Created on 2010-07-23.00:00:00 last changed 174 months ago
What should the following code output?
#include <string> #include <iostream> #include <iomanip> int main() { std::string test("0X1Y2Z"); std::cout.fill('*'); std::cout.setf(std::ios::internal, std::ios::adjustfield); std::cout << std::setw(8) << test << std::endl; }
I would expect "**0X1Y2Z", and this is what the compilers I have access to (VC++, g++ and Sun CC) do. But according to the standard, it should be "0X**1Y2Z":
[string.io]/5:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns true when converted to a value of type bool, determines padding as described in [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() and str.size(); then calls os.width(0).
[facet.num.put.virtuals]/5:
[...]
Stage 3: A local variable is initialized as
fmtflags adjustfield= (flags & (ios_base::adjustfield));The location of any padding is determined according to Table 88.
If str.width() is nonzero and the number of charT's in the sequence after stage 2 is less than str.width(), then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence to str.width(). str.width(0) is called.
Table 88 — Fill padding State Location adjustfield == ios_base::left pad after adjustfield == ios_base::right pad before adjustfield == internal and a sign occurs in the representation pad after the sign adjustfield == internal and representation after stage 1 began with 0x or 0X pad after x or X otherwise pad before
Although it's not 100% clear what "the sequence after stage 2" should mean here, when there is no stage 2, the only reasonable assumption is that it is the contents of the string being output. In the above code, the string being output is "0X1Y2Z", which starts with "0X", so the padding should be inserted "after x or X", and not before the string. I believe that this is a defect in the standard, and not in the three compilers I tried.
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Date | User | Action | Args |
2010-07-23 00:00:00 | admin | create |