[Dune] [#622] Semantics of Conversion

[Dune]

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User who did this - Martin Nolte (nolte) 

Attached to Project - Dune
Summary - Semantics of Conversion
Task Type - Support Request
Category - Common
Status - New
Assigned To - 
Operating System - Unspecified / All
Severity - Low
Priority - Normal
Reported Version - SVN
Due in Version - Undecided
Due Date - Undecided
Details - What is the exact semantics of Conversion< From, To > in typetraits.hh?

Problem: In the current implementation, class From needs a copy constructor (unless From = To) and hence may not be pure virtual.

Possible semantics:
1) Conversion::exists == true, iff a static_cast will succeed
2) Conversion::exists == true, iff To is a base class of From

ad 1) In this case, the constructor From( const To & ), but the check should not fail if it does not exist. In the current implementation From also needs a copy constructor (which could be fixed by 'static From &makeFrom()'). Moreover, the specialization of Conversion< T, T > is wrong, since it returns true, even if the copy constructor is private.

ad 2) In this case, the copy constructors need not exist and we really should check if a reference can be converted, i.e., 'static Small test( To & )' and 'static From &makeFrom()'.

P.S.: In both suggested fixes, we need to assure that From and To respectively are no reference types.

More information can be found at the following URL:
http://www.dune-project.org/flyspray/index.php?do=details&task_id=622

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