Ivor Horton's Beginning Visual C++ 2012

SHARING MEMORY BETWEEN VARIABLES

As a relic of the days when 64KB was quite a lot of memory, you have a facility in C++ that allows more than one variable to share the same memory (but, obviously, not at the same time). This is called a union, and there are four basic ways in which you can use one:

• You can use it so that a variable, A, occupies a block of memory at one point in a program, which is later occupied by another variable, B, of a different type, because A is no longer required. I recommend that you don’t do this. It’s not worth the risk of error that is implicit in such an arrangement. You can achieve the same effect by allocating memory dynamically.
• Alternatively, you could have a situation in a program where a large array of data is required, but you don’t know in advance of execution what the data type will be — it will be determined by the input data. I also recommend that you don’t use unions in this case, since you can achieve the same result using a couple of pointers of different types and, again, allocating the memory dynamically.
• A third possible use for a union is one that you may need now and again — when you want to interpret the same data in two or more different ways. This could happen when you have a variable that is of type long, and you want to treat it as two values of type short. Windows will sometimes package two short values in a single parameter of type long passed to a function. Another instance arises when you want to treat a block of memory ...