Operator Overloading
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Thread: Operator Overloading

  1. #1
    Join Date
    Aug 2011
    Posts
    4

    Question Operator Overloading

    Hi there,

    I'm trying to write a smart pointer class for an int variable but I've gotten a little stuck. Firstly, to declare a smart pointer to an int, I MUST use this line of code (As seen in the main method below):

    smart_pointer s = new int();

    To do this, I try to overload the "=" operator and set a pointer to an int (called "value" in the smart_pointer class) to the address of this new int. However, I get this error:

    "conversion from ‘int*’ to non-scalar type ‘smart_pointer’ requested"

    What am I doing wrong? I thought "new int()" returned a memory address which I could set the smart_pointer's int pointer to by overloading the "=" sign...

    Thank you.

    -------Code-------

    #include<stdio.h>
    #include<stdlib.h>

    class smart_pointer{
    private:
    int *value;

    public:
    smart_pointer();

    // Overwrite "=" operator.
    smart_pointer &operator=(int *val){
    value = val;
    return *this;
    }

    ~smart_pointer(){
    delete value;
    }
    };

    smart_pointer::smart_pointer(){
    value = NULL;
    }

    int main(void){
    smart_pointer s = new int();
    return 0;
    }

  2. #2
    Join Date
    Jan 2006
    Location
    Singapore
    Posts
    6,391

    Re: Operator Overloading

    Quote Originally Posted by ace_of_pentacles
    To do this, I try to overload the "=" operator
    You are mistaken. This:
    Code:
    smart_pointer s = new int();
    is equivalent to:
    Code:
    smart_pointer s(new int());
    i.e., you should write a constructor instead (or perhaps, in addition to overloading operator= since you should also define or disable the copy constructor and copy assignment operator)
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  3. #3
    Join Date
    Jun 2009
    Location
    France
    Posts
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    Re: Operator Overloading

    Looking at your implementation, you might be interested in just using auto_ptr instead.

    If you have access to boost or C++0x, then you could look into unique_ptr and shared_ptr.
    Is your question related to IO?
    Read this C++ FAQ LITE article at parashift by Marshall Cline. In particular points 1-6.
    It will explain how to correctly deal with IO, how to validate input, and why you shouldn't count on "while(!in.eof())". And it always makes for excellent reading.

  4. #4
    Join Date
    Aug 2011
    Posts
    4

    Smile Re: Operator Overloading

    Ah, I see. Of course, the constructor would have to be the first thing to be called.
    I feel a little silly now.

    Thank you for taking the time to help me out.

  5. #5
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    Jun 2009
    Location
    France
    Posts
    2,359

    Re: Operator Overloading

    [EDIT] Double post
    Last edited by monarch_dodra; August 17th, 2011 at 06:31 AM.
    Is your question related to IO?
    Read this C++ FAQ LITE article at parashift by Marshall Cline. In particular points 1-6.
    It will explain how to correctly deal with IO, how to validate input, and why you shouldn't count on "while(!in.eof())". And it always makes for excellent reading.

  6. #6
    Join Date
    Jun 2009
    Location
    France
    Posts
    2,359

    Re: Operator Overloading

    Remember to make the constructor explicit. Inadvertently placing a pointer into an object that claims ownership of the pointer's life is one of the last things you want to do.

    [EDIT]Whilst were on the subject, having "smart_pointer &operator=(int *val)" and being able to write "smart_pointer s = new int();" are both very bad ideas, for the same reason.

    You'll want a named member function to do that.

    PS: There is a flaw in your operator= implementation: What if your smart_pointer already had a value?
    Is your question related to IO?
    Read this C++ FAQ LITE article at parashift by Marshall Cline. In particular points 1-6.
    It will explain how to correctly deal with IO, how to validate input, and why you shouldn't count on "while(!in.eof())". And it always makes for excellent reading.

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