Why can a class not do this?

This is a discussion on Why can a class not do this? within the C++ Programming forums, part of the General Programming Boards category; Hi this isn't really a problem as there are ways around this. I was wondering why this: Code: private: std::string ...

  1. #1
    Its hard... But im here swgh's Avatar
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    Why can a class not do this?

    Hi this isn't really a problem as there are ways around this.

    I was wondering why this:

    Code:
    private:
       std::string m_Name;
       std::vector<std::string> nameList;
       std::vector<int> gradeList;
    };
    
    GradeBook::GradeBook ( std::string name )
    {
       setName ( name );
       nameList ( 10, "" ); // this is illegal
    }
    Is not allowed in C++? A constructor initalzes data members to starting values so why are you not allowed to tell the constructor I want the vector to hold 10 names and then make all the elements empty? ( "" )

    It allowed me to declare the vector as a private member and name it but the only way to do what I need in the constructor is to remove it and create it in new function like this:

    Code:
     std::vector<std::string> nameList ( 10, "" );
    This of course would work. I am not moaning I am only wondering why you cannot do such a simple thing inside a class constructor. Any help appreciated.
    I'm just trying to be a better person - My Name Is Earl

  2. #2
    Sweet
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    Code:
    GradeBook::GradeBook ( std::string name ) : nameList(10, "")
    {
    }
    Should accomplish what you want.

  3. #3
    Its hard... But im here swgh's Avatar
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    Thank you! I thought there was a way of doing it but I could find nothing to get around the error. I appreicate that.
    I'm just trying to be a better person - My Name Is Earl

  4. #4
    The superhaterodyne twomers's Avatar
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    Yeah. Member initialisation lists are how I generally initialise my variables. Not sure if it's good practise or faster, or slower or anything but I tend to do it.
    For your initialiser I'd do something like:

    Code:
    GradeBook::GradeBook( std::string name )
      : nameList( 10, "" ),
        m_Name( name ) {
    }
    It probably is better to do it this way because it sets values on construction rather than afterwards... might be more efficient to do it there... I really don't know.

  5. #5
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by twomers View Post
    Yeah. Member initialisation lists are how I generally initialise my variables. Not sure if it's good practise or faster, or slower or anything but I tend to do it.
    It is good practice, and it is faster in some cases, as for example means using copy-construction instead of default construction followed by assignment. Furthurmore when it comes to references it's actually a necessity.

    In debug builds in particular it is faster because it stops the compiler from filling the variable with dummy values like 0xCCCCCCCC prior to them being assigned real values. Instead they get set directly to what you tell them to be.

    Constructor initialisation lists are very much a good thing to use.
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