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, union and/or structure:
“The difference between de jure and de facto segregation is the difference open, forthright bigotry and the shamefaced kind that works through unwritten agreements between real estate dealers, school officials, and local politicians.”
—Shirley Chisholm (b. 1924)
“These semi-traitors [Union generals who were not hostile to slavery] must be watched.—Let us be careful who become army leaders in the reorganized army at the end of this Rebellion. The man who thinks that the perpetuity of slavery is essential to the existence of the Union, is unfit to be trusted. The deadliest enemy the Union has is slavery—in fact, its only enemy.”
—Rutherford Birchard Hayes (1822–1893)
“Agnosticism is a perfectly respectable and tenable philosophical position; it is not dogmatic and makes no pronouncements about the ultimate truths of the universe. It remains open to evidence and persuasion; lacking faith, it nevertheless does not deride faith. Atheism, on the other hand, is as unyielding and dogmatic about religious belief as true believers are about heathens. It tries to use reason to demolish a structure that is not built upon reason.”
—Sydney J. Harris (1917–1986)