Copy constructor error, 'this' pointer conversion

**bennyandthejets** · 01-08-2004

Making a copy constructor, I am getting a problem.

Code:

CPost::CPost(const CPost& rhs)
{
	rhs.GetMessageLength(); //error on this line
}

error C2662: 'GetMessageLength' : cannot convert 'this' pointer from 'const class CPost' to 'class CPost &'

Some discussions I found about this recommended taking away the const, but when I do that, MS VC++ 6.0 wont recognize it as a copy constructor. Is anyone aware of a solution to this problem?

**Shakti** · 01-08-2004

Try to make the function const aswell, like

Code:

class c
{
public:
    c(const c& rhs);
    int GetSomething() const { return something; }
private:
    int something;
};

c::c(const c& rhs)
{
    rhs.GetSomething();
}

Not sure if that is it but according to msdn this should solve your problem.

**bennyandthejets** · 01-08-2004

That worked, thanks. But what if the function I want to call CANNOT be const, ie, it modifies the object?

**Shakti** · 01-08-2004

Then you cant send the object as a const. At least this is how I think it is.

**Prelude** · 01-08-2004

>But what if the function I want to call CANNOT be const, ie, it modifies the object?
Look up the mutable keyword. I can't guarantee that this is what you want since I haven't seen your code, but it's likely.

**jlou** · 01-08-2004

If the function that cannot be const is logically const - meaning it might change the value of some private member variables but the logical state of the object is unchanged - then like Prelude suggested you can use the mutable keyword on the variables that might change and still mark the function as const.

However, if the function that cannot be const actually changes the state of the object, then you shouldn't be using it inside the copy constructor. It doesn't make sense to modify the argument passed into the constructor. That is why the convention is to use a const reference parameter.

**bennyandthejets** · 01-08-2004

then you shouldn't be using it inside the copy constructor. It doesn't make sense to modify the argument passed into the constructor.

I never thought about it that way. Cheers

.

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