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Showing content from https://en.cppreference.com/w/cpp/language/../keyword/../language/variadic_arguments.html below:

Variadic arguments - cppreference.com

Allows a function to accept any number of extra arguments.

A function is a variadic if the last parameter of its parameter list is an ellipsis (...).

// the function declared as follows
int printx(const char* fmt, ...);
int printx(const char* fmt...); // same as above, but deprecated since C++26
 
// may be called with one or more arguments:
printx("hello world");
printx("a=%d b=%d", a, b);
 
int printy(..., const char* fmt); // error: ... can only be the last parameter
int printz(...); // valid, but the arguments cannot be accessed portably

This is different from a function parameter pack expansion, which is indicated by an ellipsis that is a part of a parameter declarator, rather than an ellipsis being a parameter alone. Both parameter pack expansion and the “variadic” ellipsis may appear in the declaration of a function template, as in the case of std::is_function.

When a variadic function is called, after lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions, each argument that is a part of the variable argument list undergoes additional conversions known as default argument promotions:

• float arguments are converted to double as in floating-point promotion.
• bool, char, short, and unscoped enumerations are converted to int or wider integer types as in integral promotion.

Non-POD class types(until C++11)Scoped enumerations and class types with an eligible non-trivial copy constructor, an eligible non-trivial move constructor, or a non-trivial destructor(since C++11) are conditionally-supported in potentially-evaluated calls with implementation-defined semantics (these types are always supported in unevaluated calls).

Because variadic parameters have the lowest rank for the purpose of overload resolution, they are commonly used as the catch-all fallbacks in SFINAE.

Within the body of a function that uses variadic arguments, the values of these arguments may be accessed using the <cstdarg> library facilities:

The behavior of the va_start macro is undefined if the last parameter before the ellipsis has reference type, or has type that is not compatible with the type that results from default argument promotions.

• Variadic templates can also be used to create functions that take variable number of arguments. They are often the better choice because they do not impose restrictions on the types of the arguments, do not perform integral and floating-point promotions, and are type safe.
• If all variable arguments share a common type, a std::initializer_list provides a convenient mechanism (albeit with a different syntax) for accessing variable arguments. In this case however the arguments cannot be modified since std::initializer_list can only provide a const pointer to its elements.

In the C programming language until C23, at least one named parameter must appear before the ellipsis parameter, so R printz(...); is not valid until C23. In C++, this form is allowed even though the arguments passed to such function are not accessible, and is commonly used as the fallback overload in SFINAE, exploiting the lowest priority of the ellipsis conversion in overload resolution.

This syntax for variadic arguments was introduced in 1983 C++ without the comma before the ellipsis. When C89 adopted function prototypes from C++, it replaced the syntax with one requiring the comma. For compatibility, C++98 accepts both C++-style f(int n...) and C-style f(int n, ...). The original C++-style grammar is deprecated since C++26.

The comma can be used in abbreviated function templates to make the ellipsis signify a variadic function instead of a variadic template:

void f1(auto...);   // same as template<class... Ts> void f3(Ts...)
void f2(auto, ...); // same as template<class T> void f3(T, ...)

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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