Type conversion

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In computer science, type conversion, [1] [2] type casting, [1] [3] type coercion, [3] and type juggling [4] [5] 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 (automatically) or explicitly, [1] [6] 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. [6] [7] 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 (like 5 + 0.1), 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 (e.g. adding type identifiers or calling built-in routines) 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 (as opposed to an implicit 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 (or class) is located by a pointer and how a pointer may be adjusted by the compiler in cases like object (class) inheritance.

Language comparison

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:


Although d, l, and i 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 (rounded toward zero). Conversely, precision can be lost when converting representations from integer to floating-point, since a floating-point type may be unable to exactly represent an integer type. For example, float 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<stdio.h>intmain(void){inti_value=16777217;floatf_value=16777216.0;printf("The integer is: %d\n",i_value);printf("The float is:   %f\n",f_value);printf("Their equality: %d\n",i_value==f_value);}

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 i_value to float when it is compared with f_value. The conversion causes loss of precision, which makes the values equal before the comparison.

Important takeaways:

  1. float to int causes truncation, i.e., removal of the fractional part.
  2. double to float causes rounding of digit.
  3. long to int causes dropping of excess higher order bits.
Type promotion

One special case of implicit type conversion is type promotion, where an object is automatically converted into another data type representing a superset of the original type. Promotions are commonly used with types smaller than the native type of the target platform's arithmetic logic unit (ALU), before arithmetic and logical operations, to make such operations possible, or more efficient if the ALU can work with more than one type. C and C++ perform such promotion for objects of boolean, character, wide character, enumeration, and short integer types which are promoted to int, and for objects of type float, which are promoted to double. Unlike some other type conversions, promotions never lose precision or modify the value stored in the object.

In Java:

intx=3;doubley=3.5;System.out.println(x+y);// 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 (instead of being done automatically according to the rules of the language for implicit type conversion). It is defined by the user in the program.

doubleda=3.3;doubledb=3.3;doubledc=3.4;intresult=(int)da+(int)db+(int)dc;// result == 9// if implicit conversion would be used (as with "result = da + db + dc"), result would be equal to 10

There are several kinds of explicit conversion.

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.
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 (i.e., C-like) manner, the former called checked type cast. [8]

Animalanimal=newCat();Bulldogb=(Bulldog)animal;// if (animal is Bulldog), stat.type(animal) is Bulldog, else an exceptionb=animalasBulldog;// if (animal is Bulldog), b = (Bulldog) animal, else b = nullanimal=null;b=animalasBulldog;// b == null

In C++ a similar effect can be achieved using C++-style cast syntax.

Animal*animal=newCat;Bulldog*b=static_cast<Bulldog*>(animal);// compiles only if either Animal or Bulldog is derived from the other (or same)b=dynamic_cast<Bulldog*>(animal);// if (animal is Bulldog), b = (Bulldog*) animal, else b = nullptrBulldog&br=static_cast<Bulldog&>(*animal);// 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 avoidedanimal=nullptr;b=dynamic_cast<Bulldog*>(animal);// b == nullptrdeleteanimal;// always free resources


In Eiffel the notion of type conversion is integrated into the rules of the type system. The Assignment Rule says that an assignment, such as:


is valid if and only if the type of its source expression, y in this case, is compatible with the type of its target entity, x in this case. 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 familiar 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.

Definition of type conversion in Eiffel

The actions of type conversion in Eiffel, specifically converts to and converts from are defined as:

A type based on a class CU converts to a type T based on a class CT (and T converts from U) if either

CT has a conversion procedure using U as a conversion type, or
CU has a conversion query listing T as a conversion type


Eiffel is a fully compliant language for Microsoft .NET Framework. Before development of .NET, Eiffel already had extensive class libraries. Using the .NET type libraries, particularly with commonly used types such as strings, poses a conversion problem. Existing Eiffel software uses the string classes (such as STRING_8) from the Eiffel libraries, but Eiffel software written for .NET must use the .NET string class (System.String) in many cases, for example when calling .NET methods which expect items of the .NET type to be passed as arguments. So, the conversion of these types back and forth needs to be as seamless as possible.

my_string:STRING_8-- Native Eiffel stringmy_system_string:SYSTEM_STRING-- Native .NET string...my_string:=my_system_string

In the code above, two strings are declared, one of each different type (SYSTEM_STRING is the Eiffel compliant alias for System.String). Because System.String does not conform to STRING_8, then the assignment above is valid only if System.String converts to STRING_8.

The Eiffel class STRING_8 has a conversion procedure make_from_cil for objects of type System.String. Conversion procedures are also always designated as creation procedures (similar to constructors). The following is an excerpt from the STRING_8 class:


The presence of the conversion procedure makes the assignment:


semantically equivalent to:


in which my_string is constructed as a new object of type STRING_8 with content equivalent to that of my_system_string.

To handle an assignment with original source and target reversed:


the class STRING_8 also contains a conversion query to_cil which will produce a System.String from an instance of STRING_8.


The assignment:


then, becomes equivalent to:


In Eiffel, the setup for type conversion is included in the class code, but then appears to happen as automatically as explicit type conversion in client code. The includes not just assignments but other types of attachments as well, such as argument (parameter) substitution.


Rust provides no implicit type conversion (coercion) between primitive types. But, explicit type conversion (casting) can be performed using the as keyword. [9]

println!("1000 as a u16 is: {}",1000asu16);

Security issues

In hacking, typecasting is the misuse of type conversion to temporarily change a variable's data type from how it was originally defined. [10] This provides opportunities for hackers since in type conversion after a variable is "typecast" to become a different data type, the compiler will treat that hacked variable as the new data type for that specific operation. [11]

See also

Related Research Articles

Eiffel is an object-oriented programming language designed by Bertrand Meyer and Eiffel Software. Meyer conceived the language in 1985 with the goal of increasing the reliability of commercial software development; the first version becoming available in 1986. In 2005, Eiffel became an ISO-standardized language.

Double-precision floating-point format is a computer number format, usually occupying 64 bits in computer memory; it represents a wide dynamic range of numeric values by using a floating radix point.

Data type Attribute of data

In computer science and computer programming, a data type or simply type is an attribute of data which tells the compiler or interpreter how the programmer intends to use the data. Most programming languages support basic data types of integer numbers, floating-point numbers, characters and Booleans. A data type constrains the values that an expression, such as a variable or a function, might take. This data type defines the operations that can be done on the data, the meaning of the data, and the way values of that type can be stored. A data type provides a set of values from which an expression may take its values.

In programming languages, a type system is a logical system comprising a set of rules that assigns a property called a type to the various constructs of a computer program, such as variables, expressions, functions or modules. These types formalize and enforce the otherwise implicit categories the programmer uses for algebraic data types, data structures, or other components. The main purpose of a type system is to reduce possibilities for bugs in computer programs by defining interfaces between different parts of a computer program, and then checking that the parts have been connected in a consistent way. This checking can happen statically, dynamically, or as a combination of both. Type systems have other purposes as well, such as expressing business rules, enabling certain compiler optimizations, allowing for multiple dispatch, providing a form of documentation, etc.

In computer science, an object type is a datatype that is used in object-oriented programming to wrap a non-object type to use it more like a dynamic object.

Pointer (computer programming) Object which stores memory addresses in a computer program

In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying computer architecture.

In computer science, a union is a value that may have any of several representations or formats within the same position in memory; that consists of a variable that may hold such a data structure. Some programming languages support special data types, called union types, to describe such values and variables. In other words, a union type definition will specify which of a number of permitted primitive types may be stored in its instances, e.g., "float or long integer". In contrast with a record, which could be defined to contain a float and an integer; in a union, there is only one value at any given time.

In computer science, type safety is the extent to which a programming language discourages or prevents type errors. A type error is erroneous program behaviour caused by a discrepancy between differing data types for the program's constants, variables, and methods (functions), e.g., adding a string to an integer. 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 use other type-unsafe facilities in the same language. The formal type-theoretic definition of type safety is considerably stronger than what is understood by most programmers.

In class-based object-oriented programming, a constructor is a special type of subroutine called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.

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In the C programming language, data types constitute the semantics and characteristics of storage of data elements. They are expressed in the language syntax in form of declarations for memory locations or variables. Data types also determine the types of operations or methods of processing of data elements.

C Sharp (programming language) Multi-paradigm (object-oriented) programming language

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In computer programming, an enumerated type is a data type consisting of a set of named values called elements, members, enumeral, or enumerators of the type. The enumerator names are usually identifiers that behave as constants in the language. An enumerated type can be seen as a degenerate tagged union of unit type. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value. In other words, an enumerated type has values that are different from each other, and that can be compared and assigned, but are not specified by the programmer as having any particular concrete representation in the computer's memory; compilers and interpreters can represent them arbitrarily.

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Generics are a facility of generic programming that were added to the Java programming language in 2004 within version J2SE 5.0. They were designed to extend Java's type system to allow "a type or method to operate on objects of various types while providing compile-time type safety". The aspect compile-time type safety was not fully achieved, since it was shown in 2016 that it is not guaranteed in all cases.

In computer science, a type punning is any programming technique that subverts or circumvents the type system of a programming language in order to achieve an effect that would be difficult or impossible to achieve within the bounds of the formal language.

This article describes the syntax of the C# programming language. The features described are compatible with .NET Framework and Mono.

The computer programming languages C and Pascal have similar times of origin, influences, and purposes. Both were used to design their own compilers early in their lifetimes.


  1. 1 2 3 Mehrotra, Dheeraj (2008). S. Chand's Computer Science. pp. 81–83. ISBN   978-8121929844.
  2. Programming Languages - Design and Constructs. 2013. p. 35. ISBN   978-9381159415.
  3. 1 2 Reilly, Edwin (2004). Concise Encyclopedia of Computer Science. pp.  82, 110. ISBN   0470090952.
  4. Fenton, Steve (2017). Pro TypeScript: Application-Scale JavaScript Development. pp. xxiii. ISBN   978-1484232491.
  5. "PHP: Type Juggling - Manual". php.net. Retrieved 27 January 2019.
  6. 1 2 Olsson, Mikael (2013). C++ Quick Syntax Reference. pp. 87–89. ISBN   978-1430262770.
  7. Kruse, Rudolf; Borgelt, Christian; Braune, Christian; Mostaghim, Sanaz; Steinbrecher, Matthias (16 September 2016). Computational Intelligence: A Methodological Introduction. p. 269. ISBN   978-1447172963.
  8. Mössenböck, Hanspeter (25 March 2002). "Advanced C#: Checked Type Casts" (PDF). Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik. p. 5. Retrieved 4 August 2011. at C# Tutorial
  9. "Casting - Rust By Example". doc.rust-lang.org.
  10. Jon Erickson Hacking, 2nd Edition: The Art of Exploitation 2008 1593271441 p51 "Typecasting is simply a way to temporarily change a variable's data type, despite how it was originally defined. When a variable is typecast into a different type, the compiler is basically told to treat that variable as if it were the new data type, but only for that operation. The syntax for typecasting is as follows: (typecast_data_type) variable ..."
  11. Arpita Gopal Magnifying C 2009 8120338618 p. 59 "From the above, it is clear that the usage of typecasting is to make a variable of one type, act like another type for one single operation. So by using this ability of typecasting it is possible for create ASCII characters by typecasting integer to its ..."