Move constructor and char array argument - c++

Question

struct Foo
{
char data[100];
template<int T>
Foo(char (&&var)[T])
{
data = std::move(var);
var = 0;
}
};
int main()
{
char v[100];
//..init v
Foo f( std::move(v) ); //error C2664: 'Foo::Foo<100>(char (&&)[100])' : cannot convert parameter 1 from 'char [100]' to 'char (&&)[100]'
return 0;
}
I don't get why MSVC is not happy with the line Foo f( std::move(v) ) ? (Maybe this code is senseless)

Answer 1

You could use the std::move algorithm, from the <algorithm> header.
auto it = std::move(data, data + T, var);
after checking that T isn't larger than 100. But moving char doesn't have many benefits over just copying.

Answer 2

The line
Foo f( std::move(v) );
is legal for GCC, Clang and Intel. Visual Studio however complains and yields
error C2664: 'Foo::Foo(const Foo &)' : cannot convert parameter 2 from 'char [100]' to 'char (&&)[100]'
1> You cannot bind an lvalue to an rvalue reference
This seems a bug to me. Indeed, v is an lvalue but std::move cast it to an rvalue reference.
Since arrays of chars are not movable, although legal, I don't see much use for taking them by rvalue reference. If you really want the code to compile, a workaround is making f taking a regular (lvalue) reference and remove the std::move at the call site. Or even better, make f take a pointer to char* (f doesn't need to be a template). In this case, calling f(v) passes the address of v[0] (i.e. the first element of v) to f. This is the so called array-to-pointer conversion which is implicitly performed by the compiler.
However, as others have pointed out, the lines
data = std::move(var);
var = 0;
are illegal for all compilers. data and var are arrays (or references to arrays) and arrays are not assignable. For instance, the error raised by var = 0; by each compiler is as follows:
GCC:
error: incompatible types in assignment of 'int' to 'char [100]'
Clang:
error: array type 'char [100]' is not assignable
Intel:
error: expression must be a modifiable lvalue
Visual Studio:
error C2440: '=' : cannot convert from 'int' to 'char [100]'