how can I convert istream * to string or just print it? - c++

Question

The below function part of connector/C++, it returns a istream*. if i just try and print it, it shows hex or a memory location because its a * type.
istream *stream = res->getBlob(1);
I tried to read & print it with this:
string s;
while (getline(*stream, s))
cout << s << endl;
But this crashes with access violation though. any other way i can print it or convert to string?
the value of stream before the getline:
stream 0x005f3e88 {_Chcount=26806164129143632 } std::basic_istream > *
so it seems that its valid to me. I think it would be null or 0 if it failed

Answer 1

You can extract and print a std::istream by using its stream buffer:
std::cout << in->rdbuf();
Of course, this will consume the input and you may not be able to get it again. If you want to keep the content, you could write it an std::ostringstream instead and print the content using the str() method. Alternatively, you can directly construct a std::string from a stream, too, e.g.:
std::string content{ std::istreambuf_iterator<char>(*in),
std::istreambuf_iterator<char>() };
BTW, when you printed your stream pointer, you actually used the output operator for void const*: it prints the address the pointer is referring to. In C++03 you could even restore a correspondingly printed pointer by reading a void* using an std::istream: as long as the pointed to object wasn't deleted, you could get a pointer back that way! In C++11 pointer hiding is prohibited, however, to support optional garbage collection which may or may not be added to the language in the future. The guarantee about non-hidden pointers also helps member debuggers, though.

Answer 2

You can use the std::getline function in the while loop to display the data in the istream. Here is an example that I ran and it worked correctly:
#include <iostream>
#include <sstream>
#include <istream>
#include <string>
int main()
{
std::stringstream s1("This is a test string\nWith two lines");
std::istream s2(s1.rdbuf()); //just creates the istream to start with
std::string stt;
while(std::getline(s2,stt)) //can also have delimiter in getline
{
std::cout<<stt<<std::endl;
}
return 0;
}
Run it and it displays:
This is a test string
With two lines
I also tried this so that I was using pointer to the istream like you have:
#include <iostream>
#include <sstream>
#include <istream>
#include <string>
int main()
{
std::stringstream s1("This is a test string\nWith three lines);
std::istream s2(s1.rdbuf()); //just creates istream to start with
std::istream *s3 = &s2; //and use a pointer to istream like code at top
std::string stt;
while(std::getline(*s3,stt,'\n'))
{
std::cout<<stt<<std::endl; //result.
}
return 0;
}
This code ran and gave the same result as I got without using the pointer. I was unable to reproduce your error. So the problem looks like the creation of your istream (e.g. istream *stream = res->getBlob(1);).