What are the rules for the "..." token in the context of variadic templates?

Question

In C++11 there are variadic templates like this one:

template< class T, class... Args > unique_ptr<T> make_unique( Args&&... args ) {     return unique_ptr<T>(new T(std::forward<Args>(args)...)); } 

There are some curiosities about this: The expression std::forward<Args>(args)... uses both Args and args but only one ... token. Furthermore std::forward is a non-variadic template function taking only one template parameter and one argument. What are the syntax rules for that (roughly)? How can it be generalized?

Also: In the function implementation the ellipsis (...) is at the end of the expression of interest. Is there a reason that in the template argument list and the parameter list the ellipsis is in the middle?

Answer 1

In the context of variadic template, the ellipsis ... is used to unpack the template parameter pack if it appears on the right side of an expression (call this expression pattern for a moment), or it's a pack argument if it appears on left side of the name:

...thing  // pack   : appears as template arguments thing...  // unpack : appears when consuming the arguments 

The rule is that whatever pattern is on the left side of ... is repeated — the unpacked patterns (call them expressions now) are separated by comma ,.

It can be best understood by some examples. Suppose you have this function template:

template<typename ...T> //pack void f(T ... args)      //pack {    // here are unpack patterns     g( args... );        //pattern = args    h( x(args)... );     //pattern = x(args)    m( y(args...) );     //pattern = args (as argument to y())    n( z<T>(args)... );  //pattern = z<T>(args) } 

Now if I call this function passing T as {int, char, short}, then each of the function call is expanded as:

g( arg0, arg1, arg2 );            h( x(arg0), x(arg1), x(arg2) ); m( y(arg0, arg1, arg2) ); n( z<int>(arg0), z<char>(arg1), z<short>(arg2) ); 

In the code you posted, std::forward follows the fourth pattern illustrated by n() function call.

Note the difference between x(args)... and y(args...) above!

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You can use ... to initialize an array also as:

struct data_info {      boost::any  data;      std::size_t type_size; };  std::vector<data_info> v{{args, sizeof(T)}...}; //pattern = {args, sizeof(T)} 

which is expanded to this:

std::vector<data_info> v  {     {arg0, sizeof(int)},    {arg1, sizeof(char)},    {arg2, sizeof(short)} }; 

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I just realized a pattern could even include access specifier such as public, as shown in the following example:

template<typename ... Mixins> struct mixture : public Mixins ...  //pattern = public Mixins {     //code }; 

In this example, the pattern is expanded as:

struct mixture__instantiated : public Mixin0, public Mixin1, .. public MixinN   

That is, mixture derives publicly from all the base classes.

Hope that helps.