20.2. Structure Definitions

Consider the following structure definition:

Keyword struct introduces the definition for structure Card. The identifier Card is the structure name and is used in C++ to declare variables of the structure type (in C, the type name of the preceding structure is struct Card). Card’s definition contains two string members—face and suit.

The following declarations

declare oneCard to be a structure variable of type Card, deck to be an array with 52 elements of type Card and cardPtr to be a pointer to a Card structure. Variables of a given structure type can also be declared by placing a comma-separated list of the variable names between the closing brace of the structure definition and the semicolon that ends the structure definition. For example, the preceding declarations could have been incorporated into the Card structure definition as follows:

As with classes, structure members are not necessarily stored in consecutive bytes of memory. Sometimes there are “holes” in a structure, because some computers store specific data types only on certain memory boundaries for performance reasons, such as half-word, word or double-word boundaries. A word is a standard memory unit used to store data in a computer—usually two, four or eight bytes and typically eight bytes on today’s popular 64-bit systems. Consider the following structure definition in which structure objects sample1 and sample2 of type Example are declared:

A computer with two-byte words might require that each of the members of Example be aligned on a word boundary (i.e., at the beginning of a word—this is machine dependent). Figure 20.1 shows a sample storage alignment for an object of type Example that’s been assigned the character 'a' and the integer 97 (the bit representations of the values are shown). If the members are stored beginning at word boundaries, there is a one-byte hole (byte 1 in the figure) in the storage for objects of type Example. The value in the one-byte hole is undefined. If the values in sample1 and sample2 are in fact equal, the structure objects might not be equal, because the undefined one-byte holes are not likely to contain identical values.

Fig. 20.1. Possible storage alignment for an Example object, showing an undefined byte.