Variadic function

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In computer programming, a variadic function is a function of variable arity; that is, one which can take different numbers of arguments. Support for variadic functions differs widely among programming languages.

There are many mathematical and logical operations which come across naturally as variadic functions. For instance, the summing of numbers or the concatenation of strings or other sequences are operations that can logically apply to any number of operands.

Another operation which has been implemented as a variadic function in many languages is output formatting. The C function printf and the Common Lisp function format are two such examples. Both take one argument which specifies the formatting of the output, and any number of arguments which provide the values to be formatted.

Variadic functions can expose type-safety problems in some languages. For instance, C's printf, if used uncautiously, can give rise to a class of security holes known as format string attacks. The attack is possible because the language support for variadic functions is not type-safe; it permits the function to attempt to pop more arguments off the stack than were placed there -- corrupting the stack and leading to unexpected behavior.

Contents 1 Variadic functions in C and C++ 2 Variadic functions in PHP 3 Variadic functions in C# and Java 4 Variadic functions in Python 5 See also

[edit] Variadic functions in C and C++

To portably implement variadic functions in the C programming language, the standard <stdarg.h> header file should be used. The older <varargs.h> header has been deprecated in favor of <stdarg.h>.

The variadic function feature is going to be present in the upcoming C++ language standard, C++0x; this feature is called "Variadic templates". This allows the creation of variadic template classes and variadic template functions.

For a simple tutorial refer to the Variable Argument Functions tutorial.

      template<typename T>
      void print( const T& t )
      {
              std::cout << t;
      }

      template< typename T, typename... Args>
      void print( const T& t, const Args&... args )
      {
              print( t );
              print( args... );
      }

      template<typename... T>
      class tuple;
      template<typename T, typename... Ts>
      class tuple<T,Ts...>;
      template<>
      class tuple<>;

[edit] Variadic functions in PHP

In PHP, variable-length argument lists are natively supported (without security risk) since version 4; dedicated functions (func_num_args, func_get_arg, func_get_args) allow the programmer to determine the number and values of unspecified arguments.

[edit] Variadic functions in C# and Java

Other languages, such as C# and Java use a different approach - they just allow that a variable number of arguments of the same (super)type may be passed to a variadic function. Inside the method they are simply collected in an array. Using autoboxing, this can also be used to pass primitive arguments to for example a print function, as can be seen in the Java example below:

public static void printSpaced(Object... objects) {
  for (Object o : objects)
    System.out.print(o + " ");
}

// Can be used to print:
// printSpaced(1, 2, "three");

[edit] Variadic functions in Python

Python supports very flexible variadic functions. By marking variables with one asterisk (e.g. *var) the given variable is defined to be a tuple of all the extra arguments. By marking variables with two asterisks (e.g. **var) the given variable is a dictionary of all extra keyword arguments; the keys are strings, which are the names that were . Conventionally these are called "args" and "kwargs" respectively, but they may be something else, and packages often make good use of this ability to improve readability (e.g. BeautifulSoup). If they exist, these arguments must be the last one in the list.

      def f(*args, **kwargs):
          print args
          print kwargs
      
      f(1,2,"cow","kitty")
      > (1, 2, "cow", kitty)
      > {}
      
      f(arg1=1,sample=2,name="cow",hero="kitty")
      > ()
      > {"arg1":1, "sample":2, "name":"cow", "hero":"kitty"}
      
      f(1,2,name="cow",hero="kitty")
      > (1, 2)
      > {"name":"cow", "hero":"kitty"}
      
      f(arg1=1,sample=2,name="cow", "kitty") -> SyntaxError "Non-keyword arg after keyword arg"

Conversely you may also pass in a tuple or dictionary using the same asterisk-notation and have it automatically expand to fill.

      def g(a, b, c):
          print a, b, c
      
      mytuple = 1,2,3
      mydict = {"a":"first", "b":"second", "c":"third"}
      g(*mytuple) -> 1 2 3
      g(**mydict) -> first second third
      g(**{"a":"first"}) -> TypeError "g() takes exactly 3 non-keyword arguments (got 1)"
      g(**{"a":"first", "b":"second", "c":"third", "d":"fourth"}) -> TypeError "g() got an unexpected keyword argument 'd'"

[edit] See also

• Variadic macro (C programming language)
• Java syntax
• A very simple tutorial on Variable Argument Functions for C++ Variable Argument Functions