tagged pointerとは 意味・読み方・使い方

In computer science, a tagged pointer is a common example of a tagged union, where the primary type of data to be stored in the union is a pointer. Often, the tag in a tagged pointer will be "folded" into the data representing the pointer, taking advantage of one of the following properties of pointers:

• Pointers to certain types of data will often be aligned to the size of the data (4 bytes, 8 bytes, etc.), which leaves a few bits of the pointer unused. As long as code that uses the pointer properly masks out these bits, the pointer can be tagged with extra information.
• The virtual memory system in most modern operating systems reserves a block of logical memory around address 0 as unusable. This means that, for example, a pointer to 0 is never a valid pointer and can be used as a special null pointer value.