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I tried overloading the operator<<() for ostream as a static member
function in a class, but the compiler (gcc) complained that it should
be non-static or non-member. Why is this not possible? Since I want
to pass a private type of the class as argument, this is a problem.

What I tried to do is something like this:

#include <iostream>

using namespace std;

class Foo {
public:
void foo() {
#if 0
cout << "yadda yadda " << b << endl;
#endif
}
private:
struct Bar {
public:
#if 1
static ostream &operator<<(ostream &os, const Bar &b)
{
b.print(os);
return os;
}
#endif
private:
int bar;
void print(ostream &os) const { os << bar; }
};

Bar b;
};

I cannot make the operator<< a non-member function since its parameter
type Bar is private and I'd prefer to leave it so. How should I solve
this?

When I replace the "operator<<" by some other function name the code
compiles but I still cannot use << like in foo().

urs

On 11/4/2011 10:01 AM, Urs Thuermann wrote:
> I tried overloading the operator<<() for ostream as a static member
> function in a class, but the compiler (gcc) complained that it should
> be non-static or non-member. Why is this not possible?

Because that's how operators are required to be defined: either
non-static members or non-members.

> Since I want
> to pass a private type of the class as argument, this is a problem.
>
> What I tried to do is something like this:
>
> #include<iostream>
>
> using namespace std;
>
> class Foo {
> public:
> void foo() {
> #if 0
> cout<< "yadda yadda "<< b<< endl;
> #endif
> }
> private:
> struct Bar {
> public:
> #if 1
> static ostream&operator<<(ostream&os, const Bar&b)

Try replacing the keyword 'static' with 'friend'...

> {
> b.print(os);
> return os;
> }
> #endif
> private:
> int bar;
> void print(ostream&os) const { os<< bar; }
> };
>
> Bar b;
> };
>
> I cannot make the operator<< a non-member function since its parameter
> type Bar is private and I'd prefer to leave it so. How should I solve
> this?

Define a non-member operator << and make it call your static member
function (if that's what you need to access that static data member).
The alternative is to declare that non-member operator<< a friend.

> When I replace the "operator<<" by some other function name the code
> compiles but I still cannot use<< like in foo().

So, combine the two...

V
--
I do not respond to top-posted replies, please don't ask

On 04.11.2011 15:01, Urs Thuermann wrote:
> I tried overloading the operator<<() for ostream as a static member
> function in a class, but the compiler (gcc) complained that it should
> be non-static or non-member. Why is this not possible? Since I want
> to pass a private type of the class as argument, this is a problem.

> class Foo {
> struct Bar {
> public:
> static ostream&operator<<(ostream&os, const Bar&b)
> {
> b.print(os);
> return os;
> }

> I cannot make the operator<< a non-member function since its parameter
> type Bar is private and I'd prefer to leave it so. How should I solve
> this?

What's wrong with
friend ostream&operator<<(ostream&os, const Bar&b);
inside class Foo?


Marcel

Marcel Müller <> writes:

> On 04.11.2011 15:01, Urs Thuermann wrote:
> > I tried overloading the operator<<() for ostream as a static member
> > function in a class, but the compiler (gcc) complained that it should
> > be non-static or non-member. Why is this not possible? Since I want
> > to pass a private type of the class as argument, this is a problem.
>
> > class Foo {
> > struct Bar {
> > public:
> > static ostream&operator<<(ostream&os, const Bar&b)
> > {
> > b.print(os);
> > return os;
> > }
>
> > I cannot make the operator<< a non-member function since its parameter
> > type Bar is private and I'd prefer to leave it so. How should I solve
> > this?
>
> What's wrong with
> friend ostream&operator<<(ostream&os, const Bar&b);
> inside class Foo?

I am surprised that that works, because I have tried it before I
posted and gcc reported an error since the type Foo::Bar is private
and unusable outside class Foo. OK, I must have done something wrong,
since it now works, both, inline and outside of the class.

But now I've got the next question: How can I make that non-member
friend function static in foo.cc so it's not visible in other
translation units? (I know I can't use it anyway because I cannot
have object of type Foo::Bar, but I'd still prefer to have that
function staitc.)

When I try

friend static ostream &operator<<(ostream &os, const Bar &b);

in class Foo and

static ostream &operator<<(ostream &os, const Foo::Bar &b)
{
b.print(os);
return os;
}

outside any class in foo.cc I get the compiler message

error: storage class specifiers invalid in friend function declarations

and when I remove the static from the friend declaration I get

error: 'std:stream& operator<<(std:stream&, const Foo::Bar&)' was declared 'extern' and later 'static'

Without static in both places, everylooks looks fine.
But is there a way to have that operator<<() static?

urs

On 11/7/2011 2:19 AM, Urs Thuermann wrote:
> Marcel Müller<> writes:
>
>> On 04.11.2011 15:01, Urs Thuermann wrote:
>>> I tried overloading the operator<<() for ostream as a static member
>>> function in a class, but the compiler (gcc) complained that it should
>>> be non-static or non-member. Why is this not possible? Since I want
>>> to pass a private type of the class as argument, this is a problem.
>>
>>> class Foo {
>>> struct Bar {
>>> public:
>>> static ostream&operator<<(ostream&os, const Bar&b)
>>> {
>>> b.print(os);
>>> return os;
>>> }
>>
>>> I cannot make the operator<< a non-member function since its parameter
>>> type Bar is private and I'd prefer to leave it so. How should I solve
>>> this?
>>
>> What's wrong with
>> friend ostream&operator<<(ostream&os, const Bar&b);
>> inside class Foo?
>
> I am surprised that that works, because I have tried it before I
> posted and gcc reported an error since the type Foo::Bar is private
> and unusable outside class Foo. OK, I must have done something wrong,
> since it now works, both, inline and outside of the class.
>
> But now I've got the next question: How can I make that non-member
> friend function static in foo.cc so it's not visible in other
> translation units? (I know I can't use it anyway because I cannot
> have object of type Foo::Bar, but I'd still prefer to have that
> function staitc.)
>
> When I try
>
> friend static ostream&operator<<(ostream&os, const Bar&b);

Drop the 'static'.

>
> in class Foo and
>
> static ostream&operator<<(ostream&os, const Foo::Bar&b)

Drop the 'static'.

> {
> b.print(os);
> return os;
> }
>
> outside any class in foo.cc I get the compiler message
>
> error: storage class specifiers invalid in friend function declarations

That's what your compiler is telling you: drop the 'static'.

> and when I remove the static from the friend declaration I get
>
> error: 'std:stream& operator<<(std:stream&, const Foo::Bar&)' was declared 'extern' and later 'static'

Huh?

> Without static in both places, everylooks looks fine.

So, what's the problem?

> But is there a way to have that operator<<() static?

WTF for?

V
--
I do not respond to top-posted replies, please don't ask