GCC 4.6 and missing variadic-templates expansions

Question

I'm using this code to create multiple functions wrappers using variadic templates:

// Compile with g++ -std=c++0x $(pkg-config sigc++-2.0 --cflags --libs) test.cpp -o test
#include <iostream>
#include <type_traits>
#include <sigc++/sigc++.h>

template <typename R, typename G, typename... Ts>
class FuncWrapper
{
public:
  FuncWrapper(G object, std::string const& name, sigc::slot<R, Ts...> function) {};
};

int main()
{
  FuncWrapper<void, int, int, bool, char> tst(0, "test", [] (int a, bool b, char c) {});

  return EXIT_SUCCESS;
}

This code correctly compiles with clang++, but not with g++ due to the known issue:

test.cpp:9:73: sorry, unimplemented: cannot expand ‘Ts ...’ into a fixed-length argument list

I know that gcc-4.7 should handle this correctly, but I can't upgrade for now... So I'd like to have a workaround to make Ts... to unpack correctly. I've tested what suggested here in questions like this one, but they don't seem to solve the issue here.

I've once seen this same error when I actually tried to misuse variadic templates, and upgrading didn't help (fixing the code did). Just mentioning — Kos, Jul 02 '12 at 16:24
@Kos I'm working on a framework where we use sigc::slot as base... — Treviño, Jul 02 '12 at 18:19

score 5 · Accepted Answer · answered Jul 02 '12 at 16:41

You can workaround the bug with:

template<template <typename...> class T, typename... Args>
struct Join
{ typedef T<Args...> type; };

then replace sigc::slot<R, Ts...> with typename Join<sigc::slot, R, Ts...>::type

(Thanks to Chris Jefferson's suggestion on the GCC bug report)

GCC 4.6 and missing variadic-templates expansions

1 Answers1