ostringstream(string) constructor

  • Thread starter Thread starter Bob Altman
  • Start date Start date
B

Bob Altman

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"
 
Bob said:
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
Click to expand...

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
Click to expand...

Right, since you've misunderstood what the ostringstream constructor does.
cout << strm.str(); // but the stream internally contains "25"
Click to expand...

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
 
string s;
Where did you get the idea that the comment above is correct?
Click to expand...

"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.
 
Bob said:
"Beginning C++" by Ivor Horton, 1998 ed., page 802, states (apparently
incorrectly):
Click to expand...

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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