Understanding Pointers, Arrays, and References

A declaration is a short line of code that tells the language what a name refers to and what kind of data it holds. Two of the most common declarations in C look almost identical on the page, yet they do very different jobs. One creates a collection of values. The other creates a variable that refers to a single object somewhere else. This article teaches you to read both, so that you can tell them apart before you see any further code.

Take the declaration char v[6]; and follow what it sets up. The name is v. The type char says what each element holds. The number inside the brackets, 6, says how many elements the array contains. In the declaration, the brackets mean array of, so the whole statement can be read as v is an array of six char. The source also says that the size given in an array declaration must be a constant expression.

The diagram lists every valid index for v. Read it from left to right: six slots exist, labeled v[0] through v[5]. The label v[6] belongs to no slot in v at all. Writing to or reading from v[6] would fall outside the array.

Every array in this description starts at index 0, which is its lower bound. The lower bound is the first valid index, and it is not 1. Because of this, the valid indices for an array of size $n$ run from $0$ to $n-1$. The last valid index is always one less than the number of elements. For char v[6], that means $n = 6$, the first index is $0$, and the last valid index is $6-1 = 5$.

A pointer is declared in a similar form, but the star symbol does the work that the brackets did for the array. The source reads the star as pointer to. The declaration char *p; therefore creates a pointer variable that is meant to hold the address of an object of type char. The name p does not hold a char itself. It holds the location of a char object somewhere else.

The diagram shows the relationship. The pointer p stores an address, and that address refers to one object elsewhere. The source says a pointer variable can hold the address of an object of the appropriate type. The type in the declaration tells you what kind of object the address is supposed to point to. A pointer declared as char * is meant for objects of type char, and not for objects of some other type.

The two declarations use nearly the same pattern, and that is why they are easy to confuse. The difference lies in what each declaration says about the name. The array declaration says the name is a collection of six values. The pointer declaration says the name holds the location of one object of the given type. Reading the symbol is the key step: brackets signal array of, and the star signals pointer to.