Can someone explain exactly what happens if an exception is thrown during the process of allocating an array of objects on the heap?-Collection of common programming errors

The “atomic” C++ allocation and construction functions are correct and exception-safe: If new T; throws, nothing leaks, and if new T[N] throws anywhere along the way, everything that’s already been constructed is destroyed. So nothing to worry there.

Now a digression:

What you always must worry about is using more than one new expression in any single unit of responsibility. Basically, you have to consider any new expression as a hot potato that needs to be absorbed by a fully-constructed, responsible guardian object.

Consider new and new[] strictly as library building blocks: You will never use them in high-level user code (perhaps with the exception of a single new in a constructor), and only inside library classes.

To wit:

// BAD:
A * p = new A;
B * q = new B;  // Ouch -- *p may leak if this throws!

// Good:
std::unique_ptr p(new A);
std::unique_ptr q(new B); // who cares if this throws
std::unique_ptr r(new C[3]); // ditto

As another aside: The standard library containers implement a similar behaviour: If you say resize(N) (growing), and an exception occurs during any of the constructions, then all of the already-constructed elements are destroyed. That is, resize(N) either grows the container to the specified size or not at all. _{(E.g. in GCC 4.6, see the implementation of _M_fill_insert() in bits/vector.tcc for a library version of exception-checked range construction.)}