Update:
Thank you, Jamboree.
This is the final struct A
.
struct A
{
template<class ... Args,class=std::enable_if_t<(sizeof...(Args)!=1)>>
A(Args &&...args)
{
cout<<'v';
}
template<class Arg,class=std::enable_if_t<!std::is_base_of<std::remove_reference_t<Arg>,A>::value>>
A(Arg &&arg)
{
cout<<'v';
}
A(const A &)
{
cout<<'c';
}
A(A &&)
{
cout<<'m';
}
};
Origin:
About this code,
#include<iostream>
#include<type_traits>
#include<utility>
using namespace std;
struct A
{
template<class ... Args,class=std::enable_if_t<
sizeof...(Args)!=1
||!std::is_same<std::remove_cv_t<std::remove_reference_t<Args>>,A>::value>>
A(Args &&...args)
{
cout<<'v';
}
A(const A &)
{
cout<<'c';
}
A(A &&)
{
cout<<'m';
}
};
int main()
{
A a{10};
A b{10,20};
A c{b};
const A d{c};
A e{move(c)};
}
The output is vvvvm
in VC++ 14.0.
But why is the output not vvccm
?
(I want c and d
to use copy constructor. And I know Effective Modern C++ Item 27 use only one forwarding reference.)
Because b
and c
, is A&
to the compiler (not const-qualified) when you pass them to the ctor of c
and d
, Args &&...args
is thus a better match than const A &
.
To achieve what you want, you could do it this way:
struct A
{
A() = default;
template<class ... Args,std::enable_if_t<(sizeof...(Args)>1), bool> = true>
A(Args &&...args)
{
cout<<'v';
}
template<class Arg,std::enable_if_t<!std::is_base_of<A, std::remove_reference_t<Arg>>::value, bool> = true>
A(Arg && arg)
{
cout<<'v';
}
A(const A &)
{
cout<<'c';
}
A(A &&)
{
cout<<'m';
}
};