#include <iostream>
using namespace std;

class X
{
class Y
Y *y;
public:
X() { y = new Y; }
~X() { delete y; }
void func()
{
y->n = 10;
y->display();
}
};

class X::Y
{
public:
int n;
void display() { cout<<n; }
};

int main(int argc, char *argv[])
{
X x;
x.func();
system("pause");
}
上面的不行。

嵌套的类Y在X内部定义没问题，要想在X外部定义，应该怎么写？？

X::去掉

X去掉也不行啊。

似乎只有这样了。
#include <iostream>
using namespace std;

class X
{
class Y
{
public:
int n;
void display();
};
Y *y;

public:

X() { y = new Y; }
~X() { delete y; }
void func()
{
y->n = 10;
y->display();
}
};

void X::Y::display()
{
cout<<n;
}

int main(int argc, char *argv[])
{
X x;
x.func();
system("pause");
}

哦。。。
对了，你为什么要那样啊？把一个类型声明在一个类里？在哪个时候可以用到啊？还没碰到过

书上的例题，还没做完，Srep类是实际的容器，Cref是帮助String实现下标的操作。
把String类的实现分解，这些类可以被限制在String的空间。可能显得比较麻烦，我觉得有些意义的。

String.h

[CODE]#ifndef MYSTRING_H
#define MYSTRING_H

#include <cstring>
#include <iostream>
using namespace std;

class String
{
struct Srep
{
char *s;
int sz;
int n; //reference counter

Srep(int nsz, const char *p):sz(nsz),n(1)
{
s = new char[sz+1];
strcpy(s, p);
}

~Srep() { delete[] s; }

Srep* get_own_copy()
{
if(n==1) return this;
n--;
return new Srep(sz, s);
}

void assign(int nsz, const char *p)
{
if(sz != nsz)
{
delete[] s;
sz = nsz;
s = new char[sz+1];
}
strcpy(s, p);
}
};

Srep *rep;

public:

class Cref
{
friend class String;
String &str;
int i;
Cref(String &ss, int ii): str(ss),i(ii) {}
public:
void operator=(const char ch) { str.write(i, ch); }
operator char() { return str.read(i); }
};
class Range {};

String();
String(const char *p);
String(const String &s);
~String();

void check(int i)const { if( i<0 || i>=rep->sz ) throw Range(); }
char read(int i) { return rep->s[i]; }
void write(int i, char ch) { rep = rep->get_own_copy(); rep->s[i]=ch; }
Cref operator[](int i) { check(i); return Cref(*this, i); }
char operator[](int i)const { check(i); return rep->s[i]; }

String& String::operator=( const String& );
String& String::operator=( const char* );


String& operator+=( const String& );
String& operator+=( const char* );

friend ostream& operator<<(ostream&, const String&);
friend istream& operator>>(istream&, String&);

friend bool operator==(const String &x, const String &y)
{ return strcmp(x.rep->s, y.rep->s) == 0; }
friend bool operator==(const String &x, const char *p)
{ return strcmp(x.rep->s, p) == 0; }

friend bool operator!=(const String &x, const String &y)
{ return strcmp(x.rep->s, y.rep->s) != 0; }
friend bool operator!=(const String &x, const char *p)
{ return strcmp(x.rep->s, p) != 0; }

friend bool operator>(const String &x, const String &y)
{ return strcmp(x.rep->s, y.rep->s) > 0; }
friend bool operator<(const String &x, const String &y)
{ return strcmp(x.rep->s, y.rep->s) < 0; }
};

String operator+(const String&, const String&);
String operator+(const String&, const char*);
String operator+(const char*, const String&);

#endif
[/CODE]

String.cpp

[CODE]#include "String.h"

String::String()
{
rep = new Srep(0, "");
}

String::String(const String &x)
{
rep = x.rep;
rep->n++;
}

String& String::operator=(const String &x)
{
(x.rep->n)++;
if(--(rep->n) == 0) delete rep;
rep = x.rep;
return *this;
}

String::~String()
{
if(--(rep->n) == 0) delete rep;
}

String::String(const char *p)
{
rep = new Srep(strlen(p), p);
}

String& String::operator=(const char *p)
{
if(rep->n == 1)
rep->assign(strlen(p), p);
else
{
rep->n--;
rep = new Srep(strlen(p), p);
}
return *this;
}

String& String::operator+=(const String &x)
{
Srep *tmp = new Srep(rep->sz + x.rep->sz, rep->s);
strcpy( (tmp->s + rep->sz), x.rep->s);
if(--(rep->n) == 0)
delete rep;
rep = tmp;
return *this;
}

String& String::operator+=(const char *p)
{
Srep *tmp = new Srep( (rep->sz + strlen(p)), rep->s );
strcpy( (tmp->s + rep->sz), p);
if(--(rep->n) == 0)
delete rep;
rep = tmp;
return *this;
}

istream& operator>>(istream &in, String &x)
{
char tmp[100];
in.get(tmp, 100);
if(in)
x = tmp;
in.get(tmp, 100);
while(in)
{
x += tmp;
in.get(tmp, 100);
}
in.clear();
in.sync();
return in;
}

ostream& operator<<(ostream &out, const String &x)
{
if(x.rep->sz > 0)
out<<x.rep->s;
return out;
}

String operator+(const String &x, const String &y)
{
String tmp(x);
return tmp += y;
}

String operator+(const String &x, const char *p)
{
String tmp(x);
return tmp += p;
}

inline String operator+(const char *p, const String &x)
{
return x + p;
}[/CODE]

main.cpp

[CODE]#include <iostream>
#include "String.h"

using namespace std;

String f(String a, String b)
{
a[2] = 'x';
char c = b[3];
cout<<a<<' '<<b<<' '<<c<<endl;
return b;
}


int main(int argc, char *argv[])
{

String x, y;

cout<<"please enter two strings\n";
cin>>x>>y;
cout<<"input : "<<x<<' '<<y<<endl;


String z = x;
y = f(x, y);
if(x != z) cout<<"x corrupted\n";
x[0] = '!';

if(x == z) cout<<"write failed\n";
cout<<"exit : "<<x<<' '<<y<<' '<<z<<endl;
cout<<x+y<<endl;

system("PAUSE");
return EXIT_SUCCESS;
}[/CODE]