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Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 98 and para 5 in 29.5.3.4 [rand.req.eng] specify the IO insertion and extraction semantic of random number engines. It can be shown, v.i., that the specification of the extractor cannot guarantee to fulfill the requirement from para 5:
If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v.
The problem is, that the extraction process described in table 98 misses to specify that it will initially set the if.fmtflags to ios_base::dec, see table 104:
dec: converts integer input or generates integer output in decimal base
Proof: The following small program demonstrates the violation of requirements (exception safety not fulfilled):
#include <cassert>
#include <ostream>
#include <iostream>
#include <iomanip>
#include <sstream>
class RanNumEngine {
int state;
public:
RanNumEngine() : state(42) {}
bool operator==(RanNumEngine other) const {
return state == other.state;
}
template <typename Ch, typename Tr>
friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& os, RanNumEngine engine) {
Ch old = os.fill(os.widen(' ')); // Sets space character
std::ios_base::fmtflags f = os.flags();
os << std::dec << std::left << engine.state; // Adds ios_base::dec|ios_base::left
os.fill(old); // Undo
os.flags(f);
return os;
}
template <typename Ch, typename Tr>
friend std::basic_istream<Ch, Tr>& operator>>(std::basic_istream<Ch, Tr>& is, RanNumEngine& engine) {
// Uncomment only for the fix.
//std::ios_base::fmtflags f = is.flags();
//is >> std::dec;
is >> engine.state;
//is.flags(f);
return is;
}
};
int main() {
std::stringstream s;
s << std::setfill('#'); // No problem
s << std::oct; // Yikes!
// Here starts para 5 requirements:
RanNumEngine x;
s << x;
RanNumEngine v;
s >> v;
assert(x == v); // Fails: 42 == 34
}
A second, minor issue seems to be, that the insertion description from table 98 unnecessarily requires the addition of ios_base::fixed (which only influences floating-point numbers). Its not entirely clear to me whether the proposed standard does require that the state of random number engines is stored in integral types or not, but I have the impression that this is the indent, see e.g. p. 3
The specification of each random number engine defines the size of its state in multiples of the size of its result_type.
If other types than integrals are supported, then I wonder why no requirements are specified for the precision of the stream.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]