When is memory allocated when declaring an array in C plus plus?

It depends how the array is declared (fixed size or variable size) and where it is declared (global scope or local scope).

If the array is declared in global scope (outside a function) and is fixed size, it will be allocated in static memory. If it is variable size, the pointer is stored in static memory while the array itself is allocated on the heap. The pointer in static memory points to the start address of the array in heap memory.

If the array is declared in local scope (inside a function) and is fixed size, it will be allocated on the stack in whichever thread the function was called. If it is variable size, the local pointer is stored on the stack while the array is allocated on the heap. The pointer will fall from scope when the function returns so the array must not be allowed to outlive the function in which the pointer is declared. If the array must outlive the function that allocates the array, the pointer must be declared at a higher scope in the call stack and must be passed by reference to or returned by value from the function that allocates the array.

If you provide your own memory manager, however, an array may be allocated wherever the memory manager's memory pool is allocated, be it in static memory, the stack or the heap. A memory manager essentially allocates an array of bytes which you can then utilise as you see fit (the array of bytes will be allocated as per the previous description for arrays in general).