[eigen] Eigen::Ref and GCC's -Wzero-as-null-pointer-constant

[Hervé Audren <h.audren@xxxxxxxxxx>]

Hello everyone,

I am a quite long-time user of Eigen, but I encountered a weird problem 
today. I was converting an old code that used ints to a newer API taking 
std::string (So, C++11). Unfortunately, the literal 0 can be promoted to 
char*, and thus to std::string, triggering an error, namely 
"basic_string::_S_construct null not valid". I noticed that GCC has 
zero-as-null-pointer-constant warning designed to catch such programming 
errors. Unfortunately, as I turned it on, I noticed that methods taking 
an Eigen::Ref as an argument triggered the warning (Code sample below):
warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
   Eigen::VectorXd myvec;
   my_method(myvec);

I think that somewhere in RefBase/MapBase/Memory a literal zero is used 
instead of NULL (Which is correctly not flagged by GCC). But maybe it is 
GCC incorrectly reporting the warning ?

I would really like to be able to enable that flag in my codebase to 
track down places where I forgot to change the ints to strings.

Thanks,
Hervé

---- Contents ----

Minimal code example:
#include <Eigen/Core>

void fill_with_ones(Eigen::Ref<Eigen::VectorXd> vec)
{
  vec.setOnes();
}

int main()
{
  Eigen::VectorXd myvec(7);
  fill_with_ones(myvec);
  return 0;
}

Compiled with (Ubuntu 14.04, gcc4.8.4 - Same behaviour on Arch with gcc 
5.3.0):
g++ -std=c++11 -o test_eigen -I/usr/include/eigen3 test_eigen.cpp

Full Warning Log:
In file included from /usr/include/eigen3/Eigen/Core:256:0,
                 from test_eigen.cpp:1:
/usr/include/eigen3/Eigen/src/Core/util/Memory.h: In function ‘void* 
Eigen::internal::handmade_aligned_malloc(std::size_t)’:
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:107:19: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (original == 0) return 0;
                   ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:107:29: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (original == 0) return 0;
                             ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h: In function ‘void* 
Eigen::internal::handmade_aligned_realloc(void*, std::size_t, std::size_t)’:
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:126:14: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (ptr == 0) return handmade_aligned_malloc(size);
              ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:130:19: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (original == 0) return 0;
                   ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:130:29: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (original == 0) return 0;
                             ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h: In function ‘void* 
Eigen::internal::generic_aligned_realloc(void*, std::size_t, std::size_t)’:
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:154:12: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (ptr==0)
            ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:160:12: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
     return 0;
            ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:164:17: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (newptr == 0)
                 ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:169:12: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
     return 0;
            ^
/usr/include/eigen3/Eigen/src/Core/util/Memory.h:172:14: warning: zero 
as null pointer constant [-Wzero-as-null-pointer-constant]
   if (ptr != 0)
              ^
In file included from /usr/include/eigen3/Eigen/Core:326:0,
                 from test_eigen.cpp:1:
/usr/include/eigen3/Eigen/src/Core/products/GeneralBlockPanelKernel.h: 
At global scope:
/usr/include/eigen3/Eigen/src/Core/products/GeneralBlockPanelKernel.h:28:91: 
warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
 inline void manage_caching_sizes(Action action, std::ptrdiff_t* l1=0, 
std::ptrdiff_t* l2=0)
^
/usr/include/eigen3/Eigen/src/Core/products/GeneralBlockPanelKernel.h:28:91: 
warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
/usr/include/eigen3/Eigen/src/Core/products/GeneralBlockPanelKernel.h:28:91: 
warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
/usr/include/eigen3/Eigen/src/Core/products/GeneralBlockPanelKernel.h:28:91: 
warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
test_eigen.cpp: In constructor ‘Eigen::Ref<PlainObjectType, Options, 
StrideType>::Ref(Eigen::PlainObjectBase<OtherDerived>&, typename 
Eigen::internal::enable_if<(bool)(typename 
Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, 
_StrideType> >::match<Derived>::MatchAtCompileTime), Derived>::type*) 
[with Derived = Eigen::Matrix<double, -1, 1>; PlainObjectType = 
Eigen::Matrix<double, -1, 1>; int Options = 0; StrideType = 
Eigen::InnerStride<1>; typename 
Eigen::internal::enable_if<(bool)(typename 
Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, 
_StrideType> >::match<Derived>::MatchAtCompileTime), Derived>::type = 
Eigen::Matrix<double, -1, 1>]’:
test_eigen.cpp:11:23: warning: zero as null pointer constant 
[-Wzero-as-null-pointer-constant]
   fill_with_ones(myvec);
                       ^
test_eigen.cpp:11:23: warning: zero as null pointer constant 
[-Wzero-as-null-pointer-constant]
In file included from /usr/include/eigen3/Eigen/Core:308:0,
                 from test_eigen.cpp:1:
/usr/include/eigen3/Eigen/src/Core/Ref.h: In instantiation of 
‘Eigen::RefBase<Derived>::RefBase() [with Derived = 
Eigen::Ref<Eigen::Matrix<double, -1, 1> >]’:
/usr/include/eigen3/Eigen/src/Core/Ref.h:196:5:   required from 
‘Eigen::Ref<PlainObjectType, Options, 
StrideType>::Ref(Eigen::PlainObjectBase<OtherDerived>&, typename 
Eigen::internal::enable_if<(bool)(typename 
Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, 
_StrideType> >::match<Derived>::MatchAtCompileTime), Derived>::type*) 
[with Derived = Eigen::Matrix<double, -1, 1>; PlainObjectType = 
Eigen::Matrix<double, -1, 1>; int Options = 0; StrideType = 
Eigen::InnerStride<1>; typename 
Eigen::internal::enable_if<(bool)(typename 
Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, 
_StrideType> >::match<Derived>::MatchAtCompileTime), Derived>::type = 
Eigen::Matrix<double, -1, 1>]’
test_eigen.cpp:11:23:   required from here
/usr/include/eigen3/Eigen/src/Core/Ref.h:150:100: warning: zero as null 
pointer constant [-Wzero-as-null-pointer-constant]
StrideType::InnerStrideAtCompileTime==Dynamic?0:StrideType::InnerStrideAtCompileTime)
^