A structure is a user-defined data type containing a collection of logically related data which may be of different types such as int, double, char, and so on. All of them are encapsulated (packed) in a single unit with a single name. The classes of C++ are in fact generalization of C-structures. A class in C++ with only public members is similar to a C-structure. All members of a structure in C are by default public. However, in a class declaration in C++, if there is no access specifier, the members are private by default. We know that arrays can be used to represent a group of data items that belong to the same type, such as int or float. This restriction is not there in structures.

The C language permits structures to be nested, in which a structure can contain structure members. A structure may be declared inside another structure. Also, a structure may be a data member of another structure. The important thing to note is how to initialize the data members of inner structures and how to access the data members of inner structures. The use of such nested structures facilitates representation and processing of complex data.

A program to determine the number of words and average word length is given in Program. It uses the strtok function to separate the words in a given string. The program given below uses the same technique to separate the words in a given string and determine and print the frequency of these words.

Let us use structure complex to represent a complex number and write a simple program containing only the main function. Thus, we can define structure complex either in the main function or before it as a global definition.

Since structures are multivalue, multitype data objects, they can be considered to form a useful tool in manipulation of quantities such as vectors, complex variables, etc. For this we define functions of the type struct, i.e., the return value of the function is a structure. The function may have arguments of type struct besides other arguments. Let us define a structure with three components of vector as its data members as shown below.