CODEWITHSUNDEEP
c++
Nim
Nim

Reputation: 33655

Odd(?) behaviour of a temporary std::ostringstream

I was messing around with std::ostringstream whilst looking at this question: sprintf in c++?, and noticed the stringbuilder() wrapper by Nawaz and thought, well that ought to work with std::ostringstream.

So my first attempt was the following:

std::cout << (std::ostringstream("select * from foo limit") << max_limit).str() << std::endl;

Now this obviously fails to compile (correctly) as the result of the operator<< is a std::ostream - which doesn't have the member str(). So I thought a cast should do the trick, and specifically a cast to a const reference (works with a cast to a normal reference too), so second attempt:

std::cout << static_cast<std::ostringstream const&>(std::ostringstream("select * from foo limit") << max_limit).str() << std::endl;

Now this compiles fine and runs, however the output is, well, not what I was expecting.

10lect * from foo limit

Now - here's the question, am I invoking some undefined behaviour somewhere - and if so where? And how is this different to the approach that Nawaz has taken (I guess aside from the result of his operator is the stringbuilder itself rather than std::ostream).

EDIT: here is the ideone code.

EDIT: oops - forgot to specify, max_limit is int.

Upvotes: 4

Views: 319

Answers (3)

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361412

You could also simply use flush as:

static_cast<std::ostringstream const&>(std::ostringstream().flush() << "select * from foo limit " << max_limit).str() << std::endl;

But then the code is not same. Its using default constructor.

As a sidenote : isn't dynamic_cast more appropriate here?

Demo : http://www.ideone.com/06xNS

Upvotes: 0

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

What's maxLimit?

Some of the ostream operator<< overloads are free functions, like:

ostream& operator<<(ostream& os, T const&);

If the stream is a temporary (which in your case it is), it cannot bind to that ref-to-non-const, and the overload cannot be chosen.

So you may be using a non-preferred overload by accident; possibly something like the overload for char const*. Hard to tell without knowing what maxLimit is.

This is a limitation when trying to do this serialisation on a single line, and you can't get around it.

You're also attempting to stream std::cout to std::cout, which is obviously not what you intended to do.

Update Vijay figured it out.

std::cout << static_cast<std::ostringstream const&>(
   std::ostringstream("select * from foo limit", std::ios_base::app) << max_limit
).str() << std::endl

The above is still definitely worth bearing in mind, though.

Upvotes: 2

Vijay Mathew
Vijay Mathew

Reputation: 27174

You need to move the stream's position to the end of the internal buffer used by ostringstream:

  std::ostringstream out("select * from foo limit ", std::ios_base::app);
  out << max_limit;
  std::cout << out.str () << std::endl;

See the documentation on ostringstream constructor.

Upvotes: 4

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