Type conversion
In computer science, type conversion, type casting, type coercion, and type juggling are different ways of changing an expression from one data type to another. An example would be the conversion of an integer value into a floating point value or its textual representation as a string, and vice versa. Type conversions can take advantage of certain features of type hierarchies or data representations. Two important aspects of a type conversion are whether it happens implicitly or explicitly, and whether the underlying data representation is converted from one representation into another, or a given representation is merely reinterpreted as the representation of another data type. In general, both primitive and compound data types can be converted.
Each programming language has its own rules on how types can be converted. Languages with strong typing typically do little implicit conversion and discourage the reinterpretation of representations, while languages with weak typing perform many implicit conversions between data types. Weak typing language often allow forcing the compiler to arbitrarily interpret a data item as having different representations—this can be a non-obvious programming error, or a technical method to directly deal with underlying hardware.
In most languages, the word coercion is used to denote an implicit conversion, either during compilation or during run time. For example, in an expression mixing integer and floating point numbers, the compiler will automatically convert integer representation into floating point representation so fractions are not lost. Explicit type conversions are either indicated by writing additional code or by coding conversion routines for the compiler to use when it otherwise would halt with a type mismatch.
In most ALGOL-like languages, such as Pascal, Modula-2, Ada and Delphi, conversion and casting are distinctly different concepts. In these languages, conversion refers to either implicitly or explicitly changing a value from one data type storage format to another, e.g. a 16-bit integer to a 32-bit integer. The storage needs may change as a result of the conversion, including a possible loss of precision or truncation. The word cast, on the other hand, refers to explicitly changing the interpretation of the bit pattern representing a value from one type to another. For example, 32 contiguous bits may be treated as an array of 32 Booleans, a 4-byte string, an unsigned 32-bit integer or an IEEE single precision floating point value. Because the stored bits are never changed, the programmer must know low level details such as representation format, byte order, and alignment needs, to meaningfully cast.
In the C family of languages and ALGOL 68, the word cast typically refers to an explicit type conversion, causing some ambiguity about whether this is a re-interpretation of a bit-pattern or a real data representation conversion. More important is the multitude of ways and rules that apply to what data type is located by a pointer and how a pointer may be adjusted by the compiler in cases like object inheritance.
Explicit casting in various languages
Ada
provides a generic library function Unchecked_Conversion.C-like languages
Implicit type conversion
Implicit type conversion, also known as coercion or type juggling, is an automatic type conversion by the compiler. Some programming languages allow compilers to provide coercion; others require it.In a mixed-type expression, data of one or more subtypes can be converted to a supertype as needed at runtime so that the program will run correctly. For example, the following is legal C language code:
double d;
long l;
int i;
if
if
if
Although,, and belong to different data types, they will be automatically converted to equal data types each time a comparison or assignment is executed. This behavior should be used with caution, as unintended consequences can arise. Data can be lost when converting representations from floating-point to integer, as the fractional components of the floating-point values will be truncated. Conversely, precision can be lost when converting representations from integer to floating-point, since a floating-point type may be unable to exactly represent all possible values of some integer type. For example, might be an IEEE 754 single precision type, which cannot represent the integer 16777217 exactly, while a 32-bit integer type can. This can lead to unintuitive behavior, as demonstrated by the following code:
- include
On compilers that implement floats as IEEE single precision, and ints as at least 32 bits, this code will give this peculiar print-out:
The integer is: 16777217
The float is: 16777216.000000
Their equality: 1
Note that 1 represents equality in the last line above. This odd behavior is caused by an implicit conversion of to float when it is compared with. The conversion causes loss of precision, which makes the values equal before the comparison.
Important takeaways:
- to causes truncation, i.e., removal of the fractional part.
- to causes rounding of digit.
- to causes dropping of excess higher order bits.
Type promotion
In Java:
int x = 3;
double y = 3.5;
System.out.println; // The output will be 6.5
Explicit type conversion
Explicit type conversion, also called type casting, is a type conversion which is explicitly defined within a program. It is requested by the user in the program.double a = 3.3;
double b = 3.3;
double c = 3.4;
int result = static_cast
// result 9
// if implicit conversion would be used, result would be equal to 10
There are several kinds of explicit conversion.
; checked: Before the conversion is performed, a runtime check is done to see if the destination type can hold the source value. If not, an error condition is raised.
; unchecked: No check is performed. If the destination type cannot hold the source value, the result is undefined.
; bit pattern: The raw bit representation of the source is copied verbatim, and it is re-interpreted according to the destination type. This can also be achieved via aliasing.
In object-oriented programming languages, objects can also be downcast : a reference of a base class is cast to one of its derived classes.
C# and C++
In C#, type conversion can be made in a safe or unsafe manner, the former called checked type cast.Animal animal = new Cat;
// if, stat.type is Bulldog, else an exception
Bulldog b = animal;
// if, b = animal, else b = null
b = animal as Bulldog;
// remove the reference to Cat, marking it for garbage collection
animal = null;
// b null
b = animal as Bulldog;
In C++ a similar effect can be achieved using C++-style cast syntax.
Animal* animal = new Cat;
// compiles only if either Animal or Bulldog is derived from the other
Bulldog* b = static_cast
// if, b = animal, else b = nullptr
b = dynamic_cast
// same as above, but an exception will be thrown if a nullptr was to be returned
// this is not seen in code where exception handling is avoided
Bulldog& br = static_cast
// deallocate animal after use
delete animal;
animal = nullptr;
// b nullptr
b = dynamic_cast
Eiffel
In Eiffel the notion of type conversion is integrated into the rules of the type system. The Assignment Rule says that an assignment, such asx := y, is valid if and only if the type of its source expression is compatible with the type of its target entity. In this rule, compatible with means that the type of the source expression either conforms to or converts to that of the target. Conformance of types is defined by the rules for polymorphism in object-oriented programming. For example, in the assignment above, the type of y conforms to the type of x if the class upon which y is based is a descendant of that upon which x is based.Rust
provides no implicit type conversion between most primitive types. But, explicit type conversion can be performed using theas keyword.let x: i32 = 1000;
println!;