The Pier Shareware 6/The_Pier_Shareware_Number_6_(The_Pier_Exchange)_(1995).iso/038/cppq1194.exe/EX.CPP

C/C++ Source or Header | 1994-11-01 | 6KB | 238 lines

////////////////////////////////////////////////////////////////
// EX.CPP
// Written by Paul DiLascia for Microsoft Systems Journal 1994
//
// This program illustrates how to avoid losing memory when exceptions
// are thrown. It comes in two flavors: the "GOOD" and "bad" versions.
// To compile the bad version, type:            bcc ex.cpp
// To compile the good version, type:            bcc -DGOOD ex.cpp
// (requires Borland 4.0 C++ compiler)
//

#include <malloc.h>
#include <stdio.h>
#include <assert.h>

//////////////////
// Primitive memory tracking
//
const MAXALLOCPTRS = 50;                        // size of alloc tracking array
static void*    AllocPtrs[MAXALLOCPTRS];    // alloc tracking array
static size_t    AllocSizes[MAXALLOCPTRS];    // size of tracking array
static int        AllocCount = 0;                // total num allocations

//////////////////
// Override global new operator to keep track of allocations.
//
void* operator new(size_t size)
{
    void *p = calloc(size, 1);
    assert(p);

    // Store pointer in tracking array if there's room
    for (int i=0; i<MAXALLOCPTRS; i++) {
        if (AllocPtrs[i]==NULL) {
            AllocPtrs[i] = p;
            AllocSizes[i] = size;
            break;
        }
    }
    AllocCount++;                // increment total count
    return p;
}

//////////////////
// Override global delete operator to keep track of allocations.
//
void operator delete(void * p)
{
    free(p);

    // If this is one of the allocations tracked,
    // remove it from tracking array.
    //
    for (int i=0; i<MAXALLOCPTRS; i++) {
        if (AllocPtrs[i]==p) {
            AllocPtrs[i] = NULL;
            break;
        }
    }
    AllocCount--;                // decrement total allocations extant
}

////////////////
// Base class mother-of-all-objects.
// Destructor must be virtual for "Deleter" class to work
//
class Object {
public:
    virtual ~Object() { }        // virtual destructor
};

//////////////////
// Some other object derived from base class Object.
// This example allocates its own array from the free store.
//
class CharArray : public Object {
    char* array;                    // allocated by constructor
    enum { arraysize=512 };    // array size in bytes
public:
    CharArray()        { array = new char [arraysize]; }
    ~CharArray()    { delete [] array; }
};

//////////////////
// Divide by zero exception class, thrown by Divide function.
//
class DivideByZero : public Object {
public:
    enum ErrType { DivByZero=1, ZeroOverZero } errType;
    DivideByZero(ErrType e) : errType(e) { }
};

#ifdef GOOD

//////////////////
// "Deleter" class is used in the "good" version of the program to
// delete objects that might otherwise be left as garbage when an
// exception is thrown.
//
// This class requires a constructor for each different kind of object
// that might be deleted. Since Object has a virtual destructor,
// only one Deleter constrcutor is required for all Object-derived classes.
//
class Deleter {
    void* ptr;                                    // object to be deleted
    enum { tvoid, tobject } iObjType;    // type
public:
    Deleter(void *pv);                        // "raw" memory
    Deleter(Object* pobj);                    // object-derived classes
//    Deleter(Other* poth);                    // add for other classes used...
    ~Deleter();
};

//////////////////
// Deleter constructors are straightforward
//
Deleter::Deleter(void *pv) : ptr(pv), iObjType(tvoid)
{
}
Deleter::Deleter(Object* pobj) : ptr(pobj), iObjType(tobject)
{
}

//////////////////
// Destructor destroys object "tracked" based on type.
//
Deleter::~Deleter()
{
    if (iObjType==tvoid)
        delete ptr;                // delete as void*
    else if (iObjType==tobject)
        delete (Object*)ptr;    // delete as Object*
    else {
        assert(0);
    }
}
#endif

//////////////////
// Low-level divide function may throw an exception.
//
float Divide(float numerator, float denominator)
{
    if (denominator==0) {    // Oops!
        if (numerator==0)
            throw DivideByZero(DivideByZero::ZeroOverZero);
        else
            throw DivideByZero(DivideByZero::DivByZero);
    }
    return numerator/denominator;
}

//////////////////
// DoDivide divides two numbers.
// It acts as the "intermediary" function that allocates storage
// which may be lost when an exception is thrown across the great divide
// (pardon the pun).
//
float DoDivide(float numerator, float denominator)
{
    // The following objects never get deleted in the bad version,
    // if Divide throws an exception:
    //
    char* p            = new char[40];        // ordinary character array
    CharArray *po    = new CharArray;        // object created in free store

    // The following object is destroyed even in the bad version,
    // because it's created on the stack.
    //
    CharArray o;

#ifdef GOOD
    // "Good" version of program uses Deleter class to
    // ensure memory objects are deleted
    //
    Deleter d1(p);                // delete character array p
    Deleter d2(po);            // delete object pointer po
#endif

    // Do the divide:
    float result = Divide(numerator, denominator);

#ifndef GOOD
    // Bad version of program uses delete to destroy allocations--But
    // these lines are never reached if Divide throws an exception!
    //
    delete [] p;                // may never get deleted
    delete po;                    // ditto
#endif

    return result;
}

//////////////////
// Some numbers to divide
//
const NPAIRS=5;
float DivPairs[NPAIRS][2] =
{
    { 10, 2 }, { 3.14159, 2 }, { 10, 0 }, { 0,  0 }, { 2.71828, 2.71828 }
};

//////////////////
// Main program loop:
// Display results of dividing above pairs.
//
int main()
{
    assert(AllocCount==0);    // should have no allocations to start

    for (int i=0; i<NPAIRS; i++) {
        float numerator   = DivPairs[i][0];
        float denominator = DivPairs[i][1];
        printf("%f \t/ %f \t= ", numerator, denominator);

        try {
            float result = DoDivide(numerator, denominator);
            printf("%f\n", result);

        } catch (DivideByZero dbz) {
            printf("DivideByZero(%d)\n", dbz.errType);
        }
    }

    // Display nasty message of any garbage is left
    //
    if (AllocCount!=0) {
        printf("\nGARBAGE LEFT! [%d objects]:\n", AllocCount);

        for (int i=0; i<MAXALLOCPTRS; i++) {
            if (AllocPtrs[i]!=NULL)
                printf(" Array %p [%d]\n", AllocPtrs[i], AllocSizes[i]);
        }
    }

    return 0;
}