Re: correct usage of ostringstream
mzdude <jsanga@cox.net> wrote:
The two functions foo() and bar() are trying to do the same thing. foo
doesn't work but bar does. My question "Is bar a leagal function or is
it treading into the undefined behaviour and I'm simply getting away
with it"? I have tried the code both in VC 6 and VC 2005. Both exhibit
the same behaviour.
As long as I use the ostringstream::str() function as the first output
variable it seems to work. As soon as it appears in a different
location, the code will crash.
void foo()
{
const char stx = '\2';
const char etx = '\3';
std::ostringstream os;
os << stx
<< "One"
<< SomeFunc(os.str().substr(1)) // crashes here
<< etx;
A compiler can evaluate subexpressions of an expression in any order,
subject to certain limitations. In particular, it can evaluate
os.str().substr(1) before os << stx. Which is what it apparently does in
this case: by the time substr(1) is called, os.str() is still an empty
string, and substr() throws out_of_range exception.
std::string done = os.str();
}
void bar()
{
const char stx = '\2';
const char etx = '\3';
std::ostringstream os;
os << stx
<< "One";
os << SomeFunc(os.str().substr(1)) // works but why?
<< etx;
Semicolon introduces a sequence point: os << stx << "One" is always
executed before os.str().substr(1), so os.str() cannot be empty.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925