In computer science and computer programming, a type system is said to feature strong typing when it specifies one or more restrictions on how operations involving values of different data types can be intermixed. The opposite of strong typing is weak typing.
Most generally, "strong typing" implies that the programming language places severe restrictions on the intermixing that is permitted to occur, preventing the compiling or running of source code which uses data in what is considered to be an invalid way. For instance, an addition operation may not be used with an integer and string values; a procedure which operates upon linked lists may not be used upon numbers. However, the nature and strength of these restrictions is highly variable.
Some of the factors which writers have qualified as "strong typing" include:
Note that some of these definitions are contradictory, others are merely orthogonal, and still others are special cases (with additional constraints) of other, more "liberal" (less strong) definitions. Because of the wide divergence among these definitions, it is possible to defend claims about most programming languages that they are either strongly or weakly typed. For instance: