const parameter question

This is a discussion on const parameter question within the C++ Programming forums, part of the General Programming Boards category; I get the following error(marked in the code): error C2662: 'species' : cannot convert 'this' pointer from 'const class Duck' ...

  1. #1
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    const parameter question

    I get the following error(marked in the code):
    error C2662: 'species' : cannot convert 'this' pointer from 'const class Duck' to 'class Duck &'
    Conversion loses qualifiers

    Code:
    #include<iostream>
    using namespace std;
    
    class Duck
    {
    public:
    	virtual void species() = 0;
    };
    
    class Mallard : public Duck
    {
    public:
    	void species()
    	{
    		cout<<"Mallard"<<endl;
    	}
    };
    
    
    void operateDuck(const Duck* pDuck)
    {
    	pDuck->species();
    }
    	
    int main()
    {
    	Mallard myDuck;
    	operateDuck(&myDuck);
    	
    	cout<<endl;
    	return 0;
    }

  2. #2
    Yes, my avatar is stolen anonytmouse's Avatar
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    You need to tell the compiler that the species function doesn't alter the object:
    Code:
    class Duck
    {
    public:
    	virtual void species() const = 0;
    };
    
    class Mallard : public Duck
    {
    public:
    	void species() const
    	{
    		cout<<"Mallard"<<endl;
    	}
    };
    C++ FAQ Lite: What is a "const member function"?

  3. #3
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    Ah hah. Const objects can only call const member functions(they have to be assured they will not be changed inside the function). Since I declared the parameter const, then inside the function it is a const object and species() is not a const member function. Adding const to the species() function in the Mallard class just causes more errors because the abstract base class version was not declared as const. So, you have to declare the base class version const.

  4. #4
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    lol. I guess I was writing my post when you posted. Thanks.

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