Question

I have a list of type Instruction*. Instruction is a class that I made. This class has a function called execute().

I create a list of Instruction*

list<Instruction*> instList;

I create an Instruction*

Instruction* instPtr;
instPtr = new Instruction("test",10);

If I call

instPtr.execute();

the function will be executed correctly, however if I store instPtr in the instList I cannot call the execute() function anymore from the list.

//add to list
instList.push_back(instPtr);

//create iterator for list
list<Instruction*>::iterator p = instList.begin();
//now p should be the first element in the list
//if I try to call execute() function it will not work
p -> execute();

I get the following error:

error: request for member ‘execute’ in ‘* p.std::_List_iterator<_Tp>::operator-> [with _Tp = Instruction*]()’, which is of non-class type ‘Instruction*’

Answer 1

p is an iterator of Instruction * pointers. You can think of it as if it were of type Instruction **. You need to double dereference p like so:

(*p)->execute();

*p will evaluate to an Instruction *, and further applying the -> operator on that will dereference the pointer.