Error in deleting a dynamic object? - c++

Question

I am allocating a memory for union in heap and i need to delete the object of union when the element Id of union is 900.
Please help me to delete groupUnion[i] object when the Id is 900
below is my code.
groupUnion = (SettingsUnion *) malloc(sizeof(SettingsUnion) * (NumAttrs + 1));
if(groupUnion == (SettingsUnion *) NULL)
{
return (FALSE);
}
for (unsigned int i=0; i < NumAttrs; i++)
{
inFile.read(reinterpret_cast<char*>(&groupUnion[i]),sizeof(SettingsUnion));
if(groupUnion[i].Id == 900)
{
free groupUnion[i]; // this is bad how can i delete groupUnion[i] when groupUnion[i].Id == 900
groupUnion[i] = NULL;
}
}
inFile.close()
Thanks in Advance!!

Answer 1

Your code fragment free groupUnion[i];groupUnion[i] = NULL lets me assume that you actually want to express an array of pointers to SettingUnion-objects rather than an array of SettingUnion-objects.
So your code could look like as follows (I used your malloc/free-style, though in C++ you actually would use new/delete):
groupUnion = (SettingsUnion **) malloc(sizeof(SettingsUnion*) * (NumAttrs + 1));
if(groupUnion == NULL)
{
return (FALSE);
}
for (unsigned int i=0; i < NumAttrs; i++)
{
groupUnion[i] = malloc(sizeof(SettingsUnion));
inFile.read(reinterpret_cast<char*>(&groupUnion[i]),sizeof(SettingsUnion));
if(groupUnion[i]->Id == 900)
{
free groupUnion[i];
groupUnion[i] = NULL;
}
}
inFile.close()

Answer 2

You cannot free part of the allocated memory: free groupUnion[i]
What you can do however is to allocate individually the elements and then free them individually:
// not sure why you need the +1 (anyway you allocate an array of pointers to the struct here. Consider using new operator)
groupUnion = (SettingsUnion **) malloc(sizeof(SettingsUnion *) * (NumAttrs + 1));
if(groupUnion == (SettingsUnion *) NULL)
{
return (FALSE);
}
for (unsigned int i=0; i < NumAttrs; i++)
{
// you allocate the individual groupUnion here:
groupUnion[i] = (SettingsUnion *) malloc(sizeof(SettingsUnion));
if(groupUnion[i] == (SettingsUnion *) NULL)
{
return (FALSE);
}
inFile.read(reinterpret_cast<char*>(&groupUnion[i]),sizeof(SettingsUnion));
if(groupUnion[i].Id == 900)
{
free groupUnion[i]; // this is bad how can i delete groupUnion[i] when groupUnion[i].Id == 900
groupUnion[i] = NULL;
}
}
inFile.close()