Understanding the Expression ((size_t *)ptr - 1) in C/C++

The expression '( (size_t *)ptr - 1)' casts the pointer 'ptr' to a 'size_t' pointer and then subtracts 1 from it.

This expression is commonly used when dealing with memory allocation functions like 'malloc' or 'calloc'. In these functions, the memory allocated is typically preceded by a header that stores information about the allocated block, such as its size. By subtracting 1 from the pointer, we can access this header information.

For example, if 'ptr' points to the beginning of a dynamically allocated block of memory, '( (size_t *)ptr - 1)' will give us a pointer to the header information just before the block.