GCC error with variadic templates: “Sorry, unimplemented: cannot expand 'Identifier…' into a fixed-length argument list”

Question

While doing variadic template programming in C++11 on GCC, once in a while I get an error that says \"Sorry, unimplemented: cannot expand \'Identifier...\' into a fixed-leng

Answer 1

There is a trick to get this to work with gcc. The feature isn't fully implemented yet, but you can structure the code to avoid the unimplemented sections. Manually expanding a variadic template into a parameter list won't work. But template specialization can do that for you.

template< char head, char ... rest >
struct head_broken
{
   static const char value = head;
};

template< char ... all >
struct head_works; // make the compiler hapy

template< char head, char ... rest >
struct head_works // specialization
{
   static const char value = head;
};

template
struct do_head
{
   static const char head = head_works::value;
   //Sorry, unimplemented: cannot expand 'all...' into a fixed-length arugment list
   //static const char head = head_broken::value;
};

int main
{
   std::cout << head_works<'a','b','c','d'>::value << std::endl;
   std::cout << head_broken<'a','b','c','d'>::value << std::endl;
   std::cout << do_head<'a','b','c','d'>::head << std::endl;
}

I tested this with gcc 4.4.1