[Insight-developers] important dashboard warning...

[David Cole]

Hello ITK developers,

This warning:
/.../Insight/Code/Common/itkQuaternionRigidTransform.txx: In member
function `const typename
itk::QuaternionRigidTransform<TScalarType>::InverseMatrixType&
itk::QuaternionRigidTransform<TScalarType>::GetInverseMatrix() const
[with TScalarType = double]':
/.../DEBUG_64_64/Insight/Code/Common/Templates/itkQuaternionRigidTransformD.cxx:4:
  instantiated from here
/.../Insight/Code/Common/itkQuaternionRigidTransform.txx:220: warning:
returning reference to temporary

(found here: http://www.itk.org/Testing/Sites/chiarugi.uiowa/Linux64-g++-3.4-dbg/20070329-0100-Nightly/BuildWarning.html
)

...indicates a "bad thing". It's an important warning to fix by
changing the code (as opposed to ignoring it or masking the warning
from dashboard results). The method
QuaternionRigidTransform::GetInverseMatrix is declared like so:
  const InverseMatrixType & GetInverseMatrix( void ) const;
in the protected section of QuaternionRigidTransform.

In the implementation of that method, however, the return statement is:
  return this->GetVarInverseMatrix();

...but GetVarInverseMatrix is defined like this:
  InverseMatrixType GetVarInverseMatrix( void ) const

Just like you shouldn't return a local variable by reference, neither
should you return a temporary variable by reference. The copy only
exists as long as that temporary is in scope, so the caller may not
actually be referencing a valid object when he goes to use it. The
caller's reference variable is given a reference to something that may
go out of scope prior to the reference variable itself going out of
scope......

So my questions for the group are these:
Who's the right person to fix this?
What's the right fix?
Is it ok to change the protected method GetVarInverseMatrix so that it
returns a "const&" rather than a copy of the data member?
And if we did that, is that the right thing to do in this case?

Thanks for wading through this email if you've made it this far...

David Cole
Kitware, Inc.