I have a simple but confusing question here. Is it legal to have a different return value type for overridden methods than the abstact ones defined in the base class?? I did that and the compiler didn't complain... could someone please explain?

class MyBaseClass
{
    int value;

public:
    virtual int getValue() = 0;
};

class MyClass : public MyBaseClass
{
    double value;

public:
    virtual double getValue(); // here!!! return is double, not int
};

double MyClass::getValue()
{
   return this->value;
}

The compiler totally accepted something similar (MSVC und MinGW)... could anyone please exaplain to what extent this is legal?