This program fails as written, but a fix
is shown in comments:

#include <iostream.h>
#include <strstream.h>

struct mine
{
    int a;
};

ostream& operator << (ostream& out, mine& m)
{
    return out << m.a;
}

istream& operator >> (istream& in, mine& m)
{
    return in >> m.a;
}

int main()
{
    mine mm;

//these two lines work
//  istrstream in("106");
//  if (in >> mm)

//but this one line alternative does not work
    if (istrstream("106") >> mm)

        cout << "mm= " << mm << endl;
    else
        cout << "extract fails" << endl;
}


The bad version gets this error message:
    cannot generate copy constructor for class
    'istream' because of private 'ios'

Why does the compiler need to generate a copy
constructor? The code creates a temporary istrstream,
passes it by reference to operator>>(), then uses the
returned reference to call member function ios::operator
void*(). Neither passing by reference nor calling a
member function involves copy constructing.

Why does the commented-out version work? Giving the istrstream a name instead of using a temp should not influence the need for a copy constructor.

Hello Steve,

Steve Strand wrote...
> #include <iostream.h>
> #include <strstream.h>
> 
> struct mine
> {
>     int a;
> };
> 
> ostream& operator << (ostream& out, mine& m)
> {
>     return out << m.a;
> }
> 
> istream& operator >> (istream& in, mine& m)
> {
>     return in >> m.a;
> }
> 
> int main()
> {
>     mine mm;
> 
> //these two lines work
> //  istrstream in("106");
> //  if (in >> mm)
> 
> //but this one line alternative does not work
>     if (istrstream("106") >> mm)

You create a temporary istrstream obj here. Temporarys can only be passed to const references.

> 
>         cout << "mm= " << mm << endl;
>     else
>         cout << "extract fails" << endl;
> }

> The bad version gets this error message:
>     cannot generate copy constructor for class
>     'istream' because of private 'ios'

I think the error message is misleading here. I tried your program with PC-Lint and got this error message:

--- Module:   stru.cpp

    if (istrstream("106") >> mm)
stru.cpp  28  Info 1776: Converting a string literal
to char * is not const safe (arg. no. 1)
stru.cpp  28  Error 1058: Initializing a non-const reference
'istream &' with a non-lvalue


PC-Lint tells you that you can't pass a temporary to a non const reference which you do in your operator >> for type mine.

If you add const to your first (working) line than you will also get a error (also misleading error message but due to const). In that case my PC-Lint tells me

--- Module:   stru.cpp
                           _
  const istrstream in("106");
stru.cpp  24  Info 1776: Converting a string literal
to char * is not const safe (arg. no. 1)
              _
  if (in >> mm)
stru.cpp  25  Warning 1561: Reference initialization causes loss of
    const/volatile integrity (arg. no. 1)

I don't yet know why this is only flagged as a warning and not a error which it should be in my opinion.

- Heinz

Thank you Heinz! Your answer is helpful and insightful.

Steve