The C++ wrappers create a temporary variable for a parameter to be passed to a
function. This is initially default constructed and then copy assigned from the
instance being passed in from the target language. This is unchanged, however,
when the temporary variable is passed to wrapped function, it is now done using
std::move. If the type is move constructible, the move constructor will be used
instead of the copy constructor.
Note that the implementation calls std::move for all user-defined types
(non-primitive types passed by value), this excludes anything passed by pointer,
reference and arrays. It does also include any type that has not been
defined/parsed by SWIG, that is, unknown types. std::move is called via the
SWIG_STD_MOVE macro which only calls std::move for C++11 and later code.
This is the current state of play where the copy constructor and copy
assignment operators are called, even for movable types passed as
function parameters.
Enhance SWIGTYPE "out" typemaps to use std::move when copying
objects, thereby making use of move semantics when wrapping a function returning
by value if the returned type supports move semantics.
Wrapping functions that return move only types 'by value' now work out the box
without having to provide custom typemaps.
The implementation removed all casts in the "out" typemaps to allow the compiler to
appropriately choose calling a move constructor, where possible, otherwise a copy
constructor. The implementation alsoand required modifying SwigValueWrapper to
change a cast operator from:
SwigValueWrapper::operator T&() const;
to
#if __cplusplus >=201103L
SwigValueWrapper::operator T&&() const;
#else
SwigValueWrapper::operator T&() const;
#endif
This is not backwards compatible for C++11 and later when using the valuewrapper feature
if a cast is explicitly being made in user supplied "out" typemaps. Suggested change
in custom "out" typemaps for C++11 and later code:
1. Try remove the cast altogether to let the compiler use an appropriate implicit cast.
2. Change the cast, for example, from static_cast<X &> to static_cast<X &&>, using the
__cplusplus macro if all versions of C++ need to be supported.
Issue #999Closes#1044
More about the commit:
Added some missing "varout" typemaps for Ocaml which was falling back to
use "out" typemaps as they were missing.
Ruby std::set fix for SwigValueWrapper C++11 changes.
William S Fulton