Set array of object to null in C++

Question

Suppose I have an array of objects of type Foo in C++:

Foo array[10];

In Java, I can set an object in this array to null simply by:

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Answer 1

so is there away to "remove" an element in the array without creating a new one? i.e making a hole in the array of some sort?

In C++, if you have an array:

Foo array[10];

Then all 10 elements will be default-constructed. In general, there's no concept of array elements being in use vs. "removed". One way to track this idea is to make having actual values in the array elements optional, as in:

boost::optional array[10];

You can read about the boost library's optional<> library at http://www.boost.org/doc/libs/release/libs/optional/

For the sake of documenting alternatives, I'll cover a hideous one. You could call the destructor on one of the elements: array[3].~Foo(); This is very dangerous as when the array itself goes out of scope, the destructor for each element will be called, and a precondition for that is to have a properly constructed object, so you have to be sure to have constructed a Foo in that element again beforehand (using "placement" new). There is nothing in the array itself that will help you keep track of which elements have had their destructors called by you - you'd need to track that yourself (having some record of this inside the objects would probably work in practice, but accessing the object after destruction is undefined behaviour). You'd need to be very careful that in all code paths - including those due to exceptions - you tracked the momentarily unused array elements. You really don't want to do this.

If your concern is to remove an element from the array to save memory, then use smart pointers:

shared_ptr array[10];

You can then control specific array elements independently, with only still-populated elements being destructed when the element goes out of scope. Suitable smart pointers are available in boost or C++11, or you could use std::auto_ptr<> but should carefully read about its semantics beforehand to see if it's suitable and how to safely use it.

Separately, if there's a Foo::operator=(Some_Type*) then you could set array[3] = NULL;, and it would do whatever that assignment operator did to array[3]. Still, if Foo is not a pointer, it is probably a bad idea for it to allow someone to assign NULL to it.