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In programming languages, ad hoc polymorphism〔C. Strachey, Fundamental concepts in programming languages. Lecture notes for International Summer School in Computer Programming, Copenhagen, August 1967〕 is a kind of polymorphism in which polymorphic functions can be applied to arguments of different types, because a polymorphic function can denote a number of distinct and potentially heterogeneous implementations depending on the type of argument(s) to which it is applied. It is also known as function overloading or operator overloading. The term ad hoc in this context is not intended to be pejorative; it refers simply to the fact that this type of polymorphism is not a fundamental feature of the type system. This is in contrast to parametric polymorphism, in which polymorphic functions are written without mention of any specific type, and can thus apply a single abstract implementation to any number of types in a transparent way. This classification was introduced by Christopher Strachey in 1967. ==Early binding== Ad hoc polymorphism is a dispatch mechanism: control moving through one named function is dispatched to various other functions without having to specify the exact function being called. Overloading allows multiple functions taking different types to be defined with the same name; the compiler or interpreter automatically ensures that the right function is called. This way, functions appending lists of integers, lists of strings, lists of real numbers, and so on could be written, and all be called ''append''—and the right ''append'' function would be called based on the type of lists being appended. This differs from parametric polymorphism, in which the function would need to be written ''generically'', to work with any kind of list. Using overloading, it is possible to have a function perform two completely different things based on the type of input passed to it; this is not possible with parametric polymorphism. Another way to look at overloading is that a routine is uniquely identified not by its name, but by the combination of its name and the number, order and types of its parameters. This type of polymorphism is common in object-oriented programming languages, many of which allow operators to be overloaded in a manner similar to functions (see operator overloading). Some languages that are not dynamically typed and lack ad hoc polymorphism (including type classes) have longer function names such as print_int, print_string, etc. This can be seen as advantage (more descriptive) or a disadvantage (overly verbose) depending on one's point of view.An advantage that is sometimes gained from overloading is the appearance of specialization, e.g., a function with the same name can be implemented in multiple different ways, each optimized for the particular data types that it operates on. This can provide a convenient interface for code that needs to be specialized to multiple situations for performance reasons. The downside is that the type system cannot guarantee the consistency of the different implementations. Since overloading is done at compile time, it is not a substitute for late binding as found in subtyping polymorphism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ad hoc polymorphism」の詳細全文を読む スポンサード リンク
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