Defect report [N2134]: Missing IO roundtrip for random number engines

"=?iso-8859-1?q?Daniel_Kr=FCgler?=" <>
Thu, 8 Mar 2007 12:56:20 CST
Table 98 and para 5 in [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;
  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|
    os.fill(old); // Undo
    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;
    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.

Proposed resolution:

1) In table 98 from in column "pre/post-condition",
row expression "is >> x" change

"Sets v's state as determined by
reading its textual representation
from is.[..]"


"Sets v's state as determined by
reading its textual representation
with is.fmtflags set to ios_base::dec
from is.[..]"

2) In table 98 from in column "pre/post-condition",
row expression "os << x" change

"With os.fmtflags set to
fixed|ios_base::left and[..]"


"With os.fmtflags set to
left and[..]"

Greetings from Bremen,

Daniel Kr?gler

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