I would like to inherit from a class with the const
specifier like this:
class Property
{
int get() const;
void set(int a);
};
class Co
I would like to inherit from a class with the const specifier"
However much you want to is irrelevant. You cannot. That is not valid C++.
Can I somehow use the const keyword for inheritance?"
No.
I have a trick, not a clean solution.
class ConstChild : private Property
{
operator const Property () { return *this; }
};
then
ConstChild cc;
cc.set(10); // ERROR
cc().get();
If you create a const
member function set
, you will get what you need.
class Property
{
int get() const;
void set(int a);
};
class ConstChild : public Property
{
void set(int a) const {}
};
The only caveat is that a sly user can circumvent your intention by using:
ConstChild child;
child.set(10); // Not OK by the compiler
Property& base = child;
base.set(10); // OK by the compiler
You can use a template class and a specialization for a constant type:
template<typename T> class base_property {
protected:
T value;
};
template<typename T> class property : public base_property<T> {
public:
const T& get()const { return value; }
void set(const T& v){ value = v; }
};
template<typename T> class property<const T> : public base_property<T> {
public:
const T& get()const { return value; }
};
class ConstChild : public property<const int>{ };
I had the same need and I ended up with this quite manual approach :
class A
{
public:
void fooMutable(void) {}
void fooConst(void) const {}
};
class B : private A
{
public:
using A::fooConst;
const A& getParent(void) const
{
return *this;
}
};
void fooParent(const A&) {}
int main(void)
{
auto b = B{};
b.fooConst(); // Ok
b.fooMutable(); // Fail
fooParent(b); // Fail
fooParent(b.getParent()); // Ok
return 0;
}
Note that the using keyword would not work with overloads const/mutable :
class A
{
public:
void foo(void) {}
void foo(void) const {}
};
class B : private A
{
public:
using A::foo; // Expose the const and the mutable version
};
To solve this you could redefine the function yourself and call the parent :
class B : private A
{
public:
void foo(void) const
{
A::foo();
}
};
It can become pretty time consuming if you're inheriting a large hierarchy, but if it's for a not-so-big class it should be very reasonable and being quite natural for the user.
I had the need for a related problem, which is: to really control/highlight mutable and const access on some class. I did it with this simple reusable template wrapper:
template <typename T>
class TAccessor : private T
{
public:
const T& Const() const { return *this; }
T& Mutable() { return *this; }
};
// Example of use:
template <typename T>
using MyVector = TAccessor<std::vector<T>>;
void main()
{
MyVector<int> vector;
vector.Mutable().push_back(10);
int x = vector.Const()[1];
...
}