safeassign.cpp

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//: C01:SafeAssign.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// An Exception-safe operator=.
#include <iostream>
#include <new>       // For std::bad_alloc
#include <cstring>
#include <cstddef>
using namespace std;

// A class that has two pointer members using the heap
class HasPointers {
  // A Handle class to hold the data
  struct MyData {
    const char* theString;
    const int* theInts;
    size_t numInts;
    MyData(const char* pString, const int* pInts,
      size_t nInts)
    : theString(pString), theInts(pInts), numInts(nInts) {}
  } *theData;  // The handle
  // Clone and cleanup functions:
  static MyData* clone(const char* otherString,
      const int* otherInts, size_t nInts) {
    char* newChars = new char[strlen(otherString)+1];
    int* newInts;
    try {
      newInts = new int[nInts];
    } catch(bad_alloc&) {
      delete [] newChars;
      throw;
    }
    try {
      // This example uses built-in types, so it won't
      // throw, but for class types it could throw, so we
      // use a try block for illustration. (This is the
      // point of the example!)
      strcpy(newChars, otherString);
      for(size_t i = 0; i < nInts; ++i)
        newInts[i] = otherInts[i];
    } catch(...) {
      delete [] newInts;
      delete [] newChars;
      throw;
    }
    return new MyData(newChars, newInts, nInts);
  }
  static MyData* clone(const MyData* otherData) {
    return clone(otherData->theString, otherData->theInts,
                 otherData->numInts);
  }
  static void cleanup(const MyData* theData) {
    delete [] theData->theString;
    delete [] theData->theInts;
    delete theData;
  }
public:
  HasPointers(const char* someString, const int* someInts,
              size_t numInts) {
    theData = clone(someString, someInts, numInts);
  }
  HasPointers(const HasPointers& source) {
    theData = clone(source.theData);
  }
  HasPointers& operator=(const HasPointers& rhs) {
    if(this != &rhs) {
      MyData* newData = clone(rhs.theData->theString,
        rhs.theData->theInts, rhs.theData->numInts);
      cleanup(theData);
      theData = newData;
    }
    return *this;
  }
  ~HasPointers() { cleanup(theData); }
  friend ostream&
  operator<<(ostream& os, const HasPointers& obj) {
    os << obj.theData->theString << ": ";
    for(size_t i = 0; i < obj.theData->numInts; ++i)
      os << obj.theData->theInts[i] << ' ';
    return os;
  }
};

int main() {
  int someNums[] = { 1, 2, 3, 4 };
  size_t someCount = sizeof someNums / sizeof someNums[0];
  int someMoreNums[] = { 5, 6, 7 };
  size_t someMoreCount =
  sizeof someMoreNums / sizeof someMoreNums[0];
  HasPointers h1("Hello", someNums, someCount);
  HasPointers h2("Goodbye", someMoreNums, someMoreCount);
  cout << h1 << endl;  // Hello: 1 2 3 4
  h1 = h2;
  cout << h1 << endl;  // Goodbye: 5 6 7
} ///:~