Working with fixed-size sequences using array

When you create an array, you often already know what its contents should be. One option is to declare an empty container and then assign each slot one at a time. A more direct option is to supply the starting values together with the declaration itself. The source material for this topic calls this technique initializing an array with an initializer list.

Before reading further, look at a declaration and ask yourself three questions: what name refers to the array, how many values the declaration asks for, and where those values come from. The rest of this article answers each of those questions and then gives you a rule for deciding whether a declaration is valid.

The declaration has three parts. The name a1 is the identifier you will use to refer to the array later in the program. 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.

Keep the two numbers in the type separate in your mind. The 3 in array<int,3> is the declared size, which is a property of the array itself. The values inside the braces are the supplied values, which are what you are asking the declaration to store. The counting rule in the next section compares these two quantities.

Notice that this rule is a comparison, not an exact match. An initializer with exactly the declared number of values passes, and an initializer with fewer values also passes. Only a count larger than the declared size breaks the rule. So for array<int,3>, the braces may hold three values, two values, one value, or none at all, but never four.

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, which is 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 10 is the first one written in the braces, so it occupies position 0. The value 20 occupies position 1, and the value 30 occupies position 2. The diagram shows the three slots that the declaration sets up for a1, with each slot holding one value in the order the values were written.

To check a declaration, count the values in the braces and compare that count with the declared size. If the count is less than or equal to the size, the declaration satisfies the rule. If the count is larger, the declaration breaks the rule, and the problem sits in the declaration line, so that is where you should look first.