Linxutopia - Thinking in C++ - 5: Hiding the Implementation

Thinking in C++
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Nested friends

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Making a structure nested doesn’t automatically give it access to private members. To accomplish this, you must follow a particular form: first, declare (without defining) the nested structure, then declare it as a friend, and finally define the structure. The structure definition must be separate from the friend declaration, otherwise it would be seen by the compiler as a non-member. Here’s an example:

//: C05:NestFriend.cpp
// Nested friends
#include <iostream>
#include <cstring> // memset()
using namespace std;
const int sz = 20;

struct Holder {
private:
  int a[sz];
public:
  void initialize();
  struct Pointer;
  friend struct Pointer;
  struct Pointer {
  private:
    Holder* h;
    int* p;
  public:
    void initialize(Holder* h);
    // Move around in the array:
    void next();
    void previous();
    void top();
    void end();
    // Access values:
    int read();
    void set(int i);
  };
};

void Holder::initialize() {
  memset(a, 0, sz * sizeof(int));
}

void Holder::Pointer::initialize(Holder* rv) {
  h = rv;
  p = rv->a;
}

void Holder::Pointer::next() {
  if(p < &(h->a[sz - 1])) p++;
}

void Holder::Pointer::previous() {
  if(p > &(h->a[0])) p--;
}

void Holder::Pointer::top() {
  p = &(h->a[0]);
}

void Holder::Pointer::end() {
  p = &(h->a[sz - 1]);
}

int Holder::Pointer::read() {
  return *p;
}

void Holder::Pointer::set(int i) {
  *p = i;
}

int main() {
  Holder h;
  Holder::Pointer hp, hp2;
  int i;

  h.initialize();
  hp.initialize(&h);
  hp2.initialize(&h);
  for(i = 0; i < sz; i++) {
    hp.set(i);
    hp.next();
  }
  hp.top();
  hp2.end();
  for(i = 0; i < sz; i++) {
    cout << "hp = " << hp.read()
         << ", hp2 = " << hp2.read() << endl;
    hp.next();
    hp2.previous();
  }
} ///:~

Once Pointer is declared, it is granted access to the private members of Holder by saying:

friend struct Pointer;

The struct Holder contains an array of ints and the Pointer allows you to access them. Because Pointer is strongly associated with Holder, it’s sensible to make it a member structure of Holder. But because Pointer is a separate class from Holder, you can make more than one of them in main( ) and use them to select different parts of the array. Pointer is a structure instead of a raw C pointer, so you can guarantee that it will always safely point inside the Holder.

The Standard C library function memset( ) (in <cstring>) is used for convenience in the program above. It sets all memory starting at a particular address (the first argument) to a particular value (the second argument) for n bytes past the starting address (n is the third argument). Of course, you could have simply used a loop to iterate through all the memory, but memset( ) is available, well-tested (so it’s less likely you’ll introduce an error), and probably more efficient than if you coded it by hand.

Thinking in C++
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