Question

I have a region of memory, which will be used for multiple queues. For example, I allocate 1024 bytes of memory and I need two queues. The first queue will occupy the first 512 bytes and the second the next 512 bytes.

However, my queue is represented by a C++ class. Using the placement new operator, how can I construct each queue object. Is the following approach correct?

Queue *q1, *q2;
void *mem = malloc( 1024 );

*q1 = new (mem) Queue;
*q2 = new (mem+512)Queue;

Answer 1

I would do the following:

Queue* mem = static_cast<Queue*>(malloc(2 * sizeof(Queue));
Queue* q1 = new (mem) Queue;
Queue* q2 = new (mem+1) Queue;

If it’s important that the two queues are exactly 512 bytes from each other your original suggestion is almost correct:

char* mem = static_cast<char*>(malloc(1024));
Queue* q1 = new (mem) Queue;
Queue* q2 = new (mem+512) Queue;

This assumes sizeof(Queue) <= 512. The reason for the cast to char* is that pointer arithmetic with void* is illegal in C++.