What is the meaning of “… …” token? i.e. double ellipsis operator on parameter pack

by

Every instance of that oddity is paired with a case of a regular single ellipsis.

  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes...)>
    { typedef _Res result_type; };

  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes......)>
    { typedef _Res result_type; };

  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes...) const>
    { typedef _Res result_type; };

  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes......) const>
    { typedef _Res result_type; };

My guess is that the double ellipsis is similar in meaning to _ArgTypes..., ..., i.e. a variadic template expansion followed by a C-style varargs list.

Here’s a test supporting that theory… I think we have a new winner for worst pseudo-operator ever.

Edit: This does appear to be conformant. §8.3.5/3 describes one way to form the parameter list as

parameter-declaration-listopt …opt

So the double-ellipsis is formed by a parameter-declaration-list ending with a parameter pack, followed by another ellipsis.

The comma is purely optional; §8.3.5/4 does say

Where syntactically correct and where “…” is not part of an abstract-declarator, “, …” is synonymous with “…”.

This is within an abstract-declarator, [edit] but Johannes makes a good point that they are referring to an abstract-declarator within a parameter-declaration. I wonder why they didn’t say “part of a parameter-declaration,” and why that sentence isn’t just an informative note…

Furthermore, va_begin() in <cstdarg> requires a parameter before the varargs list, so the prototype f(...) specifically allowed by C++ is useless. Cross-referencing with C99, it is illegal in plain C. So, this is most bizarre.

Usage note

By request, here is a demonstration of the double ellipsis:

#include <cstdio>
#include <string>

template< typename T >
T const &printf_helper( T const &x )
    { return x; }

char const *printf_helper( std::string const &x )
    { return x.c_str(); }

template< typename ... Req, typename ... Given >
int wrap_printf( int (*fn)( Req... ... ), Given ... args ) {
    return fn( printf_helper( args ) ... );
}

int main() {
    wrap_printf( &std::printf, "Hello %s\n", std::string( "world!" ) );
    wrap_printf( &std::fprintf, stderr, std::string( "Error %d" ), 5 );
}

More Related Contents:

  1. How to reverse the order of arguments of a variadic template function?
  2. C++11: Number of Variadic Template Function Parameters?
  3. “unpacking” a tuple to call a matching function pointer
  4. make_unique and perfect forwarding
  5. Variadic template pack expansion
  6. How to call a function on all variadic template args?
  7. How do I get the argument types of a function pointer in a variadic template class?
  8. Specifying one type for all arguments passed to variadic function or variadic template function w/out using array, vector, structs, etc?
  9. C++11: I can go from multiple args to tuple, but can I go from tuple to multiple args? [duplicate]
  10. What are the rules for the “…” token in the context of variadic templates?
  11. Calling a function for each variadic template argument and an array
  12. Check at Compile-Time if Template Argument is void
  13. How to store variadic template arguments?
  14. Is it possible to “store” a template parameter pack without expanding it?
  15. How to create the Cartesian product of a type list?
  16. Parameter pack must be at the end of the parameter list… When and why?
  17. What does this variadic template code do?
  18. effective way to select last parameter of variadic template
  19. C++ parameter pack, constrained to have instances of a single type?
  20. C++11 “overloaded lambda” with variadic template and variable capture
  21. Check traits for all variadic template arguments
  22. How can I have multiple parameter packs in a variadic template?
  23. How can I expand call to variadic template base classes?
  24. How to make a variadic is_same?
  25. generic member function pointer as a template parameter
  26. how to avoid undefined execution order for the constructors when using std::make_tuple
  27. Generating one class member per variadic template argument
  28. Is there a way to write make_unique() in VS2012?
  29. How to combine std::bind(), variadic templates, and perfect forwarding?
  30. template parameter packs access Nth type and Nth element

Leave a Comment