Defining const in a class

This is a discussion on Defining const in a class within the C++ Programming forums, part of the General Programming Boards category; Sorry if the question is rather elementary. You see, I've never defined constants with "const" keyword. I've always defined it ...

  1. #1
    In the Land of Diddly-Doo g4j31a5's Avatar
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    Defining const in a class

    Sorry if the question is rather elementary. You see, I've never defined constants with "const" keyword. I've always defined it in a macro. But apparently KDevelop can't recognize macro constant in the "CTRL+Space" hotkey (which is rather annoying). And defining the constants in global variables is a bad practice. So, the question is simple really, how do I define constants with "const typedef" keyword in a class? I've tried defining it in the ".h" and failed e.g.:
    Code:
    class a
    {
    private:
      const int c_iTheConst = 10000; //Failed!!!
    };
    Thanks in advance.

  2. #2
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    I think you have to initialize it with a member initializer like:

    Code:
    class a : c_iTheConst(10000)
    {
    private:
      const int c_iTheConst; 
    };
    Edit: Ignore that, you use the member initializer in the constructor instead. My bad.
    Last edited by robatino; 11-19-2006 at 09:52 PM.

  3. #3
    Registered User Tonto's Avatar
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    Translation:

    Code:
    class a
    {
    public:
            a() : c_iTheConst(10000) { }
    private:
            const int c_iTheConst;
    };
    Note that I used a() : c_iTheConst(10000) { } and not a() { c_iTheConst = 100000; }

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  4. #4
    In the Land of Diddly-Doo g4j31a5's Avatar
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    Thanks. But what if the constant was a struct?

  5. #5
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    You should give the struct a constructor that initialises its members in an initialisation list, then use something like this:
    Code:
    struct s
    {
        s( const int m1_, const int m2_ ) : m1(m1_) , m2(m2_)
        {}
        const int m1;
        const int m2;
    };
    
    class a
    {
    public:
            a() : c_iTheConst(10000) , s(3030, 10349) { }
    private:
            const struct s const_struct;
            const int c_iTheConst;
    };

  6. #6
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    If the const has the same value for all instances of the class, then you should make it static as well. If it is a static const int, you should be able to initialize it in the class body (although some older compilers might not allow it). For the struct, I don't remember if you can initialize it in the class body or not, but either way if all instances are using the same const value it should be static.

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