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The Metacode extension to C++ (Vandevoorde 2003) [1] was an early experimental system to allow compile-time function evaluation (CTFE) and code injection as an improved syntax for C++ template metaprogramming. In earlier versions of C++, template metaprogramming is often used to compute values at compile time, such as:
Parameters appear in procedure definitions; arguments appear in procedure calls. In the function definition f(x) = x*x the variable x is a parameter; in the function call f(2) the value 2 is the argument of the function. Loosely, a parameter is a type, and an argument is an instance.
In C/C++, it is possible to declare the parameter of a function or method as constant. This is a guarantee that this parameter cannot be inadvertently modified after its initialization by the caller. If the parameter is a pre-defined (built-in) type, it is called by value and cannot be modified. If it is a user-defined type, the variable is the ...
C++ supports function, class, alias, and variable templates. Templates may be parameterized by types, compile-time constants, and other templates. Templates are implemented by instantiation at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class ...
A function definition starts with the name of the type of value that it returns or void to indicate that it does not return a value. This is followed by the function name, formal arguments in parentheses, and body lines in braces. In C++, a function declared in a class (as non-static) is called a member function or method.
Both the C99 and C++11 standards require at least one argument, but since C++20 this limitation has been lifted through the __VA_OPT__ functional macro. The __VA_OPT__ macro is replaced by its argument when arguments are present, and omitted otherwise. Common compilers also permit passing zero arguments before this addition, however.
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According to the standard, varadic functions without any named parameters are not allowed in C17 and earlier, but in C++ and C23 [2] such a declaration is permitted. In C, a comma must precede the ellipsis if a named parameter is specified, while in C++ it is optional. Some K&R C style function declarations do not use ellipses. [3]