Re: [Boost-users] [boost] [multi_array] Everyone, please make your value-types Assignable!

On Jul 18, 2008, at 8:03 AM, David Abrahams wrote:

On Jul 18, 2008, at 7:30 AM, Daryle Walker wrote:

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[SNIP]

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Yes, the OP was having a problem with a boost class (template) that is like std::valarray: you can only do assignments if the source and destination objects _already_ had the same layout.

I understand that

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And my first response to this thread explained why this is problematic for our users,

I understand that too. My problem is with your implicit declaration that such types are not to be considered Assignable. That concept is supposed to be compatible with the builtins.

My use of "Assignable" was more colloquial, and not one of the official meanings in various C++ literature. What does practical assignability mean? It means that any object of a certain type can be either initialized by or receive an assignment from any other valid object of that type (Uninitialized PODs can't count because a program that uses one as an r-value is illegal.) and the destination object becomes observationally identical to the source object. If an object only lets certain objects of the same type be allowed to be assignment-source objects, then its utility becomes a lot lower because the user has to add more checks on his/her code. It might be OK for a mixed-operation-assignment to discriminate (with either an assertion or exception) because there's no way to match two objects with incompatible parameters for the desired operation. But the regular copy-assignment is different. It has a special place in the C++ type/object model, so much so that said operator is automatically defined if it's not explicitly provided. Anything that messes up the assumptions that C++ has about regular assignment, like multiple assignability classes or non-splatter semantics, cascades until an ugliness-hiding wrapper is made.

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and so we shouldn't do this. In this setup, all objects are validly initialized; it's just there isn't a single assignment- compatibility class that all valid object states belong to. Note that you have to go out of your way to create a class like this.

Not really. It's pretty easy to end up with uninitialized members. But that's really beside the point.

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It's a pain because the user who wants to use this class internally has either make sure all wrapping objects keep all sub- objects of this type within the same assignment-compatibility class or write a custom assignment routine. Said routine can be at most the basic guarantee if the resizing or copy steps may throw.

So? I don't see how that's related or why it's a problem

It's an example of the hoops a user has to do if they need to "correct" the assignment on their end. The problem is that the user's code doesn't have access to the inside information needed to increase the safety guarantee's strength. If the author provided the assignment, s/he could have used the inside information to strengthen the safety guarantee.

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(A strong guarantee could be done if the type provides a swap that can cross assignment-compatibility classes.) An author- supplied full-assignment routine could take advantage of the internal implementation and add rollback.

-- Daryle Walker Mac, Internet, and Video Game Junkie darylew AT hotmail DOT com