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In the C, C++, and D programming languages, const is a type qualifier: a keyword applied to a data type that indicates that the data is constant (does not vary). While this can be used to declare constants, const in the C family of languages differs from similar constructs in other languages in being part of the ''type,'' and thus has complicated behavior when combined with pointers, references, composite data types, and type-checking.== Introduction == When applied in an object declaration, it indicates that the object is a constant: its value does not change, unlike a variable. This basic use – to declare constants – has parallels in many other languages. However, unlike in other languages, in the C family of languages the const is part of the ''type,'' not part of the ''object.'' For example, in C, const int x = 1; declares an object x of const int type – the const is part of the type, as if it were parsed “(const int) x” – while in Ada, X : constant INTEGER := 1_ declares a constant (a kind of object) X of INTEGER type: the constant is part of the ''object,'' but not part of the ''type.''This has two subtle results. Firstly, const can be applied to parts of a more complex type – for example, int const declares a constant pointer to a constant integer, while int const declares a variable pointer to a constant integer, and int declares a constant pointer to a variable integer. Secondly, because const is part of the type, it must match as part of type-checking. For example, the following code is invalid:void f(int& x); // ... const int i; f(i); because the argument to f must be a ''variable'' integer, but i is a ''constant'' integer. This matching is a form of program correctness, and is known as const-correctness. This allows a form of programming by contract, where functions specify as part of their type signature whether they modify their arguments or not, and whether their return value is modifiable or not. This type-checking is primarily of interest in pointers and references – not basic value types like integers – but also for composite data types or templated types such as containers. It is concealed by the fact that the const can often be omitted, due to type coercion (implicit type conversion) and C being call-by-value (C++ and D are either call-by-value or call-by-reference).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「Const (computer programming)」の詳細全文を読む スポンサード リンク
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