Question

I’m trying to get a regular pointer from an iterator, but a really peculiar thing is happening regarding the types I’m getting. I’ll just post the code, I assume these two snippets are equal to eachother, but tell me if I’m wrong. The code leading up to the snippets is:

CallbackTrigger trigger(triggerParameters);
std::set<CallbackTrigger> triggerSet;
auto result = triggerSet.insert(trigger);

Snippet A:

auto whatIGet = &(*result.first);  // whatIGet is type:
                    // "const std::allocator<CallbackTrigger>::value_type *"

Snippet B:

auto arbitraryStep = *result.first; 
auto whatIWanted = &arbitraryStep;  // this is type "CallbackTrigger*"

The code that has trouble with this difference and refuses to compile with snippet A, is when I try to push the pointer onto a list

std::list<CallbackTrigger*> listing;
listing.push_back(whatIWanted); // compiles fine
listing.push_back(whatIGet);    // error: "cannot convert parameter 1 from 
                               // 'const CallbackTrigger* to 'CallbackTrigger *&&'"

What is going on here?

Answer 1

According to the c++11 standard:

template<class T> class allocator {
  ...
  typedef T value_type;
  ...
};

Therefore const std::allocator<CallbackTrigger>::value_type * is the same as const CallbackTrigger*. Elements of set<CallbackTrigger> are of type const CallbackTrigger, so that is what you get, when you try getting a pointer directly to elements of the set.

For Snippet B arbitraryStep will be of type CallbackTrigger, since *result.first would be const CallbackTrigger and the const is dropped for the type deduction. Therefore &arbitraryStep is of type CallbackTrigger* (notice the lack of const). Also notice, that whatIWanted is a pointer to a local variable (arbitraryStep), not the object in the set. Therefore using that might not give you the behaviour you want.

Considering this it should be obvious that you need to change the type of your list to list<const CallbackTrigger*>.