One of the benefits of programming in a high-level language like C++ is that we usually do not have to worry about the details of how the computer stores our data. We can just declare int x = 10; and the compiler will do the work of deciding where to place that data into memory and keeping track of the location so when we next say x the code accesses that same location. However, there are times when it is useful to work directly with the memory address of a variable - to be able to say “work with the data stored at location X”.
In C++, to access the memory address of a variable, we can use the address-of operator &. To do so, we place it to the left of a variable name. For example, if we have a variable x, we can get its memory address using &x. The example below will print the memory address of x and y:
Address of x: 0x7ffc622069c0
Address of y: 0x7ffc622069c4
The 0x indicates that these are hexadecimal values. The rest of the digits are the actual address. Note that the two values, although quite large, are only 4 apart. Since we declared x and y one after another, and in this code, an int takes 4 bytes, the address of y is 4 more than the address of x. If we diagrammed the memory at a very low level, it might look like this:
In the diagram, we can see that what we call xare the 4 bytes starting at 0x7ffc622069c0(0x7ffc622069c0-0x7ffc622069c3). It stores the binary value for 10. What we call y are the 4 bytes starting at 0x7ffc622069c4. It stores the binary for 14. Even when we are dealing with memory addresses directly, that diagram is more detailed than we actually need. Here is a higher level view of the same memory:
This version focuses on what we actually need to know. It shows us that x is at 0x7ffc622069c0 and has the value 10. It also shows us that y is at 0x7ffc622069c4 and stores 14. We are describing it as a mid-level diagram because it includes some of the low level details - the memory addresses - but not all of them. A true high-level memory diagram, like we have used in the past, would not show us the addresses at all. It would just show us that x is 10 and y is 14.
Unfortunately the & symbol can mean multiple things in C++. It can mean:
That a variable is a reference. It means this when the & is placed to the right of a type. For example, in the function header void foo(int &x), the & means that x is a reference to an integer. The & is modifying the type of x.
The address-of operator, which gives the memory address of a variable. It means this when it is to the left of a variable name and there is no type specification. In other words, when we are using a variable, not declaring one, like cout << &x;
Logical or bitwise AND. It means this when it is placed between two variables. For example, in the expression x && y, the & means to do a bitwise AND operation on the values of x and y. x amp; y means to combine the two values bit by bit to create a new value (not covered in this book).
