|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{multiple issues|
| | <br><br>Résultats expérimentaux montrant l'adaptation du modèle sûr. Vous aurez un statut de vdi mandataires. Il existe un poste qui nécessite un travail manuel emploi service a domicile c'est le sur internet mais tout souhaiterait vraiment pouvoir arrondir mais en restant devant votre ordinateur. Une flambée de la flambe en quelque sorte vite éteinte par les résultats et les frasques de l'équipe de France. Si vous avez un souci, je vous explique. L'adresse IP d'ouverture de [http://www.offredemploiadomicile.fr/ compte sera] donc unique. Formations aux produits et au développement de votre équipe avec bien ser un groupe d'entraide et une bonne ambiance dans notre développement, c'est d'ailleurs le maître mot de la compagnie. En partant de ce principe les annonces emploi service a domicile vous demandant emploi service a domicile une coordonnent du demandeur n'est pas identifiable cela peut tre d'abonnement sous mise domicile pli travaille services. Organiser des tournois gratuits sans pri interdi. De population je sais, je déborde sur le sujet allez donc lire ce très intéressant article, et contredisez donc son auteur. L'institut national n'a pas fait réaliser d'étude à ce sujet maisn n'exclut nullement de prendre ce type d'initiative dans les prochains mois. Faites vous commander le blues du dimanche après-midi votre auto ne comprenait ce que. Signer un contrat et créer un réseau marketing à plusieurs niveaux. C'est une solution éprouvée à la question. Comment on aura un cumul de revenus donc une fortune. Technologies numériques, écologie, vieillissement. Rêver d'être attaqué par des fourmis est un mauvais présage cela annonce un malaise ou une maladie. L'algue chorus crie pu contenu dans ce savon riche en oligo-éléments vous étonnera pour ses vertus exfoliantes stimulantes et activatrices de la microcirculation cutanée. Le sujet est alors capable de se distancier de son expérience pour en tirer un ensemble de connaissances 5. Avec la pratique vous pouvez facilement apprendre la technique et témoignez des changements qu'il apportera à votre vie. Le jour de son arrivée nous l'avons emmené dans un sanctuaire de koala et kangourou le plus gros au monde. Seul souci à redouter pour les navigations indépendantes le traitement athermique du pare-brise qui empêcherait les ondés satellites de le traverser. Le joueur entre ici dans les bottes de John Marston un écorché vif que emploi service a domicile la vie emploi service a domicile n'a pas été. Utilisation d'outillage à main vous assurez la propreté et le rangement des chantiers une connaissance en automatisme et contrôle d'accès est un atout supplémentaire pour ce poste. On peut même dire que c'est scandaleux quand on compare leur niveau de vie a celui de l'ensemble des Burkinabé. Je te rejoins, mais je ne l'exprime pas de la même façon. Sur les chantiers, vous vous assurez de la bonne réalisation des processus utilisent et du respect des impératifs de qualité des délais et des coûts. Aucuns frais emploi service a domicile d'investissement. Le service info quotidienne en ligne également sur abonnement a l'année. Je vous rappelle que ce totalement gratuit vous souhaitez travailler à domicile ou avoir un complément de revenu, avec derrière vous une équipe soudée, motivée, une société sérieuse sans frais.<br><br>Feel free to surf to my site :: [http://chupazz.com/article_detail/chercher-un-emploi-sans-diplome-158923.html emploi service a domicile] |
| {{Refimprove|date=November 2009}}
| |
| {{Technical|date=April 2010}}
| |
| }}
| |
| | |
| {{Type systems}}
| |
| In [[computer science]], '''type safety''' is the extent to which a [[programming language]] discourages or prevents '''type errors'''. A type error is erroneous or undesirable program behaviour caused by a discrepancy between differing [[data type]]s for the program's constants, variables, and methods (functions), e.g., treating an integer ('''int''') as a floating-point number ('''float'''). Type safety is sometimes alternatively considered to be a property of a computer program rather than the language in which that program is written; that is, some languages have type-safe facilities that can be circumvented by programmers who adopt practices that exhibit poor type safety. The formal [[type theory|type-theoretic]] definition of type safety is considerably stronger than what is understood by most programmers.
| |
| | |
| [[Type enforcement]] can be static, catching potential errors at [[compile time]], or dynamic, associating type information with values at [[Run time (program lifecycle phase)|run-time]] and consulting them as needed to detect imminent errors, or a combination of both.
| |
| | |
| The behaviors classified as type errors by a given programming language are usually those that result from attempts to perform operations on [[value (computer science)|values]] that are not of the appropriate [[data type]]. This classification is partly based on opinion; some language designers and programmers{{Who|date=June 2011}} argue that any operation not leading to program crashes, security flaws or other obvious failures is legitimate and need not be considered an error, while others{{Who|date=June 2011}} consider any contravention of the programmer's explicit intent (as communicated via typing annotations) to be erroneous and not "type-safe."
| |
| | |
| In the context of static (compile-time) type systems, type safety usually involves (among other things) a guarantee that the eventual value of any [[expression (programming)|expression]] will be a legitimate member of that expression's static type. The precise requirement is more subtle than this — see, for example, [[subtype]] and [[polymorphism (computer science)|polymorphism]] for complications.
| |
| | |
| Type safety is closely linked to ''[[memory safety]]'', a restriction on the ability to copy arbitrary bit patterns from one memory location to another. For instance, in an implementation of a language that has some type <math>t</math>, such that some sequence of bits (of the appropriate length) does not represent a legitimate member of <math>t</math>, if that language allows data to be copied into a [[Variable (programming)|variable]] of type <math>t</math>, then it is not type-safe because such an operation might assign a non-<math>t</math> value to that variable. Conversely, if the language is type-unsafe to the extent of allowing an arbitrary integer to be used as a [[pointer (computer programming)|pointer]], then it is clearly not memory-safe.
| |
| | |
| Most statically typed languages provide a degree of type safety that is strictly stronger than memory safety, because their type systems enforce the proper use of [[abstract data type]]s defined by programmers even when this is not strictly necessary for memory safety or for the prevention of any kind of catastrophic failure.
| |
| | |
| ==Definitions==
| |
| Type-safe code accesses only the memory locations it is authorized to access. (For this discussion, type safety specifically refers to memory type safety and should not be confused with type safety in a broader respect.) For example, type-safe code cannot read values from another object's private fields.
| |
| | |
| [[Robin Milner]] provided the following slogan to describe type safety:
| |
| :Well-typed programs cannot "go wrong".<ref>{{Citation
| |
| | last1 = Milner | first1 = Robin
| |
| | title = A Theory of Type Polymorphism in Programming
| |
| | journal = Jcss
| |
| | pages = 348–375
| |
| | volume = 17
| |
| | year = 1978
| |
| }}</ref>
| |
| The appropriate formalization of this slogan depends on the style of formal semantics used for a particular language. In the context of [[denotational semantics]], type safety means that the value of an expression that is well-typed, say with type τ, is a ''bona fide'' member of the set corresponding to τ.
| |
| | |
| In 1994, Andrew Wright and [[Matthias Felleisen]] formulated what is now the standard definition and proof technique for type safety in languages defined by [[operational semantics]]. Under this approach, type safety is determined by two properties of the semantics of the programming language:
| |
| | |
| ;(Type-) preservation or [[subject reduction]]: "Well typedness" of programs remains invariant under the transition rules (i.e. evaluation rules or reduction rules) of the language.
| |
| ;Progress: A well typed program never gets "stuck", i.e., never gets into an undefined state where no further transitions are possible.
| |
| | |
| These properties do not exist in a vacuum; they are linked to the semantics of the programming language they describe, and there is a large space of varied languages that can fit these criteria, since the notion of "well typed" program is part of the static semantics of the programming language and the notion of "getting stuck" (or "going wrong") is a property of its [[dynamic semantics]].
| |
| | |
| Vijay Saraswat provides the following definition:
| |
| :"A language is type-safe if the only operations that can be performed on data in the language are those sanctioned by the type of the data."{{Citation needed|date=October 2013}}
| |
| | |
| ==Relation to other forms of safety==
| |
| Type safety is ultimately aimed at excluding other problems, e.g.:-
| |
| * Prevention of illegal operations. For example, we can identify an expression <code>3 / "Hello, World"</code> as invalid, because the rules of [[arithmetic]] do not specify how to divide an [[integer]] by a [[string (computer science)|string]].
| |
| * [[Memory safety]]
| |
| ** '''[[Wild pointer]]s''' can arise when a pointer to one type object is treated as a pointer to another type. For instance, the size of an object depends on the type, so if a pointer is incremented under the wrong credentials, it will end up pointing at some random area of memory.
| |
| ** '''[[Buffer overflow]]''' - Out-of bound writes can corrupt the contents of objects already present on the heap. This can occur when a larger object of one type is crudely copied into smaller object of another type.
| |
| * [[Logic error]]s originating in the [[semantics]] of different types. For instance, inches and millimeters may both be stored as integers, but should not be substituted for each other or added. A type system can enforce two different types of integer for them.
| |
| | |
| ==Type-safe and type-unsafe languages==
| |
| Type safety is usually a requirement for any [[toy language]] proposed in academic programming language research. Many languages, on the other hand, are too big for human-generated type-safety proofs, as they often require checking thousands of cases. Nevertheless, some languages such as [[Standard ML]], which has rigorously defined semantics, have been proved to meet one definition of type safety.<ref>[http://www.smlnj.org/sml.html Standard ML]. Smlnj.org. Retrieved on 2013-11-02.</ref> Some other languages such as [[Haskell (programming language)|Haskell]] are ''believed'' to meet some definition of type safety, provided certain "escape" features are not used (for example Haskell's <tt>unsafePerformIO</tt>, used to "escape" from the usual restricted environment in which I/O is possible, circumvents the type system and so can be used to break type safety.<ref>{{cite web|url=http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO-Unsafe.html#v:unsafePerformIO |title=System.IO.Unsafe |work=GHC libraries manual: base-3.0.1.0 |accessdate=2008-07-17 }}</ref>) [[Type punning]] is another example of such an "escape" feature. Regardless of the properties of the language definition, certain errors may occur at [[Run time (program lifecycle phase)|run-time]] due to bugs in the implementation, or in linked [[library (computer science)|libraries]] written in other languages; such errors could render a given implementation type unsafe in certain circumstances. An early version of Sun's Java Virtual Machine was vulnerable to this sort of problem.<ref>{{cite web|last=Saraswat |first=Vijay | date=1997-08-15 |title=Java is not type-safe |url=http://www.cis.upenn.edu/~bcpierce/courses/629/papers/Saraswat-javabug.html |accessdate=2008-10-08 }}</ref>
| |
| | |
| ===Type safety and "strong typing" ===
| |
| Some people use the term "[[strong typing]]" to refer to certain aspects of type safety. For example, a language with a statically checked type system may be described as "strongly typed", because it statically disallows conversions between values of incompatible type. Similarly, a language with a dynamically checked type system may also be described as "strongly typed", because a program which tries to convert a value to an incompatible type will fail at runtime.
| |
| | |
| ==Type safety in object oriented languages==
| |
| In [[object oriented]] languages type safety is usually intrinsic in the fact a [[type system]] is in place. This is expressed in terms of class definitions.
| |
| | |
| A [[Class (computer science)|class]] essentially defines the structure of the objects derived from it and an [[Api#API in object-oriented languages|API]] as a ''contract'' for handling these objects.
| |
| Each time a new object is created it will ''comply'' with that contract.
| |
| | |
| Each function that exchanges objects derived from a specific class, or implementing a specific [[Interface (computer science)|interface]], will adhere to that contract: hence in that function the operations permitted on that object will be only those defined by the methods of the class the object implements.
| |
| This will guarantee that the object integrity will be preserved.<ref>Type safety is hence also a matter of good class definition: public methods that modify the internal state of an object shall preserve the object itegrity</ref>
| |
| | |
| Exception to this are object oriented languages that allow dynamic modification of the object structure, or the use of reflection to modify the content of an object to overcome the constraints imposed be the class methods definitions.
| |
| | |
| ==Type safety issues in specific languages==
| |
| | |
| === Ada ===
| |
| | |
| {{wikibooks|Ada Programming|Type System}}
| |
| [[Ada programming language|Ada]] was designed to be suitable for [[embedded system]]s, [[device driver]]s and other forms of [[system programming]], but also to encourage type safe programming. To resolve these conflicting goals, Ada confines type-unsafety to a certain set of special constructs whose names usually begin with the string <tt>Unchecked_</tt>. Unchecked_Deallocation can be effectively banned from a unit of Ada text by applying <tt>pragma Pure</tt> to this unit. It is expected that programmers will use <tt>Unchecked_</tt> constructs very carefully and only when necessary; programs that do not use them are type safe.
| |
| | |
| The [[SPARK programming language]] is a subset of Ada eliminating all its potential ambiguities and insecurities while at the same time adding [[design by contract|statically checked contracts]] to the language features available. SPARK avoids the issues with [[dangling pointer]]s by disallowing allocation at run time entirely.
| |
| | |
| Ada2012 adds [[design by contract|statically checked contracts]] to the language itself (in form of pre-, and post-conditions, as well as type invariants).
| |
| | |
| === C ===
| |
| | |
| {{Wikibooks|C Programming}}
| |
| The [[C (programming language)|C programming language]] is typesafe in limited contexts; for example, a compile-time error is generated when an attempt is made to convert a pointer to one type of structure to a pointer to another type of structure, unless an explicit cast is used. However, a number of very common operations are non-typesafe; for example, the usual way to print an integer is something like <code>printf("%d", 12)</code>, where the <code>%d</code> tells <code>printf</code> at run-time to expect an integer argument. (Something like <code>printf("%s", 12)</code>, which erroneously tells the function to expect a pointer to a character-string, will be accepted by compilers, but will produce undefined results.) This is partially mitigated by some compilers (such as gcc) checking type correspondences between printf arguments and format strings.
| |
| | |
| In addition, C, like Ada, provides unspecified or undefined explicit conversions; and unlike in Ada, idioms that use these conversions are very common, and have helped to give C a type-unsafe reputation. For example, the standard way to allocate memory on the heap is to invoke a memory allocation function, such as <code>[[malloc]]</code>, with an argument indicating how many bytes are required. The function returns an untyped pointer (type <code>void *</code>), which the calling code must cast to the appropriate pointer type. Older C specifications required an explicit cast to do so, therefore the code <code>(struct foo *) malloc(sizeof(struct foo))</code> became the accepted practice.<ref name="knr">{{cite book | last = Kernighan | author1-link = Brian Kernighan | coauthors = [[Dennis Ritchie|Dennis M. Ritchie]] | title = The C Programming Language | edition = 2nd | publisher = [[Prentice Hall]] | date = March 1988 | location = [[Englewood Cliffs, NJ]] | url = http://cm.bell-labs.com/cm/cs/cbook/ | isbn = 0-13-110362-8 | quote=In C, the proper method is to declare that malloc returns a pointer to void, then explicitly coerce the pointer into the desired type with a cast. | page=116}}</ref> However, this practice is discouraged in ANSI C as it can mask a failure to include the header file in which <code>malloc</code> is defined, resulting in downstream errors on machines where the int and pointer types are of different sizes, such as the now-ubiquitous [[x86 64]] architecture.<ref name="stackdiscussion">{{cite web |url= http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858|title= Do I cast the result of malloc?|date=7 November 2012|publisher=[[Stack Overflow]]|accessdate=21 February 2013}}</ref> A conflict arises in code that is required to compile as C++, since the cast is necessary in that language.
| |
| | |
| ===C++===
| |
| Some features of C++ that promote more type-safe code:
| |
| * The [[new (C++)|new]] operator returns a pointer of type based on operand, whereas [[malloc]] returns a void pointer.
| |
| * C++ code can use virtual functions and [[Template (programming)|templates]] to achieve polymorphism without void pointers.
| |
| * [[C macro|Preprocessor]] constants (without type) can be rewritten as [[Constant (programming)|const]] variables (typed).
| |
| * [[C macro|Preprocessor]] macro functions (without type) can be rewritten as [[Inline function|inline]] functions (typed). The flexibility of accepting and returning different types can still be obtained by function overloading.
| |
| * Safer casting operators, such as [[dynamic_cast]] that performs run-time type checking.
| |
| | |
| ===C#===
| |
| [[C Sharp (programming language)|C#]] is type-safe (but not statically type-safe). It has support for untyped pointers, but this must be accessed using the "unsafe" keyword which can be prohibited at the compiler level. It has inherent support for run-time cast validation. Casts can be validated by using the "as" keyword that will return a null reference if the cast is invalid, or by using a C-style cast that will throw an exception if the cast is invalid. See [[C Sharp Syntax#Conversion operators|C Sharp Conversion Operators]].
| |
| | |
| Undue reliance on the [[C Sharp syntax#object class|object]] type (from which all other types are derived) runs the risk of defeating the purpose of the C# type system. It is usually better practice to abandon object references in favour of [[Generic programming#Generic programming in .NET|generics]], similar to templates in C++ and [[generics in Java]].
| |
| | |
| ===Cyclone===
| |
| [[Cyclone (programming language)|Cyclone]] is a type-safe language. It does not require a virtual machine or garbage collection to achieve type safety during runtime. The syntax is very similar to C.
| |
| | |
| ===Java===
| |
| The [[Java language]] is designed to enforce type safety.
| |
| Anything in Java ''happens'' inside an [[Object (computer science)|Object]]
| |
| and each object is an instance of a [[Class (computer programming)|class]].
| |
| | |
| To implement the ''type safety'' enforcement, each object, before usage, needs to be [[Memory allocation|allocated]].
| |
| Java allows usage of [[Primitive data type|primitive types]] but only inside properly allocated objects.
| |
| | |
| Sometimes a part of the type safety is implemented indirectly: e.g. the class BigDecimal represents a floating point number of arbitrary precision, but handles only number that can be expressed with a finite representation.
| |
| The operation BigDecimal.divide() calculates a new object as the division of two numbers expressed as BigDecimal.
| |
| | |
| In this case if the division has no finite representation, as when one computees e.g. 1/3=0.33333..., the divide() method can rise an exception if no rounding mode is definied for the operation.
| |
| Hence the library, rather than the language, guarantees that the object respects the contract implicit in the class definition.
| |
| | |
| ===Standard ML===
| |
| {{Wikibooks|Standard ML Programming}}
| |
| [[Standard ML|SML]] has rigorously defined semantics and is known to be type-safe. However, some implementations of SML, including [[Standard ML of New Jersey]] (SML/NJ), its syntactic variant [[Mythryl]] and [[Mlton]], provide libraries that offer certain unsafe operations. These facilities are often used in conjunction with those implementations' [[foreign function interface]]s to interact with non-ML code (such as C libraries) that may require data laid out in specific ways. Another example is the SML/NJ [[read-eval-print loop|interactive toplevel]] itself, which must use unsafe operations to execute ML code entered by the user.
| |
| | |
| ===Pascal===
| |
| {{Wikibooks|Pascal Programming}}
| |
| [[Pascal (programming language)|Pascal]] has had a number of type safety requirements, some of which are kept in some compilers. Where a Pascal compiler dictates "strict typing", two variables cannot be assigned to each other unless they are either compatible (such as conversion of integer to real) or assigned to the identical subtype. For example, if you have the following code fragment:
| |
| | |
| <source lang="pascal">
| |
| type
| |
| TwoTypes = record
| |
| I: Integer;
| |
| Q: Real;
| |
| end;
| |
| | |
| DualTypes = record
| |
| I: Integer;
| |
| Q: Real;
| |
| end;
| |
| | |
| var
| |
| T1, T2: TwoTypes;
| |
| D1, D2: DualTypes;
| |
| </source>
| |
| | |
| Under strict typing, a variable defined as <tt>TwoTypes</tt> is ''not compatible'' with <tt>DualTypes</tt> (because they are not identical, even though the components of that user defined type are identical) and an assignment of <tt> T1 := D2; </tt> is illegal. An assignment of <tt>T1 := T2; </tt> would be legal because the subtypes they are defined to ''are'' identical. However, an assignment such as <tt>T1.Q := D1.Q;</tt> would be legal.
| |
| | |
| ===Common Lisp===
| |
| In general, [[Common Lisp]] is a type-safe language. A Common Lisp compiler is responsible for inserting dynamic checks for operations whose type safety cannot be proven statically. However, a programmer may indicate that a program should be compiled with a lower level of dynamic type-checking.<ref>{{cite web|title=Common Lisp HyperSpec|url=http://www.lispworks.com/documentation/HyperSpec/Body/d_optimi.htm|accessdate=26 May 2013}}</ref> A program compiled in such a mode cannot be considered type-safe.
| |
| | |
| == C++ Examples ==
| |
| {{Unsourced section|date=December 2013}}
| |
| The following examples illustrates how C++ cast operators can break type safety when used incorrectly. The first example shows how basic data types can be incorrectly casted:
| |
| <source lang="cpp">
| |
| #include <iostream>
| |
| using namespace std;
| |
| | |
| int main () {
| |
| int ival = 5; // integer value
| |
| float fval = reinterpret_cast<float&>(ival); // reinterpret bit pattern
| |
| cout << fval << endl; // output integer as float
| |
| return 0;
| |
| }
| |
| </source>
| |
| In this example, <code>reinterpret_cast</code> explicitly prevents the compiler from performing a safe conversion from integer to floating-point value. When the program runs it will output a garbage floating-point value. The problem could have been avoided by instead writing <code>float fval = ival;</code>
| |
| | |
| The next example shows how object references can be incorrectly downcasted:
| |
| <source lang="cpp">
| |
| #include <iostream>
| |
| using namespace std;
| |
| | |
| class Parent {
| |
| public:
| |
| virtual ~Parent() {} // virtual destructor for RTTI
| |
| };
| |
| | |
| class Child1 : public Parent {
| |
| public:
| |
| int a;
| |
| };
| |
| | |
| class Child2 : public Parent {
| |
| public:
| |
| double b;
| |
| };
| |
| | |
| int main () {
| |
| Child1 c1;
| |
| c1.a = 5;
| |
| Parent & p = c1; // upcast always safe
| |
| Child2 & c2 = static_cast<Child2&>(p); // invalid downcast
| |
| cout << c2.b << endl; // will output garbage data
| |
| return 0;
| |
| }
| |
| </source>
| |
| The two child classes have members of different types. When downcasting a parent class pointer to a child class pointer, then the resulting pointer may not point to a valid object of correct type. In the example, this leads to garbage value being printed. The problem could have been avoided by replacing <code>static_cast</code> with <code>dynamic_cast</code> that throws an exception on invalid casts.
| |
| | |
| ==See also==
| |
| *[[Type theory]]
| |
| | |
| ==Notes==
| |
| {{reflist}}
| |
| | |
| ==References==
| |
| *{{Cite book|authorlink=Benjamin C. Pierce |last=Pierce |first=Benjamin C. |title=Types and Programming Languages |publisher=MIT Press |year=2002 |isbn=0-262-16209-1 |url=http://www.cis.upenn.edu/~bcpierce/tapl/}}
| |
| *{{cite web|title=Type Safe |work=Portland Pattern Repository Wiki |url=http://c2.com/cgi/wiki?TypeSafe}}
| |
| *{{Cite journal|last=Wright |first=Andrew K. |coauthors=[[Matthias Felleisen]] |title=A Syntactic Approach to Type Soundness |journal=Information and Computation |volume=115 |issue=1 |pages=38–94 |year=1994 |url=http://citeseer.ist.psu.edu/wright92syntactic.html |doi=10.1006/inco.1994.1093}}
| |
| *{{Cite journal|first=Stavros |last=Macrakis |title=Safety and power |journal=ACM SIGSOFT Software Engineering Notes |volume=7 |issue=2 |pages=25–26 |date=April 1982 |url=http://portal.acm.org/citation.cfm?id=1005937.1005941 |format=requires subscription |doi=10.1145/1005937.1005941}}
| |
| | |
| {{DEFAULTSORT:Type Safety}}
| |
| [[Category:Programming language topics]]
| |
| [[Category:Type theory]]
| |