C++0x Variadic Parameter Pack: Syntax

Question

the following code snippet won't compile under gcc4.6.1:

template <typename... TS>
void do_stuff(TS... ts)
{
  auto f = [](TS... things) { };
}

It throws an error stating that the pack things was not expanded. The following code does compile however:

template <typename... TS>
void do_stuff(TS... ts)
{
  auto f = [](TS... things...) { };
}

Notice the extra unpacking operator after things inside the parameter list. I've never seen a situation where a variadic pack had to be expanded during its declaration. So my question to you kind folks is:

Is this legal C++0x syntax (the snippet that compiles) or is it just a quirk with GCC when it comes to dealing with variadic types?

That isn't "capturing", capturing happens inside the [] brackets. You're just specifying an argument list. (But nice question!) — Kerrek SB
– Kerrek SB, Commented Jul 26, 2011 at 14:27
Thanks, I edited it to make more sense. I knew it wasn't capturing in the variable sense, but it seems like it's an issue with how the lambda "captures" that type TS... — bstamour
– bstamour, Commented Jul 26, 2011 at 14:42
Ah, I think I found the answer, which is "no": stackoverflow.com/questions/3575901/… — Kerrek SB
– Kerrek SB, Commented Jul 26, 2011 at 14:43
But the lambda function itself is not templated. When it comes time for the lambda expression to be evaluated, those types will be well defined. — bstamour
– bstamour, Commented Jul 26, 2011 at 14:48
Hm, right, disregard that comment. Actually, it seems that you can also say doStuff(TS... ts...). I don't know if (TS... ts) is a permissible shorthand for member function declarations... — Kerrek SB
– Kerrek SB, Commented Jul 26, 2011 at 14:49

Johannes Schaub - litb · Accepted Answer · 2011-07-26 20:03:03Z

6

Two things:

Yes, GCC is wrong to reject [](TS... things) { }. It's possible that it hasn't been implemented yet.
What you declared by [](TS ... things...) { } is equivalent to [](TS... things, ...). In C++ (not in C), you can leave off the comma before the C-style variadic ellipsis. So instead of doing void printf(char const *fmt, ...) you can declare void printf(char const *fmt...). That's what happens in your lambda. The first ellipsis is the parameter pack unpacking, and the second ellipsis is the C-style variadic ellipsis.

answered Jul 26, 2011 at 20:03

Johannes Schaub - litb

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2 Comments

Kerrek SB Over a year ago

That's very good to know, but how come auto m = [](Args... p...) { printf("%d%d\n", p...); }; actually works in GCC 4.6 if I call m(1,2); and prints out 12?

Luc Danton Over a year ago

@Kerrek It's hard to second-guess. -std=c++0x is called experimental support for the next C++ Standard after all.

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