Please have a look at the following program:
#include <iostream>
template <const int array[], size_t index>
class ArrayIndex
{
public:
static const int value = array[index];
};
extern const int array[] = { 1, 2, 3, 4, 5 };
int main()
{
std::cout <<
ArrayIndex<arra y, 0>::value << "," <<
ArrayIndex<arra y, 1>::value << "," <<
ArrayIndex<arra y, 2>::value << "," <<
ArrayIndex<arra y, 3>::value << "," <<
ArrayIndex<arra y, 4>::value;
}
Output is, on my C++ compiler (VS 2003): 0,0,0,0,0 while I'd expect that the
output is 1,2,3,4,5. What actually does the C++ Standard say about this? In
other words, is the above program correct and if it is, what should be its
output with a standard-compliant compiler ? Moreover, if the program is
correct, is the expression 'array[index]' within the ArrayIndex class
regarded as a constant expression ?
(note that I had to declare the array with 'extern', which shouldn't be
necessary by the Standard, as this array has external linkage, anyway.
However, the compiler issues an error if I don't do this. So I have reasons
to suspect that the compiler is not standard-compliant).
Regards,
Rade 7 3661
template <const int array[], size_t index>
class ArrayIndex
{
public:
static const int value;
};
template <const int array[], size_t index>
const int ArrayIndex<arra y, index>::value = array[index];
-----------------------------------------------------------------
Static member definition should be separated from its declaration.
"serock" <s.******@gmail .com> wrote in message template <const int array[], size_t index> class ArrayIndex { public: static const int value; };
template <const int array[], size_t index> const int ArrayIndex<arra y, index>::value = array[index]; ----------------------------------------------------------------- Static member definition should be separated from its declaration.
This is not required for static const integral types (int, char, bool, long,
short). I think the OP's code is legal, though my version of g++ and
Borland both complain.
Siemel Naran wrote in
news:oZ******** ***********@bgt nsc05-news.ops.worldn et.att.net in
comp.lang.c++: "serock" <s.******@gmail .com> wrote in message
template <const int array[], size_t index> class ArrayIndex { public: static const int value; };
template <const int array[], size_t index> const int ArrayIndex<arra y, index>::value = array[index]; ----------------------------------------------------------------- Static member definition should be separated from its declaration.
This is not required for static const integral types (int, char, bool, long, short). I think the OP's code is legal, though my version of g++ and Borland both complain.
The problem is that the initializer "array[ index ]" *isn't* a
compile time constant (intergral constant expression).
IOW, the out of class initialization is required.
Rob.
-- http://www.victim-prime.dsl.pipex.com/
"Rob Williscroft" <rt*@freenet.co .uk> wrote in message Siemel Naran wrote in This is not required for static const integral types (int, char, bool, long, short). I think the OP's code is legal, though my version of g++ and Borland both complain.
The problem is that the initializer "array[ index ]" *isn't* a compile time constant (intergral constant expression).
IOW, the out of class initialization is required.
Do you have a quote from the standard? I agree that if if 'array' has type
"int *" then array[3] is not an integral constant, but if it has type "int
const *" then array[3] should be a constant so long as array itself is
constant, which is the case because it is extern const.
In any case, I made the initialization out of line for g++ to compile it,
and I still witness the original behavior that it prints "0,0,0,0,0, "
instead of "1,2,3,4,5, ". Strange. Yet when I force the instantiation of
the specific classes by using template class, it does work. Stranger.
"Rade" <no************ *@btinternet.co m> wrote in message news:covce2$e84 #include <iostream>
template <const int array[], size_t index> class ArrayIndex { public: static const int value = array[index]; };
extern const int array[] = { 1, 2, 3, 4, 5 };
int main() { std::cout << ArrayIndex<arra y, 0>::value << "," << ArrayIndex<arra y, 1>::value << "," << ArrayIndex<arra y, 2>::value << "," << ArrayIndex<arra y, 3>::value << "," << ArrayIndex<arra y, 4>::value; }
Output is, on my C++ compiler (VS 2003): 0,0,0,0,0 while I'd expect that
the output is 1,2,3,4,5.
This is quite strange. I added statements to force the compiler to
instantiate the ArraynIndex classes, and it works now, at least on g++
2.95.2-6. Add the five lines (without >) to your program and see if it
works.
extern const int array[] = { 1, 2, 3, 4, 5 };
template class ArrayIndex<arra y, 0>;
template class ArrayIndex<arra y, 1>;
template class ArrayIndex<arra y, 2>;
template class ArrayIndex<arra y, 3>;
template class ArrayIndex<arra y, 4>; int main()
What actually does the C++ Standard say about this? In other words, is the above program correct and if it is, what should be its output with a standard-compliant compiler ? Moreover, if the program is correct, is the expression 'array[index]' within the ArrayIndex class regarded as a constant expression ?
Seems to me array[index] is an integral expression because array has type T
const *const, but don't know what the standard says.
(note that I had to declare the array with 'extern', which shouldn't be necessary by the Standard, as this array has external linkage, anyway. However, the compiler issues an error if I don't do this. So I have
reasons to suspect that the compiler is not standard-compliant).
Const objects have internal linkage by default, so the extern is necessary.
> Static member definition should be separated from its declaration.
OK, it's better now, but does C++ language require it, or the requirement
comes from Microsoft ?
As far as I know, one should be able to define static consts inside the
class as well as outside, as long as they are initialized by a constant
expression. That's why I asked about whether array[index] is regarded as a
constant expression in this context. If it is constant, then my code should
work (but it doesn't). If it is not constant, then the compiler should have
warned me that I was trying to initialize a static const within a class with
a nonconstant expression (but it hasn't). Am I right ?
Rade
Siemel Naran wrote in
news:Z7******** ***********@bgt nsc05-news.ops.worldn et.att.net in
comp.lang.c++: "Rob Williscroft" <rt*@freenet.co .uk> wrote in message Siemel Naran wrote in
> This is not required for static const integral types (int, char, > bool, long, short). I think the OP's code is legal, though my > version of g++ and Borland both complain.
The problem is that the initializer "array[ index ]" *isn't* a compile time constant (intergral constant expression).
IOW, the out of class initialization is required.
Do you have a quote from the standard?
5.19 & 5.2.1
Rob.
-- http://www.victim-prime.dsl.pipex.com/ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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