C++ variadic templates - c++

Question

I'm using a vector definition:
vector<lmlink_out> os{
lmlink_out(typeid(string &)),
lmlink_out(),
lmlink_out(typeid(int),typeid(char))
};
using the following code works (defined inside the class):
class lmlink_out {
vector<const type_info*> parameters;
...
public:
template<typename... T, std::size_t N = sizeof...(T)>
lmlink_out(const T&... args) : parameters({&args...}) {}
using this other leads to compilation error (defined outside the class):
class lmlink_out {
vector<const type_info*> parameters;
...
public:
template<typename... T, std::size_t N = sizeof...(T)>
lmlink_out(const T&... args);
};
template<typename... T, std::size_t N = sizeof...(T)>
lmlink_out::lmlink_out(const T&... args)
: parameters({(&args...})
{
}
the compiler (gcc version 10.2.1 20210110 (Debian 10.2.1-6)) returns:
error: no matching function for call to ‘std::vector<const
std::type_info*>::vector()’ 374 |
: parameters({(&args...}) note: candidate: ‘std::vector<_Tp,
_Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = const std::type_info*; _Alloc = std::allocator<const
std::type_info*>; std::vector<_Tp, _Alloc>::allocator_type =
std::allocator<const std::type_info*>]’ 625 |
vector(initializer_list<value_type> __l,
...
| ^~~~~~ /usr/include/c++/10/bits/stl_vector.h:625:43: note: no known conversion for argument 1 from ‘’ to ‘std::initializer_list<const std::type_info*>’
I think that two codes are semantically equivalent, why does the second give an error?
I need to add another constructor:
template<typename... T, std::size_t N = sizeof...(T)>
lmlink_out(const string &name, const T&... args) : name(name), parameters({&args...}) {}
but it lead to the same error above.

Answer 1

Your code has two simple problems:
You can't redefine template default arguments, i.e., you'll need to drop the = sizeof...(T) rom the definition of the ctor. It can only be present in the first declraation.
There is an excess '(' when calling the parameters ctor. It should be
: parameters({&args...})
This Compiler Explorer link shows the code compiling.