Andrew Falanga Andrew Falanga - 1 month ago 6
C++ Question

Should std::unique_ptr<void> be permitted

This is a very simple question. Consider the following code:

#include <iostream>
#include <memory>

typedef std::unique_ptr<void> UniqueVoidPtr;

int main() {
UniqueVoidPtr p(new int);
return 0;
}


Compiling with cygwin (g++ 4.5.3) with the following command
g++ -std=c++0x -o prog file.cpp
works just fine. However, compiling with the microsoft compiler (either VS 2010 or 2013) I get this error:

C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\memory(2067) : error C2070: 'void': illegal sizeof operand
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\memory(2066) : while compiling class template member function 'void std::default_delete<_Ty>::operator ()(_Ty *) const'
with
[
_Ty=void
]
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\type_traits(650) : see reference to class template instantiation 'std::default_delete<_Ty>' being compiled
with
[
_Ty=void
]
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\memory(2193) : see reference to class template instantiation 'std::tr1::is_empty<_Ty>' being compiled
with
[
_Ty=std::default_delete<void>
]
foo1.cpp(7) : see reference to class template instantiation 'std::unique_ptr<_Ty>' being compiled
with
[
_Ty=void
]


Is this expected? I'm writing a class where I wanted to have a unique pointer in the in the class. While trying to work out the semantics of a move constructor for the class, I ran into this (I assume because I finally got my move constructor coded correctly: i.e. the other errors were fixed).

Answer

MSVC is right while GCC is wrong:

Standard(3.9/5):

Incompletely-defined object types and the void types are incomplete types

Standard(20.7.1.1.2/4):

If T is an incomplete type, the program is ill-formed

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