We use enum keyword to define a Enumeration. Structure is a collection of variables of different data types under a single name. It is similar to a class in that, both holds a collecion of data of different data types. Structure and union variable declarations A structure or union declaration has the same form as a definition except the declaration does not have a brace-enclosed list of members.
You must declare the structure or union data type before you can define a variable having that type. Struct keyword is used to declare the structure. Union keyword is used to declare the Union. Structure variable will allocate memory for all the structure members separately. Union variable will allocate common memory for all the union members. It is a user-defined data type, which holds its own data members and member functions, which can be accessed and used by creating an instance of that class.
Pointers must be declared before they can be used, just like a normal variable. A pointer is associated with a type such as int and double too. How do you initialize a union?
Category: technology and computing programming languages. You initialize a union to another union with the same type. Alternatively, you can initialize a union by initializing the first member of a union. How do you initialize a structure member? C Structure Initialization. Way 1 : Declare and Initialize.
Can we initialize variable in structure? Can we use static variable in structure? This definition means that at any given time, a union can contain no more than one object from its list of members. It also means that no matter how many members a union has, it always uses only enough memory to store the largest member.
A union can be useful for conserving memory when you have lots of objects and limited memory. However, a union requires extra care to use correctly. You're responsible for ensuring that you always access the same member you assigned. If any member types have a non-trivial constructor, then you must write additional code to explicitly construct and destroy that member. Before you use a union, consider whether the problem you're trying to solve could be better expressed by using a base class and derived class types.
Begin the declaration of a union by using the union keyword, and enclose the member list in curly braces:. In the previous example, any code that accesses the union needs to know which member holds the data. The most common solution to this problem is called a discriminated union. It encloses the union in a struct, and includes an enum member that indicates the member type currently stored in the union. The following example shows the basic pattern:. In the previous example, the union in the Input struct has no name, so it's called an anonymous union.
Its members can be accessed directly as if they're members of the struct. For more information about how to use an anonymous union, see the Anonymous union section. The previous example shows a problem that you could also solve by using class types that derive from a common base class. You could branch your code based on the runtime type of each object in the container. Your code might be easier to maintain and understand, but it might also be slower than using a union.
Like structures, unions are used to create new data types. It can also contain members just like structures. The syntax of defining a union, creating union variables and accessing members of the union is same as that of structures, the only difference is that union keyword is used instead of structure. The important difference between structures and unions is that in structures each member has it's own memory whereas members in unions share the same memory.
When a variable of type union is declared the compiler allocates memory sufficient to hold the largest member of the union. Since all members share the same memory you can only use one member of a union at a time, thus union is used to save memory. The syntax of declaring a union is as follows:.
If we have a union variable then we can access members of union using dot operator. In lines , a union data is declared with three members namely var1 of type int , var2 of type double and var3 of type char.
When the compiler sees the definition of union it will allocate sufficient memory to hold the largest member of the union. In this case, the largest member is double , so it will allocate 8 bytes of memory.
In line 18, the first member of t i. The important thing to note is that at this point the other two members contain garbage values. In line 19, the value of t.
In line 21, the second member of t i. At this point, the other two members contain garbage values. In line 24, the third member of t i. In line 27, the sizeof operator is used to print the size of the union. Since we know that, in the case of a union, the compiler allocates sufficient memory to hold the largest member. The largest member of union data is var2 so the sizeof operator returns 8 bytes which is then printed using the printf statement.
In the above program, we have seen how we can initialize individual members of a union variable.
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