What is this?

This is a discussion on What is this? within the C++ Programming forums, part of the General Programming Boards category; I found the following code snippet in a book, and it solved my initial problem, but now I'm kinda wondering...what ...

  1. #1
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    What is this?

    I found the following code snippet in a book, and it solved my initial problem, but now I'm kinda wondering...what does it do?
    PHP Code:
    struct command_struct
    {
        
    string name;
        
    bool msg;
        
        
    //initializes command_struct
        // (1)
        
    command_struct(string xbool a) : name(x), msg(a){}
    };

    command_struct cmdArray[] =
    {
        
    // (2)
        
    command_struct("say"true),
        
    command_struct("NULL"false)
    }; 
    [list=1][*]What does the line right after comment (1) do?[*]What does the line right after comment (2) do?[/list=1]I've tried looking it up, but I'm not having much luck seeing as I'm not real sure what to look under. To me, the line after (2) looks a lot like a parameterized constructor, but the line after (1) has me very stumped. Any ideas?
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  2. #2
    Code Monkey Davros's Avatar
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    >What does the line right after comment (1) do?

    It's a constructor. Classes & structs in C++ are the same, except that with structs, by default, everything is public, but with classes everything is private.


    >What does the line right after comment (2) do?

    It's initailising an array of two structs, by calling the constructors.
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  3. #3
    Guest Sebastiani's Avatar
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    In C++, we have a choice of syntax for initializing variables in the constuctor. The one in your snippet is called the "preamble", but it does the same thing as this:

    command_struct(string x, bool a) {
    name = x, msg = a;
    }
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  4. #4
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    Okay, thanks. That's what it was acting like it was doing, but I just wanted to make sure. I'd never seen this 'preamble' form before. Learning things every day...
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  5. #5
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    Hello,

    In C++, we have a choice of syntax for initializing variables in the constuctor. The one in your snippet is called the "preamble", but it does the same thing as this:

    command_struct(string x, bool a) {
    name = x, msg = a;
    }
    Quite wrong. This code will call the default constructor for name then use the assignment operator to assign x to name. It is always better to initialize members in the initializer list. It doesn't matter for built-in types but I suggest doing it for them too to be consistent.
    - lmov

  6. #6
    Guest Sebastiani's Avatar
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    Thumbs up

    Excellent point.
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  7. #7
    Code Goddess Prelude's Avatar
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    >It is always better to initialize members in the initializer list.
    It's not always better, but it's never worse. The initialization list should be preferred.

    -Prelude
    My best code is written with the delete key.

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