ostringstream(string) constructor

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Bob Altman - 14 Mar 2005 17:27 GMT

Hi all,

Why doesn't the following unmanaged C++ code work as expected:

string s;
ostringstream strm(s); // This stream should store results in s
strm << 25;
cout << s << endl; // s still contains an empty string
cout << strm.str(); // but the stream internally contains "25"

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Tom Widmer - 14 Mar 2005 17:29 GMT

> Hi all,
>
> Why doesn't the following unmanaged C++ code work as expected:
>
> string s;
> ostringstream strm(s); // This stream should store results in s

Where did you get the idea that the comment above is correct? The above
is just shorthand for:
ostringstream strm;
strm << s;

> strm << 25;
> cout << s << endl; // s still contains an empty string

Right, since you've misunderstood what the ostringstream constructor does.

> cout << strm.str(); // but the stream internally contains "25"

An ostringstream doesn't hold a reference to a string, but rather is
interoperable with strings. You need:

ostringstream strm;
strm << 25;
string s(strm.str());
//etc.

Tom

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Bob Altman - 14 Mar 2005 19:24 GMT

> > string s;
> > ostringstream strm(s); // This stream should store results in s
>
> Where did you get the idea that the comment above is correct?

"Beginning C++" by Ivor Horton, 1998 ed., page 802, states (apparently
incorrectly):

You can use an ostringstream object to format data into a string. For
instance, you could create a string object and an output string stream with
the statements:

string outBuffer;
ostringstream outStr(outBuffer);

You can now use the insertion operators to write to outBuffer via outStr:

double number = 2.5;
outStr << "number = " << (number / 2);

As a result of the write to the string stream, outBuffer will contain
"Number = 1.25"... The string parameter to the string stream constructor is
a reference, so write operations for ostringstream objects act directly on
the string object.

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Carl Daniel [VC++ MVP] - 14 Mar 2005 20:39 GMT

>>> string s;
>>> ostringstream strm(s); // This stream should store results in s
[quoted text clipped - 3 lines]
> "Beginning C++" by Ivor Horton, 1998 ed., page 802, states (apparently
> incorrectly):

Dump that book and pick up a copy of "Accelerated C++" by Andrew Koenig and
Barbara Moo. It's 1/3 the length, has far more content, and is actually
correct.

-cd

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ostringstream(string) constructor

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