In computer science, a type signature or type annotation defines the inputs and outputs for a function, subroutine or method. A type signature includes the number of arguments, the types of arguments and the order of the arguments contained by a function. A type signature is typically used during overload resolution for choosing the correct definition of a function to be called among many overloaded forms.
In C and C++, the type signature is declared by what is commonly known as a function prototype. In C/C++, a function declaration reflects its use; for example, a function pointer that would be invoked as:
char c; double d; int retVal = (*fPtr)(c, d);
has the signature:
(int) (char, double);
In Erlang, type signatures may be optionally declared, as:
-spec(function_name(type1(), type2(), ...) -> out_type()).
-spec(is_even(number()) -> boolean()).
A type signature in the Haskell programming language is generally written in the following format:
functionName :: arg1Type -> arg2Type -> ... -> argNType
Notice that the type of the result can be regarded as everything past the first supplied argument. This is a consequence of currying, which is made possible by Haskell's support for first-class functions; this function requires two inputs where one argument supplied and the function is "curried" to produce a function for the argument not supplied. Thus calling
f x, where
f :: a -> b -> c, yields a new function
f2 :: b -> c that can be called
f2 b to produce
The actual type specifications can consist of an actual type, such as
Integer, or a general type variable that is used in parametric polymorphic functions, such as
anyType. So we can write something like:
functionName :: a -> a -> ... -> a
Since Haskell supports higher-order functions, functions can be passed as arguments. This is written as:
functionName :: (a -> a) -> a
This function takes in a function with type signature
a -> a and returns data of type
In the Java virtual machine, internal type signatures are used to identify methods and classes at the level of the virtual machine code.
Example: The method
String String.substring(int, int) is represented in bytecode as
Ljava/lang/String/substring(II)Ljava/lang/String;. The signature of
main() method looks like this:
public static void main(String args)
And in the disassembled bytecode, it takes the form of
The method signature for the
main() method contains three modifiers:
publicindicates that the
main()method can be called by any object.
staticindicates that the
main()method is a class method.
voidindicates that the
main()method has no return value.
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A function signature consists of the function prototype. It specifies the general information about a function like the name, scope and parameters. Many programming languages use name mangling in order to pass along more semantic information from the compilers to the linkers. In addition to mangling, there is an excess of information in a function signature (stored internally to most compilers) which is not readily available, but may be accessed.
Understanding the notion of a function signature is an important concept for all computer science studies.
- Modern object orientation techniques make use of interfaces, which are essentially templates made from function signatures.
- C/C++ uses function overloading with various signatures.
The practice of multiple inheritance requires consideration of the function signatures to avoid unpredictable results.
Computer science theory, and the concept of polymorphism in particular, make much use of the concept of function signature.
The term "signature" may carry other meanings in computer science. For example:
- File signatures can identify or verify the content of a file.
- Database signatures can identify or verify the schema or a version of a database.
- In the ML family of programming languages, "signature" is used as a keyword referring to a construct of the module system that plays the role of an interface.
In computer programming, especially object-oriented programming, a method is commonly identified by its unique method signature, which usually includes the method name, and the number, types and order of its parameters. A method signature is the smallest type of a method.
int printf(const char*, ... );
Manipulation of these parameters can be done by using the routines in the standard library header
void Add(out int sum, params int value); [...] Add(out sum, 3, 5, 7, 11, -1); // sum == 25
In the Java programming language, a method signature is the method name and the number and type of its parameters. Return types and thrown exceptions are not considered to be a part of the method signature.
For example, the following two methods have distinct signatures:
doSomething(int y); doSomething(String y);
The following three methods do have the same signatures and are considered the same, as only the return value differs. The name of the parameter is not part of the method signature and is ignored by the compiler for checking method uniqueness.
int doSomething(int y) String doSomething(int x) int doSomething(int z) throws java.lang.Exception
In the Objective-C programming language, method signatures for an object are declared in the interface header file. For example,
defines a method
initWithInt that returns a general object (an
id) and takes one integer argument. Objective-C only requires a type in a signature to be explicit when the type is not
id; this signature is equivalent:
- "C++ Reference: Programming terms". Retrieved 3 December 2013.
- Paul Leahy. "Method Signature". http://www.about.com/: About.com Guide. Retrieved 2011-05-31.
A method signature is part of the method declaration. It is the combination of the method name and the parameter list.
- Mössenböck, Hanspeter (2002-03-25). "Advanced C#: Variable Number of Parameters" (PDF). http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/: Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik. p. 52. Retrieved 2011-08-03.