. What if I don't work in Windows, and I need a string? Use std::string.
    I what to know why this defines at this such a form : basic_string<char, char_traits<char>, allocator<char> >
    Because a std::string is really a template.
    I what to know this line of code means that string is really a char or char_traits or allocator or something other?
    Get a book on C++ and templates. A basic_string<> is a general template class that handles sequences of characters. A character can be an 8 bit character, 16-bit character, or any type that works. A sequence of char types is a basic_string<char>.

    The char_traits<char> is another template that tells the basic_string<> template how to handle the "char" in certain places in the basic_string template function.

    The allocator template tells how to allocate memory when the basic_string template asks for memory.

    Now it's your turn to do the research and learn C++, as all of these questions are answered in depth and in full in C++ books, and by learning the C++ language. Randomly just looking at C++ source code, and then asking questions is not the way to learn the language.
    Code:
    string str1;
    basic_string<char, char_traits<char>, allocator<char> > str2;
    what are these str1 and str2 exactly ?
    Variables.

    Regards,

    Paul McKenzie

  9. #9
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    Re: what this line of code means?

    Why you need the underlying of C++ string template ? I think you better grasp basic of C++ first.
    Thanks for your help.

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