Declaring and accessing array elements

When you create an array, you often already know what its contents should be. Instead of declaring an empty container and then assigning each slot one at a time, you can supply the starting values together with the declaration. This technique is called initializing an array with an initializer list.

Before reading further, look at a declaration and work out three things: what the name refers to, how many values the declaration asks for, and where those values come from. The rest of this article answers each of these questions in turn.

The declaration has three parts. The name a1 is the identifier you use to refer to the array later. The type portion array<int, 3> carries two pieces of information: the element type int, and the number 3, which is the number of elements the array is specified to hold. The part after the equals sign, enclosed in braces, is the initializer list, the list of values that fill the array when it is created.

Read the type portion carefully. The 3 in array<int, 3> is the declared size. It is set by the declaration itself, and the braces that follow are checked against it. The values inside the braces are the supplied values.

When an initializer list matches the declared size, each value in the braces occupies one slot of the array. The first value sits in the first position, the second in the second, and so on. Counting from zero, the common convention for C++ arrays, the first value goes to position 0, the second to position 1, and the third to position 2. Later code uses these positions to read or change individual elements.

Read the diagram from left to right. The value that appears first in the braces is at the lowest index, and the value that appears last is at the highest index. The position number is always one less than the value's place in the list, because counting starts at zero.

Notice that the rule is a comparison, not an exact match. A count equal to the declared size passes, and a smaller count also passes. Only a count that exceeds the declared size breaks the rule. The initializer may match the declared size exactly, or it may supply fewer values. It may never supply more.

Compare two declarations that both declare an array of size 3. The first supplies three values, which equals the declared size, so it passes. The second supplies four values, which exceeds the declared size, so it breaks the rule.

In the invalid line, the fourth value has no slot, because the array has only positions 0, 1, and 2. The source material says that when a declaration violates the counting rule, the problem sits in the declaration line, so that is where to look first.