error in "The C++ programming language"?

[Darinm]

chapter 11.12, a string class

class String {
    struct Srep;
    Srep* rep;
public:
    class Range { };
    //...

    void check(int i) const { if(i<0 || i>=rep->sz) throw Range(); }
    //...
};

struct String::Srep {
    char* s;
    int sz;
    //...
};

int main()
{
}

Biuld msg :
error:invalid use of undefined type 'struct String::Srep'
...

why Dr.Stroustrup would write like this?

[helios]

That's perfectly valid C++ according to me, VC++, and MinGW.

[kbw]

You shouldn't use String::rep before you've actually declared it. So String::check() should be defined after the declaration of Srep.

You've used a pointer forward declaration, which is sufficient to define the layout of String, but you can't use it if you don't know what is is.

[helios]

Of course. I commented that line. I guess I should have left it the way it was.