[std-interval] Revised document available

[Steve Clamage]

George Corliss wrote:

> 
> Can you help me understand the difference between "undefined" and
> "implementation-dependent" or "implementation-defined"?


The C and C++ standards define these terms.

1. Undefined behavior means the standard places no requirements on the 
compile-time or run-time systems. Common responses to code having undefined 
behavior are
- refuse to compile
- compile, but fail to link
- link, but crash at run time
- run, but produce an undesired result
- run and produce the result the programmer wanted
Other responses are possible, such as the legendary "reformat the hard drive."
(Suppose a wild jump via an uninitialized pointer goes to the BIOS reformat 
command.)

An implementation is allowed to define the behavior of code that is formally 
undefined in the standard. Example: Divide-by-zero has undefined behavior, but 
an implementation could say that it will always raise a signal, or always abort, 
or always return a particular value.


2. Implementation defined means that the standard allows a list or range of 
possible behaviors, and the implementation is required to tell you what it does. 
Example: Type int must be capable of representing values in the range 
[-32767,32767]. The implementation must document what the actual range is, which 
almost always wider.


3. Unspecified (a category you didn't list) is similar to 
implementation-defined, but the implementation need not tell you what it does. 
Example: All the arguments to a function must be fully evaluated before the 
function is called, but the evaluation order is unspecified. An implementation 
can evaluate them in any order, and does not have to document the order. The 
order need not be consistent.


4. Implementation dependent is a generic term that means that the behavior can 
vary among systems. The term includes undefined, implementation-defined, and 
unspecified behavior.

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Steve Clamage, stephen.clamage at sun.com