What is mutable const?

Question

I didn't find any topics related to mutable const on SO. I have reduced code to minimal working code (on visual studio). If we uncomment //*data = 11;, the compiler complains about const-ness. I wonder how mutable const works.

class A
{
public:

    void func(int & a) const
    {
        pdata = &a;
        //*pdata = 11;
    }

    mutable const int * pdata;
};

int main()
{
    const A obj;

    int a = 10;
    obj.func(a);
}

Answer 1

This example is a little confusing, because the mutable keyword is not part of the type specifier const int *. It's parsed like a storage class like static, so the declaration:

mutable const int *pdata;

says that pdata is a mutable pointer to a const int.

Since the pointer is mutable, it can be modified in a const method. The value it points to is const, and cannot be modified through that pointer.