Eigen LinSpaced - 不推荐使用的复制警告

Question

In my project, compiler complains on the following (and many other similar) code snippets:

    Eigen::ArrayXf window =
            Eigen::ArrayXf::LinSpaced(2*M + 1, 0, M_PI)
            .head(2*M)
            .sin();

The warning message is long and unreadable so I will not post all of them here. The warning triggered is -Wdeprecated-copy , and the core part of warning message (in my opinion) is in follow

warning: implicitly-declared ‘Eigen::Block<const Eigen::CwiseNullaryOp<Eigen::internal::linspaced_op<float, __vector(4) float>, Eigen::Array<float, -1, 1> >, -1, 1, false>::Block(const Eigen::Block<const Eigen::CwiseNullaryOp<Eigen::internal::linspaced_op<float, __vector(4) float>, Eigen::Array<float, -1, 1> >, -1, 1, false>&)’ is deprecated [-Wdeprecated-copy]
note: because ‘Eigen::Block<const Eigen::CwiseNullaryOp<Eigen::internal::linspaced_op<float, __vector(4) float>, Eigen::Array<float, -1, 1> >, -1, 1, false>’ has user-provided ‘Eigen::BlockImpl<XprType, BlockRows, BlockCols, InnerPanel, Eigen::Dense>& Eigen::BlockImpl<XprType, BlockRows, BlockCols, InnerPanel, Eigen::Dense>::operator=(const Eigen::BlockImpl<XprType, BlockRows, BlockCols, InnerPanel, Eigen::Dense>&) [with XprType = const Eigen::CwiseNullaryOp<Eigen::internal::linspaced_op<float, __vector(4) float>, Eigen::Array<float, -1, 1> >; int BlockRows = -1; int BlockCols = 1; bool InnerPanel = false]’
  830 |     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Derived& operator=(const Derived& other) \
      |                                                    ^~~~~~~~
/somewhere/Eigen/src/Core/util/Macros.h:842:53: note: in expansion of macro ‘EIGEN_INHERIT_ASSIGNMENT_EQUAL_OPERATOR’
  842 | #define EIGEN_INHERIT_ASSIGNMENT_OPERATORS(Derived) EIGEN_INHERIT_ASSIGNMENT_EQUAL_OPERATOR(Derived)
      |                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/somewhere/Eigen/src/Core/Block.h:161:5: note: in expansion of macro ‘EIGEN_INHERIT_ASSIGNMENT_OPERATORS’
  161 |     EIGEN_INHERIT_ASSIGNMENT_OPERATORS(BlockImpl)

Does this warning lead to an incorrect(unexpected) result? How can I correct my code to eliminate this warning?

Answer 1

This is a deprecation warning with respect to the C++ standard.

Currently the copy constructor is implicitly defined even if a class has a user-provided destructor or user-provided copy assignment operator.

The message is a warning that the current C++ standard (since C++11) deprecates this behavior and that it will be removed from C++ in a future version. It indicates that the programmer should implement or default the copy constructor manually, so that the intended change in a future C++ version will not cause trouble when transitioning.

It is also a violation of the rule of 0/3/5 not to do so. But that rule is only a general guideline and does not need to always apply if the programmer knows what they are doing. It is (currently) not enforced by the language.

---

Since this warning is for library code, not your own, and it is unlikely that the library authors failed to implement all their classes correctly, I would not be concerned by it. It is more likely that they just didn't take into account the deprecation until now.

You can disable that particular warning globally with -Wno-deprecated-copy if that is ok for you or maybe you can disable it locally with a diagnostic pragma around the Eigen header include (not sure whether it will apply to template instantiations), see eg this question for GCC.

If you are bothered by the warnings, I would have suggested to open a bug report with Eigen about it, but they seem to already be aware of it and have recently fixed it on their git master branch, see this gitlab issue .

So I guess if you are bothered by the warnings and don't want to disable them, you should upgrade Eigen to the current version on git master or wait for a release that includes the fix.