The Value of a Value Class

re: The Value of a Value Class

Kevin — Sun, 25 Jul 2004 03:03:00 GMT

<Stan Lippman> When you think value class, think integer. Then things will begin to click for you. </Stan Lippman>

Doesn't int provide any copy constructor(s)and overloaded assignment operator? The following code snippet illustrates the contrary side of the story.

int a = 2.0; // copy ctor
char ch = 'a';
a = ch; // assignment

It would be interesting to see how one can implement an int value class that behaves the exactly same way as the built-in int type.

re: The Value of a Value Class

stan lippman — Sun, 25 Jul 2004 05:27:00 GMT

no -- you're not thinking blitable, and you are not thinking value. you are thinking class, and you will be disappointed with the union ...

re: The Value of a Value Class

Slawomir Lisznianski — Mon, 26 Jul 2004 14:28:00 GMT

May be "value class" should change into "value blitable"? ;-)

Well, it looks to me that the originally legitimate idea of introducing a lightweight value class met unanticipated implementation difficulties. Usage constraints were introduced and we ended up with yet another language artifact of limited value.

re: The Value of a Value Class

stan lippman — Mon, 26 Jul 2004 20:03:00 GMT

well, value types are the odd fellow out in all dynamic programming models.

i would not agree with your characterization. the `original idea' is the CLI value type -- which is valuable when contrasted with a heap based object.

you are looking at it from a C++ perspective, and are dissatisfied that the value class does not have x, y, and z characteristics, which you would expect in a native C++ class.

re: The Value of a Value Class

Nish — Tue, 27 Jul 2004 01:59:00 GMT

Hey Stan

One really good use to having a default ctor for a value class would be when you had an enum member and you wanted to initialize the enum member to a value other than the first value in the enumeration.

Say you have a enum type called Sex and you want to initialize it to female in the default ctor.

How do you work around situations like this? Is the solution then, not to use value classes if you want to do initialization of value class members to non-null values?

re: The Value of a Value Class

stan lippman — Tue, 27 Jul 2004 05:10:00 GMT

agreed -- there are instances one can take for a spin in which a default ctor would be useful.

in your example, of course, the solution is to let female have an associated value of 0, as in enum class gender { female, male };

let me just reiterate that the CLI object model does not support that abstraction.

we did add support for (a) deterministic finalization, (b) copy ctor and copy assignment operator, and (c) support for instance operators ... but all for reference types.

we haven't been all bad, after all. :-)

here is what the design guidelines in the visual studio 2005 beta have to say about value types -- unfortunately, it is written in a C#-centric mindset in which struct serves as a synonym for value type ... hopefully we can fix that before the rtm ...

from the beta documentation:

It is recommended that you use a struct for types that meet any of the following criteria:

Act like primitive types.
Have an instance size under 16 bytes.
Are immutable.
Value semantics are desirable.

Do not provide a default constructor for a struct. Note that C# does not allow a struct to have a default constructor. The runtime inserts a constructor that initializes all the values to a zero state. This allows arrays of structs to be created without running the constructor on each instance. Do not make a struct dependent on a constructor being called for each instance. Instances of structs can be created with a zero value without running a constructor. You should also design a struct for a state where all instance data is set to zero, false, or null (as appropriate) to be valid.

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