9

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

user1397417
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  • yes, i think its good, i posted what it says in the watch variable place in my update. – user1397417 Sep 05 '13 at 00:45
  • I didn't see the problem with the crash: what happens between `res->getBlob(1)` and the call to `getline(*stream, s)`? It is odd to return a `std::istream*` because they are relatively expensive and it seems to be a memory leak in the making if the stream isn't owned by `res` or something `res` points to. – Dietmar Kühl Sep 05 '13 at 01:06
  • there is nothing inbetween the `res->getBlob(1)` and the `getline(*stream, s)` – user1397417 Sep 05 '13 at 01:09

2 Answers2

14

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.

Dietmar Kühl
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  • the line `std::cout << in->rdbuf();` still crashes for me when i replace it with my while getline attemp. both crash the same way with an access violation. And I dont understand much else of your answer to try the other ways you mentioned – user1397417 Sep 05 '13 at 01:12
  • @user1397417: I didn't see that it actually crashes and answered the question on the title. The other approach will probably crash pretty much the same way. It implies that stream was probably destroyed by the time you try to use it! What happens between you getting stream and using it? I _guess_ you invalidated the result, e.g., by moving to the next element or keeping the stream for later. – Dietmar Kühl Sep 05 '13 at 01:15
  • @user1397417: What is the database type of the field you try to access? – Dietmar Kühl Sep 05 '13 at 01:17
  • there is NOTHING inbetween my two blocks of code. the database structure of the column selected is 'english varchar(225) utf8_bin ' – user1397417 Sep 05 '13 at 01:31
  • @user1397417: Looking a the source, the function `getBlob()` actually returns a new'ed `std::istringstream` (you'll need to make sure that it is deleted after you used assuming the use gets sorted...). One things I could imagine is that the standard C++ library used for the MySQL connector and for your build are actually different: if you should verify that both are build with exactly the same version of the standard C++ library. – Dietmar Kühl Sep 05 '13 at 01:56
  • i uninstalled everything mysql, and re downloaded latest versions tried to build again. it said something about not finding a boost library so i installed boost and then it build and ran without crashing. seems to work fine now. – user1397417 Sep 07 '13 at 07:17
1

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);).