Member assignment in a const function

c++ function constants variable-assignment member
Fredrik Ullner picture Fredrik Ullner · Nov 25, 2009 · Viewed 9.2k times · Source

I have a class member myMember that is a myType pointer. I want to assign this member in a function that is declared as const. I'm doing as follows:

void func() const
{
     ...
     const_cast<myType*>(myMember) = new myType();
     ...
}

Doing this works fine in VC++, but GCC gives an error with the message "lvalue required as left operand of assignment".

Making the member mutable allow me to simply remove the const_cast and assign the value. However, I'm not entirely sure that that comes with other side-effects.

Can I assign my member without having to make the member mutable? How? Are there any side-effects in making members mutable?

Answer

Steve Gilham picture Steve Gilham · Nov 25, 2009

This scenario -- an encapsulated internal state change that does not impact external state (e.g. caching results) -- is exactly what the mutable keyword is for.

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