Thread: heap memory management

I've got a program with 1 struct and 3 functions. Function 1
allocates memory to the structs. Function 2 deallocates the
memory. And function 3 tells you if the memory is allocated or not.

Code:

struct sTest
{
   int i1;
   int i2;
};

sTest *test1, *test2;

void AllocTest(sTest* TestToAlloc);
void DeallocTest(sTest* TestToDealloc);
void TestTest(sTest* TestToTest);

void AllocTest(sTest* TestToAlloc)
{
    if(!TestToAlloc)
     {
        TestToAlloc=new sTest;     
        TestToAlloc->i1=10;
        TestToAlloc->i2=11;
      }
      printf("%d allocated.",TestToAlloc);
}

void DeallocTest(sTest* TestToDealloc)
{
    printf("%d allocated.",TestToAlloc); 
    if(TestToDealloc)
     {
        delete TestToDealloc;
        TestToDealloc=NULL;
     }
}

void TestTest(sTest* TestToTest)
{
    if(TestToTest)
    {
       printf("%d allocated!",TestToTest);
    }
    else
    {
       printf("%d deallocated!",TestToTest);
    }
}

In the app the you input a number 1-7 that is used in a switch to
perform one of the 3 functions on one of the 2 struct pointers.

Code:

bool bExit=false;
int choice;
for(;;)
{
   printf("Enter your choice: " );
   scanf("%d",&choice);
   switch(choice)
   {
    case 1:
     {
        AllocTest(test1);
        system("cls");
        getch();
      }break;
    case 2:
      {
        AllocTest(test2);
        //same system("cls") and getch() lines for 1-6, left out for
       //brevity
       }break;
     case 3:
      {
        DeallocTest(test1);
      }break;
      case 4:
       {
         DeallocTest(test2);
       }break;
      case 5:
       {
         TestTest(test1);
       }break;
      case 6:
      {
         TestTest(test2);
      }break;
      case 7:
      {
         bExit=true;
         DeallocTest(test1);
         DeallocTest(test2);
      }break;
   }
   if(bExit) break;   
}
return(0);

Problems:
First off, I really need a good tutorial on heap management

When you choose either 1 or 2 it returns different #s every time.
As I understand what I'm returning here is a long that represents
a pointer to that structure in the heap memory. So if I choose 1
I get (for instance) 7867904. Then if I choose 1 again (without
choosing to deallocate it first) I get 7867840.

Also if I run AllocTest either test# and then test it with TestTest I
always get a return that it's deallocated (when I never chose to
deallocate it).

I'm pretty sure that it all boils down to the fact that I am really
lost when it comes to heap memory management. Does anyone
know of any good articles/tutorials on the subject? As I mentioned in an earlier post all that I could find in MSDN were
functions/structures that said they were unsupported in Win98.

Thanks in advance for any help.

I don't think your problem is with heap management.
Rather pointer management seems to be the problem.

You declare these global pointers without initializing them to null:

>>sTest *test1, *test2;

You can think of them as wild pointers, no one knows where they point too.

Next, you pass a wild pointer to a function and test it:

>>if(!TestToAlloc)

Since we don't know what the pointer is pointing at how can we trust the logic of the above statement?

I think you will benefit by reading up on pointers and scope rules.

Let me know what you think

Ted,
Definitely. When I first started learning C++ I skimmed the basics
on pointers, but never went deep enough into them (ie: dynamic
allocation, deallocating them, etc.). I'm kind of playing catch-up
now.

I found the tutorial listed in the FAQ (the tripod URL one) and I'm
in the middle of it's 20 or so pages on pointers right now.
Definitely helping to clear a lot of stuff up.

Thanks.

You can't change where a passed pointer points to in a function. You can only change the data it points to. To change a pointer within a function (for example allocate memory for it), use a double pointer.

Code:

#include <iostream>
#include <conio.h>
using namespace std;

struct sTest
{
   int i1;
   int i2;
};

sTest *test1 = NULL;
sTest *test2 = NULL;

void AllocTest(sTest** TestToAlloc);
void DeallocTest(sTest** TestToDealloc);
void TestTest(sTest** TestToTest);

void AllocTest(sTest** TestToAlloc)
{
    if(!*TestToAlloc)
     {
        *TestToAlloc=new sTest;     
        (*TestToAlloc)->i1=10;
        (*TestToAlloc)->i2=11;
      }
      printf("%d allocated.\n",*TestToAlloc);
}

void DeallocTest(sTest** TestToDealloc)
{
    printf("%d allocated.\n",*TestToDealloc); 
    if(*TestToDealloc)
     {
        delete *TestToDealloc;
        *TestToDealloc=NULL;
     }
}

void TestTest(sTest* TestToTest)
{
    if(TestToTest)
    {
       printf("%d allocated!\n",TestToTest);
    }
    else
    {
       printf("%d deallocated!\n",TestToTest);
    }
}

int main(void)
{
bool bExit=false;
int choice;
for(;;)
{
   printf("Enter your choice: " );
   scanf("%d",&choice);
   switch(choice)
   {
    case 1:
     {
        AllocTest(&test1);
        system("cls");
        getch();
      }break;
    case 2:
      {
        AllocTest(&test2);
        //same system("cls") and getch() lines for 1-6, left out for
       //brevity
       }break;
     case 3:
      {
        DeallocTest(&test1);
      }break;
      case 4:
       {
         DeallocTest(&test2);
       }break;
      case 5:
       {
         TestTest(test1);
       }break;
      case 6:
      {
         TestTest(test2);
      }break;
      case 7:
      {
         bExit=true;
         DeallocTest(&test1);
         DeallocTest(&test2);
      }break;
   }
   if(bExit) break;   
}
return(0);
}

Here's a clearer way to do it.

Code:

#include <iostream>
#include <conio.h>
using namespace std;

struct sTest
{
   int i1;
   int i2;
};

sTest *test1 = NULL;
sTest *test2 = NULL;

void AllocTest(sTest* &TestToAlloc);
void DeallocTest(sTest* &TestToDealloc);
void TestTest(sTest* TestToTest);

void AllocTest(sTest* &TestToAlloc)
{
    if(!TestToAlloc)
     {
        TestToAlloc=new sTest;     
        TestToAlloc->i1=10;
        TestToAlloc->i2=11;
      }
      printf("%d allocated.\n",TestToAlloc);
}

void DeallocTest(sTest* &TestToDealloc)
{
    printf("%d allocated.\n",TestToDealloc); 
    if(TestToDealloc)
     {
        delete TestToDealloc;
        TestToDealloc=NULL;
     }
}

void TestTest(sTest* TestToTest)
{
    if(TestToTest)
    {
       printf("%d allocated!\n",TestToTest);
    }
    else
    {
       printf("%d deallocated!\n",TestToTest);
    }
}

int main(void)
{
bool bExit=false;
int choice;
for(;;)
{
   printf("Enter your choice: " );
   scanf("%d",&choice);
   switch(choice)
   {
    case 1:
     {
        AllocTest(test1);
        system("cls");
        getch();
      }break;
    case 2:
      {
        AllocTest(test2);
        //same system("cls") and getch() lines for 1-6, left out for
       //brevity
       }break;
     case 3:
      {
        DeallocTest(test1);
      }break;
      case 4:
       {
         DeallocTest(test2);
       }break;
      case 5:
       {
         TestTest(test1);
       }break;
      case 6:
      {
         TestTest(test2);
      }break;
      case 7:
      {
         bExit=true;
         DeallocTest(test1);
         DeallocTest(test2);
      }break;
   }
   if(bExit) break;   
}
return(0);
}