Such a thing as C++ design pattern to avoid pointers? - c++

Question

I have a class hierarchy as shown in the example below, where a State contains a list of ZipCodes and a list of Citys, each of which contain pointers to the ZipCodes.
The goal is to be able to update the ZipCodes without needing to update Citys (or to create new instances of City).
The C++ code below meets this requirement, but it uses pointers, which I prefer to avoid because of this and that. How can I re-design this [naive] implementation so that it doesn't rely on pointers? Thanks for any help!
EDIT: Updated code below to use boost::shared_ptr instead of raw pointers. Note that State, City, and ZipCode are just example names, and they turned out to be poor choice names (I could've picked "A", "B", and "C") because the actual code allows the equivalent of City to share ZipCodes.
#include <iostream>
#include <vector>
#include <boost/shared_ptr.hpp>
using namespace std;
/**
* Zone Improvement Plan (ZIP) code
*/
class ZipCode {
public:
ZipCode() : code_(0), plus4_(0) {}
ZipCode(int code, int plus4 = 0) : code_(code), plus4_(plus4) {}
virtual ~ZipCode() {};
int code() const { return code_; }
int plus4() const { return plus4_; }
void set_code(int code) { code_ = code; }
void set_plus4(int plus4) { plus4_ = plus4; }
private:
int code_;
int plus4_;
};
typedef boost::shared_ptr<ZipCode> ZipPtr;
/**
* City points to one or more zip codes
*/
class City {
public:
const vector<ZipPtr>& zip() const { return zip_; }
void add_zip_ptr(const ZipPtr x) { if (x != NULL) zip_.push_back(x); }
private:
// TODO: this vector should be a hash set
vector<ZipPtr> zip_;
};
/**
* State contains cities, each of which has pointers to
* zip codes within the state.
*/
class State {
public:
const vector<City>& city() const { return city_; }
const vector<ZipPtr>& zip() const { return zip_; }
const ZipPtr zip_of(int code) const {
for (size_t i = 0; i < zip_.size(); i++) {
if (zip_[i]->code() == code) {
return zip_[i];
}
}
return ZipPtr();
}
void add_city(const City& x) { city_.push_back(x); }
void add_zip(int code) { zip_.push_back(ZipPtr(new ZipCode(code))); }
private:
// TODO: these vectors should be hash sets
vector<City> city_;
vector<ZipPtr> zip_;
};
int main() {
State texas;
City dallas, houston;
// create state ZIPs
texas.add_zip(75380);
texas.add_zip(75381);
texas.add_zip(77219);
texas.add_zip(77220);
// point city ZIPs to the ones we just created
dallas.add_zip_ptr(texas.zip_of(75380));
dallas.add_zip_ptr(texas.zip_of(75381));
houston.add_zip_ptr(texas.zip_of(77219));
houston.add_zip_ptr(texas.zip_of(77220));
// print all ZIPs
cout << "ZIPs in Texas: " << endl;
const vector<ZipPtr>& zips = texas.zip();
for (size_t i = 0; i < zips.size(); i++) {
cout << " " << zips[i]->code() << endl;
}
cout << "ZIPs in Dallas, Texas: " << endl;
const vector<ZipPtr> zip_ptrs1 = dallas.zip();
for (size_t i = 0; i < zip_ptrs1.size(); i++) {
cout << " " << zip_ptrs1[i]->code() << endl;
}
cout << "ZIPs in Houston, Texas: " << endl;
const vector<ZipPtr> zip_ptrs2 = houston.zip();
for (size_t i = 0; i < zip_ptrs2.size(); i++) {
cout << " " << zip_ptrs2[i]->code() << endl;
}
// change a state ZIP...
cout << "Changing Houston's ZIP 77220..." << endl;
ZipPtr z = texas.zip_of(77220);
if (z != NULL) z->set_code(88888);
// ...and show the ZIPs of the affected city
cout << "ZIPs in Houston, Texas: " << endl;
const vector<ZipPtr> zip_ptrs3 = houston.zip();
for (size_t i = 0; i < zip_ptrs3.size(); i++) {
cout << " " << zip_ptrs3[i]->code() << endl;
}
return 0;
}

Answer 1

I see the situation as two 1:n relationships
State : City == 1 : n
City : Zipcode
== 1 : n
Based on that, I think that the State containing
vector<ZipCode> zip_;
is not sound.
I might do
class State {
vector< City > cities_in_state_;
};
class City {
vector< Zipcode > zips_in_city_;
};
This does not require pointers.

Answer 2

Unless you want to duplicate your ZipCode objects, you fall into this category of usage (described in your first link):
The Bar instance is actually managed
by some other part of your program,
whereas the Foo class just needs to be
able to access it.
It seems like a legit use.
However, you might want to consider the copy option (to permanently avoid problems if the vector has to reallocate its data) or make State aggregate ZipCodes from its Cities instead of distributing them ZipCodes.
The copy simply implies that you stop using pointers in City. Aggregating the ZipCodes means that instead of giving State a list of ZipCodes, you would give City the list of ZipCode instances, and when calling zip_of, you would iterate through the cities and iterate through their ZipCode collection.