memcpy into a struct from char array - c++

Question

I have a char buffer[1024] and a struct
typedef struct {
int record;
int key;
} leaf_entry;
What I am trying to implement is the buffer array acting like a node of a tree that holds many entries of leaf_entry.
If I want to iterate down the buffer to compare an entry in the buffer to another entry
for (i = 0; i < max_ents * entry_size; i += entry_size)
{
leaf_entry curr;
memcpy (curr, buffer[i], entry_size)
if (curr == entry_to_compare)
etc...
}
Is this correct? Is there an easier/more efficient way to accomplish this?

Answer 1

You should be able to do this without copying given that your struct is POD.
I believe something like this should work where offset is the correct byte offset into the char buffer:
leaf_entry & x = static_cast<leaf_entry &>(buffer + offset);

Answer 2

I would consider using the approach that standard sorting implementations use - namely, use a user-defined compare function that receives 2 pointers. You then use these pointers to locate the element and from there, access and compare the members you're interested in. This also avoids the unnecessary memory copying.
Consider the following func used to compare ints:
int compareIntAsc(const void *int1, const void *int2)
{
int *num1 = (int*)int1;
int *num2 = (int*)int2;
return *num1 - *num2;
}
and now consider one which will compare the count member of some structs
int compareNodeCountAsc(const void *node1, const void *node1)
{
return (pHuffmanNode)(node1))->charCount - ((pHuffmanNode)(node2))->charCount;
}
If you pass the address of two elements in your array to such a function, you can compare either the record or the key.
e.g compare element 0 to every other element
code
typedef struct {
int record;
int key;
} leaf_entry, *pLeafEntry;
int compLeafKeyAsc(void *leaf1, void *leaf2)
{
leaf_entry *p1, *p2;
p1 = (leaf_entry *)leaf1;
p2 = (leaf_entry *)leaf2;
return p1->key - p2->key;
}
void printLeaf(pLeafEntry leaf)
{
printf("----- leaf -------\n");
printf("record: %d\n", leaf->record);
printf("key: %d\n", leaf->key);
}
void demo()
{
const int nElems = 16;
leaf_entry leafArray[nElems];
pLeafEntry firstElement;
int i;
for (i=0;i<nElems;i++)
{
leafArray[i].record = (rand()%51) + 100; // record is [100..150]
leafArray[i].key = (rand()%128); // key is [0..127]
printLeaf(&leafArray[i]);
}
//e.g compare element 0 to every other element
firstElement = &leafArray[0];
for (i=1; i<nElems; i++)
{
printf("%d", firstElement->key );
int result = compLeafKeyAsc(firstElement, &leafArray[i]);
if (result < 0)
printf(" is less than ");
else if (result > 0)
printf(" is greater than ");
else
printf(" is equal to ");
printf("%d\n", leafArray[i].key);
}
}
output
----- leaf -------
record: 141
key: 35
----- leaf -------
record: 110
key: 4
----- leaf -------
record: 144
key: 108
----- leaf -------
record: 103
key: 46
----- leaf -------
record: 134
key: 16
----- leaf -------
record: 144
key: 113
----- leaf -------
record: 125
key: 59
----- leaf -------
record: 116
key: 107
----- leaf -------
record: 137
key: 38
----- leaf -------
record: 133
key: 60
----- leaf -------
record: 106
key: 12
----- leaf -------
record: 126
key: 25
----- leaf -------
record: 137
key: 94
----- leaf -------
record: 130
key: 28
----- leaf -------
record: 132
key: 55
----- leaf -------
record: 141
key: 94
35 is greater than 4
35 is less than 108
35 is less than 46
35 is greater than 16
35 is less than 113
35 is less than 59
35 is less than 107
35 is less than 38
35 is less than 60
35 is greater than 12
35 is greater than 25
35 is less than 94
35 is greater than 28
35 is less than 55
35 is less than 94