how to make it working correct(libstdc++)?

[TARZAN LION]

I download a gcc-3.1 tar file.
than I excute the following command:

    tar zxvf gcc-3.1.tar.gz
    cd gcc-3.1
    ./configure --prefix = /usr
                --enable-threads=posix
                --enable-shared
    make bootstrap
    make install
    
    but some error message reported when I compiled some like sample.cpp file.

    the sample.cpp file like following:

#include <iostream.h>
#include <string.h>

int main()
{
  string str("hello world!");
  cout<<str<<endl;
  return 0;
}

  my program have not any error,but some error report like following message:
  "string" undeclared (first use this function)
I know some reason-->"the include file,something is wrong",but I don't know how to do.
  
  what can I do for it? I don't know.

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