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Why is the compiler complain in stringstream?

Posted on 2012-04-13
Medium Priority
Last Modified: 2012-04-17
the read() function in the stringstreram class returns a istream& type. So if I do a call like this...

std::istream readStream =  mDataStream.read(buffer, buffersize);

I am getting a compilation error "synthesized method ‘std::basic_istream<char, std::char_traits<char> >::basic_istream(const std::basic_istream<char, std::char_traits<char> >&)"

Then I change to like this.

std::istream* readStream =  mDataStream.read(buffer, buffersize);

Then I get this error....
invalid conversion from ‘void*’ to ‘std::istream*

What I am doing wrong?
Question by:prain
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LVL 46

Expert Comment

ID: 37845535
what language and development tool are you using?
LVL 11

Expert Comment

ID: 37846027
Take this program, change it to the way you're using it and post it back.  It is difficult to visualize what your problem is.  This one works in Visual studio

#include <sstream>

struct Dummy
   std::istringstream mResult;
   std::istream* read (char* buffer, int buffersize)
      return &mResult;
} mDataStream;

int main ()
   char buff[1024];
   std::istream* readStream = mDataStream.read (buff, sizeof(buff));
   return 0;

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Author Comment

ID: 37851304
I am developing on Unix/g++. I am confused with what the compiler says.
If I use the function without the return type, it's great.
For example...

mDataStrea.read(buffer, bufferSize);

But if I capture the return type, I get this problem...
std::istream* readStream =  mDataStream.read(buffer, buffersize);

error: invalid conversion from ‘void*’ to ‘std::istream*’

BTW, my mDataStream is a C++ stringstream instance. So unfortunately I cannot use your code in my C++ class. I have to stick to the stringstream instance.
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Author Comment

ID: 37851766
Someone try this simple example and tell me why the compiler is barking..
BTW, I using Unix/g++

#include <string>
#include <iostream>
#include <sstream>

int main ()
   std::stringstream sstr;

   sstr << "Hello" << " World" ;

   char buffer[100];
   int size = 11;

   std::istream* aIStream = sstr.read(buffer, size);

  return 0;

Author Comment

ID: 37851837
BTW, since stringstream inherits istream, the read() in istream is defined as this:

istream& read ( char* s, streamsize n );
LVL 53

Accepted Solution

Infinity08 earned 500 total points
ID: 37852429
>> std::istream readStream =  mDataStream.read(buffer, buffersize);

Copying streams is not defined.

>> std::istream* readStream =  mDataStream.read(buffer, buffersize);

You can't assign a reference to a pointer.

Try this :

std::istream& readStream =  mDataStream.read(buffer, buffersize);

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Author Closing Comment

ID: 37852447
LVL 35

Expert Comment

ID: 37854593

it is intended by design that a stream cannot be copied. they explicitly didn't provide a copy constructor (and no assignment operator) cause a stream is not a normal container but has status and position members stored for streaming purposes. in your case the read member function returns a reference to istream while the stream you used was a stringstream. so you don't get a reference to the original stream but only to the istream baseclass.

the reasons why the read member function returns a reference to istream is not mainly to assign the return value to a second variable (you still could/should use the first variable) but for two other purposes. one is that the read would return NULL in case the read operation failed (error or end-of-stream), what makes it possible to do a read like

if (!mDataStream.read(buffer, buffersize))

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or to use a read in a while statement.

the other is that you could use the returned stream reference to pass it to next stream operation:

(mDataStream.read(buffer, 4)).read(&buffer[4], 2);

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what in case of read looks strange but was often used for operator>> like in

if (mDataStream >> i >> j)

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to do a formatted read into more variables with one statement.


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