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Thread: variable scope question

  1. #1
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    Default variable scope question

    I have declared a QSpinBox in the constructor of a dialog like this

    QSpinBox *recordNumberSpinBox = new QSpinBox;

    and also declared it as public: in the header file.

    But im getting core dumps when i try to do anythting with this QSpinBox in any other methods.

    for instance setting a maximum

    recordNumberSpinBox->setMaximum(5); in the method

    void Dialog:penDataFile(QFileInfo &fileInfo){
    ....
    }

    Am i confused in thinking that setting this to public means any method can access it and that its been set globaly?

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    Default Re: variable scope question

    So in effect you have two copies, one in your constructor and one in the class.

    Perhaps what you should do is delete the one in your constructor so the class scope takes over, then you can use it in other methods.

  3. The following user says thank you to squidge for this useful post:

    jeffmetal (16th June 2011)

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    Default Re: variable scope question

    If you are still confused, after squidge's post, then the simple way say is modify your constructor like this

    Qt Code:
    1. //QSpinBox *recordNumberSpinBox = new QSpinBox;
    2. recordNumberSpinBox = new QSpinBox;
    To copy to clipboard, switch view to plain text mode 

  5. The following user says thank you to Santosh Reddy for this useful post:

    jeffmetal (16th June 2011)

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    Default Re: variable scope question

    Quote Originally Posted by Santosh Reddy View Post
    If you are still confused, after squidge's post, then the simple way say is modify your constructor like this

    Qt Code:
    1. //QSpinBox *recordNumberSpinBox = new QSpinBox;
    2. recordNumberSpinBox = new QSpinBox;
    To copy to clipboard, switch view to plain text mode 
    Whats the diffence between the two lines above and should i also leave the QSpinBox *recordNumberSpinBox; in the header ?

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    Default Re: variable scope question

    The difference is that first line is declaration of a pointer, visible only in current method, that is constructor. The recordNumberSpinBox member variable is not initialized, so you have a crash when trying to access it. If you want to use the spinbox in other methods, you should leave the declaration in the header, and initialize the member variable:
    Qt Code:
    1. this->recordNumberSpinBox = new QSpinBox();
    To copy to clipboard, switch view to plain text mode 
    Consider this example, it's just like your issue:
    Qt Code:
    1. class A{
    2. protected:
    3. int var; //member variable, every object of class A has its own var
    4. public:
    5. A(){
    6. this->var = 10; // initializes this object member variable with value 10
    7. int var = 1; // creates local variable named var, visible only in this constructor
    8. std::cout << "var = " << var << ", this->var = " << this->var << "\n";
    9. // prints: var = 1, this->var = 10
    10. }
    11. void method(){
    12. int x = var + 1; // x = 11, because this objects member variable is used
    13. }
    14. };
    To copy to clipboard, switch view to plain text mode 

  8. The following user says thank you to stampede for this useful post:

    jeffmetal (16th June 2011)

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    Default Re: variable scope question

    Leave it in the header file

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    Default Re: variable scope question

    The first line declares and initialises a variable in the local scope to point at a block of memory. This variable masks any other variable of the same name in a broader scope, e.g. a class member variable or global. The line variable becomes unavailable at the end of the scope enclosing it (although memory remains allocated). The value of the outer variable remains unchanged, i.e. it does not point at the allocated memory afterward. Compilers will sometimes issue a warning that a local variable masks another variable of the same name.

    The second line initialises a variable declared at a broader scope.

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