Difference Between Union and Structure
A union is a class all of whose data members are mapped to the same address within its object. The size of an object of a union is, therefore, the size of its largest data member.
In a structure, all of its data members are stored in contiguous memory locations. The size of an object of a struct is, therefore, the size of the sum of all its data members.
This gain in space efficiency, while valuable in certain circumstances, comes at a great cost of safety: the program logic must ensure that it only reads the field most recently written along all possible execution paths. The exception is when unions are used for type conversion: in this case, a certain field is written and the subsequently read field is deliberately different.
An example illustrating this point is:
+-----+-----+ struct { int a; float b } gives | a | b | +-----+-----+ ^ ^ | | memory location: 150 154 | V +-----+ union { int a; float b } gives | a | | b | +-----+Structures are used where an "object" is composed of other objects, like a point object consisting of two integers, those being the x and y coordinates:
typedef struct { int x; // x and y are separate int y; } tPoint;Unions are typically used in situation where an object can be one of many things but only one at a time, such as a type-less storage system:
typedef enum { STR, INT } tType; typedef struct { tType typ; // typ is separate. union { int ival; // ival and sval occupy same memory. char *sval; } } tVal;Read more about this topic: Union (computer Science)
Famous quotes containing the words difference between, difference, union and/or structure:
“The difference between Pound and Whitman is not between the democrat who in deep distress could look hopefully toward the future and the fascist madly in love with the past. It is that between the woodsman and the woodcarver. It is that between the mystic harking back to his vision and the artist whose first allegiance is to his craft, and so to the reality it presents.”
—Babette Deutsch (1895–1982)
“That was always the difference between Muhammad Ali and the rest of us. He came, he saw, and if he didn’t entirely conquer—he came as close as anybody we are likely to see in the lifetime of this doomed generation.”
—Hunter S. Thompson (b. 1939)
“We are constantly thinking of the great war ... which saved the Union ... but it was a war that did a great deal more than that. It created in this country what had never existed before—a national consciousness. It was not the salvation of the Union, it was the rebirth of the Union.”
—Woodrow Wilson (1856–1924)
“The question is still asked of women: “How do you propose to answer the need for child care?” That is an obvious attempt to structure conflict in the old terms. The questions are rather: “If we as a human community want children, how does the total society propose to provide for them?””
—Jean Baker Miller (20th century)