Type safety

In

Intuitively, type soundness is captured by Robin Milner's pithy statement that

Well-typed programs cannot "go wrong".^[2]

In other words, if a type system is sound, then expressions accepted by that type system must evaluate to a value of the appropriate type (rather than produce a value of some other, unrelated type or crash with a type error). Vijay Saraswat provides the following, related 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.^[3]

However, what precisely it means for a program to be "well typed" or to "go wrong" are properties of its

Progress

A well-typed program never gets "stuck": every expression is either already a value or can be reduced towards a value in some well-defined way. In other words, the program never gets into an undefined state where no further transitions are possible.

Preservation (or subject reduction)

After each evaluation step, the type of each expression remains the same (that is, its type is preserved).

A number of other formal treatments of type soundness have also been published in terms of

• Prevention of illegal operations. For example, a type system can reject the expression 3 / "Hello, World" as invalid, because the division operator is not defined for a string divisor.
• Memory safety
  • Type systems can prevent
    wild pointers
    that could otherwise arise from a pointer to one type of object being treated as a pointer to another type.
  • More sophisticated type systems, such as those supporting dependent types, can detect and reject out-of-bound accesses, preventing potential buffer overflows.^[7]
• Logic errors 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 (i.e.

Programming languages are often colloquially classified as strongly typed or weakly typed (also loosely typed) to refer to certain aspects of type safety. In 1974, Liskov and Zilles defined a strongly-typed language as one in which "whenever an object is passed from a calling function to a called function, its type must be compatible with the type declared in the called function."^[10] In 1977, Jackson wrote, "In a strongly typed language each data area will have a distinct type and each process will state its communication requirements in terms of these types."^[11] In contrast, a weakly typed language may produce unpredictable results or may perform implicit type conversion.^[12]

Memory management and type safety

Type safety is closely linked to memory safety. For instance, in an implementation of a language that has some type ${\displaystyle t}$ which allows some bit patterns but not others, a dangling pointer memory error allows writing a bit pattern that does not represent a legitimate member of ${\displaystyle t}$ into a dead

${\displaystyle t}$

As a minimal condition, a type-safe language must not allow dangling pointers across allocations of different types. But most languages enforce the proper use of abstract data types defined by programmers even when this is not strictly necessary for memory safety or for the prevention of any kind of catastrophic failure. Allocations are given a type describing its contents, and this type is fixed for the duration of the allocation. This allows type-based alias analysis to infer that allocations of different types are distinct.

Most type-safe languages use garbage collection. Pierce says, "it is extremely difficult to achieve type safety in the presence of an explicit deallocation operation", due to the dangling pointer problem.^[13] However Rust is generally considered type-safe and uses a borrow checker to achieve memory safety, instead of garbage collection.

Type safety in object oriented languages

In

Exceptions to this are object oriented languages that allow dynamic modification of the object structure, or the use of

Type safety issues in specific languages

Ada

The Wikibook Ada Programming has a page on the topic of: Type System

The

Ada2012 adds statically checked contracts to the language itself (in form of pre-, and post-conditions, as well as type invariants).

C

Wikibooks has a book on the topic of: C Programming

The C programming language is type-safe 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-type-safe; for example, the usual way to print an integer is something like printf("%d", 12), where the %d tells printf at run-time to expect an integer argument. (Something like printf("%s", 12), which tells the function to expect a pointer to a character-string and yet supplies an integer argument, may 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

• The
  malloc
  returns a void pointer.
• C++ code can use virtual functions and
  templates
  to achieve polymorphism without void pointers.
• Safer casting operators, such as
  dynamic cast
  that performs run-time type checking.
• C++11 strongly-typed enumerations cannot be implicitly converted to or from integers or other enumeration types.
• C++ explicit constructors and C++11 explicit conversion operators prevent implicit type conversions.

C#

Undue reliance on the 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 generics, similar to templates in C++ and generics in Java.

Java

Wikibooks has a book on the topic of: Java Programming

The

To implement the type safety enforcement, each object, before usage, needs to be

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 numbers 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 computes e.g. 1/3=0.33333..., the divide() method can raise an exception if no rounding mode is defined 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 has a book on the topic of: Standard ML Programming

Modula-2

Modula-2 is a strongly-typed language with a design philosophy to require any unsafe facilities to be explicitly marked as unsafe. This is achieved by "moving" such facilities into a built-in pseudo-library called SYSTEM from where they must be imported before they can be used. The import thus makes it visible when such facilities are used. Unfortunately, this was not consequently implemented in the original language report and its implementation.^[16] There still remained unsafe facilities such as the type cast syntax and variant records (inherited from Pascal) that could be used without prior import.^[17] The difficulty in moving these facilities into the SYSTEM pseudo-module was the lack of any identifier for the facility that could then be imported since only identifiers can be imported, but not syntax.

IMPORT SYSTEM; (* allows the use of certain unsafe facilities: *)
VAR word : SYSTEM.WORD; addr : SYSTEM.ADDRESS;
addr := SYSTEM.ADR(word);

(* but type cast syntax can be used without such import *)
VAR i : INTEGER; n : CARDINAL;
n := CARDINAL(i); (* or *) i := INTEGER(n);

The ISO Modula-2 standard corrected this for the type cast facility by changing the type cast syntax into a function called CAST which has to be imported from pseudo-module SYSTEM. However, other unsafe facilities such as variant records remained available without any import from pseudo-module SYSTEM.^[18]

IMPORT SYSTEM;
VAR i : INTEGER; n : CARDINAL;
i := SYSTEM.CAST(INTEGER, n); (* Type cast in ISO Modula-2 *)

A recent revision of the language applied the original design philosophy rigorously. First, pseudo-module SYSTEM was renamed to UNSAFE to make the unsafe nature of facilities imported from there more explicit. Then all remaining unsafe facilities where either removed altogether (for example variant records) or moved to pseudo-module UNSAFE. For facilities where there is no identifier that could be imported, enabling identifiers were introduced. In order to enable such a facility, its corresponding enabling identifier must be imported from pseudo-module UNSAFE. No unsafe facilities remain in the language that do not require import from UNSAFE.^[17]

IMPORT UNSAFE;
VAR i : INTEGER; n : CARDINAL;
i := UNSAFE.CAST(INTEGER, n); (* Type cast in Modula-2 Revision 2010 *)

FROM UNSAFE IMPORT FFI; (* enabling identifier for foreign function interface facility *)
<*FFI="C"*> (* pragma for foreign function interface to C *)

Pascal

Wikibooks has a book on the topic of: Pascal Programming

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:

type
  TwoTypes = record
     I: Integer;
     Q: Real;
  end;

  DualTypes = record
    I: Integer;
    Q: Real;
  end;

var
  T1, T2:  TwoTypes;
  D1, D2:  DualTypes;

Under strict typing, a variable defined as TwoTypes is not compatible with DualTypes (because they are not identical, even though the components of that user defined type are identical) and an assignment of T1 := D2; is illegal. An assignment of T1 := T2; would be legal because the subtypes they are defined to are identical. However, an assignment such as T1.Q := D1.Q; 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.^[19] A program compiled in such a mode cannot be considered type-safe.

C++ examples

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 cast:

#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;
}

In this example, reinterpret_cast explicitly prevents the compiler from performing a safe conversion from integer to floating-point value.^[20] When the program runs it will output a garbage floating-point value. The problem could have been avoided by instead writing float fval = ival;

The next example shows how object references can be incorrectly downcast:

#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:
    float 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;
}

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 static_cast with dynamic_cast that throws an exception on invalid casts.^[21]

See also

• Type theory

Notes