Why is the compiler complain in stringstream?

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?
prainAsked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

aikimarkCommented:
what language and development tool are you using?
cupCommented:
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;
}

Open in new window

prainAuthor Commented:
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.
-prain
CompTIA Cloud+

The CompTIA Cloud+ Basic training course will teach you about cloud concepts and models, data storage, networking, and network infrastructure.

prainAuthor Commented:
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;
}
prainAuthor Commented:
BTW, since stringstream inherits istream, the read() in istream is defined as this:

istream& read ( char* s, streamsize n );
Infinity08Commented:
>> 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);

Open in new window

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
prainAuthor Commented:
Thanks
sarabandeCommented:
FYI:

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))
{
    ...

Open in new window


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);

Open in new window


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

if (mDataStream >> i >> j)

Open in new window


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

Sara
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
C++

From novice to tech pro — start learning today.