[Python-Dev] Baffled by PyArg_ParseTupleAndKeywords modification (original) (raw)
"Martin v. Löwis" martin at v.loewis.de
Sun Feb 12 00:45:59 CET 2006
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Armin Rigo wrote:
Alas, this doesn't make gcc happy either. (I'm trying gcc 3.4.4.) In theory, it prevents the const-bypassing trick showed by Martin, but apparently the C standard (or gcc) is not smart enough to realize that.
It appears to be language-defined. Looking at the assignment
char a; const char const b; b = a;
then, in C++, 4.4p4 [conv.qual] has a rather longish formula to decide that the assignment is well-formed. In essence, it goes like this:
- the pointers are "similar": they have the same levels of indirection, and the same underlying type.
- In all places where the type of a has const/volatile qualification, the type of b also has these qualifications (i.e. none in the example)
- Starting from the first point where the qualifications differ (from left to right), all later levels also have const.
I'm unsure about C; I think the rule comes from 6.3.2.3p2:
[#2] For any qualifier q, a pointer to a non-q-qualified
type may be converted to a pointer to the q-qualified
version of the type; the values stored in the original and
converted pointers shall compare equal.So it is possible to convert a non-const pointer to a const pointer, but only if the the target types are the same. In the example, they are not: the target type of a is char*, the target of b is const char*.
Regards, Martin
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