schmidt@beaver.ics.uci.edu (Doug Schmidt) (09/15/88)
Hi, Would someone please explain to me the rationale behind C++'s allowance of ``const's'' objects for use with specifying array size declarations, as opposed to ANSI-C's reject of this construct? For example, the following is legal C++: ---------------------------------------- const int Bar = 100; int Foo[Bar]; // declares an array of 100 ints ---------------------------------------- However, this does not work with the ANSI-Cesque compilers I've tried (gcc, for example). I'm interested to know the conceptual differences between the two languages on this point. thank you, Doug Schmidt
gwyn@smoke.ARPA (Doug Gwyn ) (09/15/88)
In article <709@paris.ICS.UCI.EDU> schmidt@beaver.ics.uci.edu (Doug Schmidt) writes: >I'm interested to know the conceptual >differences between the two languages on this point. Essentially, C++ "const" means "constant"; ANSI C "const" means "readonly".
bs@alice.UUCP (Bjarne Stroustrup) (09/15/88)
Doug Schmidt of University of California, Irvine - Dept of ICS writes: > Hi, > > Would someone please explain to me the rationale behind C++'s > allowance of ``const's'' objects for use with specifying array size > declarations, as opposed to ANSI-C's reject of this construct? For > example, the following is legal C++: > > ---------------------------------------- > > const int Bar = 100; > int Foo[Bar]; // declares an array of 100 ints > > ---------------------------------------- > > However, this does not work with the ANSI-Cesque compilers I've > tried (gcc, for example). I'm interested to know the conceptual > differences between the two languages on this point. > > thank you, > > Doug Schmidt ``const'' was introduced into C++ for three reasons: (1) to provide better documentation of interfaces (In particular, consts as argument types showing that the value of objects would not change within a function) (2) to enable the greater use of symbolic constants (In particular, integer consts that do not need to be allocated as objects unless the programmer explicitly requires it by declaring a const `extern' or taking its address) This ties in with a desire to reduce the use of macros significantly. Note that C++ also introduced inline functions. (3) to enable use of read only memory for large constant structures such as the tables produced by YACC. The first two reasons was by far the most important to me at the time. I can only conjecture about the reasons of the ANSI committe, but my reading of the rumors and the various versions of the definition of `const' in the ANSI drafts is that originally they latched on to the third reason exclusively and that over the years the ANSI C definition of const moved slowly towards the original conception. Both the ANSI C and the C++ versions of `const' falls a bit short of the ideal (as is common for programming language features), but the C++ version retains the concept that unless stated otherwise a const is a relatively local entity that might be `optimized away' and a valid alternative to a #define or and enumerator in a header file. In ANSI C a `const' default has external linkage so you have to allocate storage for it (just in case) and a `const' may not be used in a constant expression.
maart@cs.vu.nl (Maarten Litmaath) (09/17/88)
In article <8500@smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
\Essentially, C++ "const" means "constant"; ANSI C "const" means "readonly".
Aha! That means the following is correct?
const volatile int * const clock; /* clock is a readonly pointer to */
/* a readonly and volatile int */
--
Alles klar, |Maarten Litmaath @ Free U Amsterdam:
Herr Kommissar? |maart@cs.vu.nl, mcvax!botter!maartbs@alice.UUCP (Bjarne Stroustrup) (09/17/88)
Free U Amsterdam writes: brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes: > Essentially, C++ "const" means "constant"; ANSI C "const" means "readonly". > > Aha! That means the following is correct? > > const volatile int * const clock; /* clock is a readonly pointer to */ > /* a readonly and volatile int */ Yes. In both languages.
bill@proxftl.UUCP (T. William Wells) (09/18/88)
In article <1411@solo3.cs.vu.nl> maart@cs.vu.nl (Maarten Litmaath) writes: : In article <8500@smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes: : \Essentially, C++ "const" means "constant"; ANSI C "const" means "readonly". : : Aha! That means the following is correct? : : const volatile int * const clock; /* clock is a readonly pointer to */ : /* a readonly and volatile int */ Almost. As specified, it is not initialized and so contains a null pointer. You should initialize it to the right address. --- Bill novavax!proxftl!bill