A structure contains an ordered group of data objects. Unlike the elements of an array, the data objects within a structure can have varied data types. Each data object in a structure is a member or field.

A union is an object similar to a structure except that all of its members start at the same location in memory. A union variable can represent the value of only one of its members at a time.

In C++, structures and unions are the same as classes except that their members and inheritance are public by default.

You can declare a structure or union type separately from the definition of variables of that type, as described in Structure and union type definition and Structure and union variable declarations; or you can define a structure or union data type and all variables that have that type in one statement, as described in Structure and union type and variable definitions in a single statement.

Structures and unions are subject to alignment considerations. For a complete discussion of alignment, see "Aligning data" in the ILE C/C++ Programmer’s Guide.