title: Can't convert STRING to INT in C++ in LINUX
body: <p>I've tried many ways, as detailed in here: <a href="http://www.cplusplus.com/forum/general/13135/" rel="nofollow">http://www.cplusplus.com/forum/general/13135/</a></p>
  
  <p>Most of them work if I run the file on Windows, but when I try to do so on LINUX, none of them works.
  For example, I try to do:</p>
  
  <pre><code>  string str = "123";
    int sp;
    istringstream ( str ) &gt;&gt; sp;
  </code></pre>
  
  <p>But it gives me the error: "invalid use of incomplete type ‘struct std::istringstream’
  /usr/include/c++/4.4/iosfwd:67: error: declaration of ‘struct std::istringstream’"</p>
  
  <p>Other option was 'atoi', but it says that 'atoi was not defined in this scope.' </p>
  
  <p>Any ideas why its happening?</p>