c++templatespointersconstantspass-by-const-reference

Can't pass 'const pointer const' to const ref

Suppose you have a set of pointers (yeah...) :

std::set<SomeType*> myTypeContainer;

Then suppose that you want to search this set from a const method of SomeType:

bool SomeType::IsContainered() const
{
    return myTypeContainer.find(this) != myTypeContainer.end();
}

This doesn't work. The this ptr in the method is a const SomeType *const, which I can't put into the find. The issue being that find takes a const-ref, which in this case would mean that the passed pointer is treated as const, but not the thing it points to.

Is there a way to resolve this smoothly (without changing the set template type)?

Solution

  • As you said, in the const member function this becomes const SomeType * (i.e. pointer to const), it can't be implicitly converted to SomeType * (i.e. pointer to non-const), which is the expected parameter type of find.

    You could use const_cast to perform explicit conversion.

    bool SomeType::IsContainered() const
{
    return myTypeContainer.find(const_cast<SomeType *>(this)) != myTypeContainer.end();
}

    It would be safe if the cast result is not used for modifying; while std::set::find won't do that.