# changing from malloc to new in C++ [closed]

So in my quest to make my AES128 code better and not to have memory leaks and the sort I came across something. Someone said "Unless you are forced to use C, you should never use malloc"

So I thought to myself, "Self, I will get rid of my malloc's". And in my Encrypt/Decrypt method I found this. Now i'm told that I should always finish with a delete[] on my byte array.. and I don't see how I would do it... maybe my methodology is wrong

here is how I call the method.

bool DESFireCard::AuthenticateOffline()
{
AES128 aes;
BYTE* RandB;
BYTE buffer[33] = {0x00};
BYTE lenRec = 0;
BYTE IV[16] = {0x00};
BYTE divkey[16];

ByteUtil::StringToHex(key.DIV, divkey);
aes.SetIV(IV);
aes.SetKey(divkey);
int state = rf_ISO14443_4_COS(APDU,2, buffer, &lenRec,2000);

aes.DecryptData(&RandB, &buffer[1], 16);
// LOTS more code HERE
delete[] RandB;
delete[] buffer; // <--crashes here
delete[] IV;
delete[] divkey;

return true;
}


then my decrypt method (which is almost identical to the encrypt method)

void AES128::DecryptData(BYTE** outBlock, const BYTE* inBlock, size_t length)
{
float blockSize = (float)(length/16);
blockSize = ceilf(blockSize);
size_t newLength = (size_t)(blockSize*16);
BYTE* temp = new BYTE[newLength];//(BYTE*)malloc(newLength);
memset(temp, 0, newLength);
for (int i=1; i<blockSize; i++)
{
}

*outBlock = new BYTE[newLength]; //(BYTE*)malloc(newLength);
memcpy((*outBlock), temp, newLength);

delete[] temp;
}
void AES128::DecryptBlock(BYTE* outBlock, const BYTE* inBlock, const BYTE* cipherBlock)
{
BYTE temp[16] = {0x00};
AES::XorBlock(temp, inBlock);

BYTE expandedKey[176] = {0x00};
BYTE* invExpandedKey;
memcpy(expandedKey, Key, 16);
Galois::expand_key(expandedKey);
Galois::InvertExpandedKey(&invExpandedKey, expandedKey);
AES::XorBlock(temp, invExpandedKey);
AES::InvShiftRows(temp);
AES::InvSubBytes(temp);
for(int i=0x10; i<0xA0; i+=0x10) //change i<0xA0
{
AES::InvDoRound(temp, &invExpandedKey[i]);
}
AES::XorBlock(temp, &invExpandedKey[160]);
AES::XorBlock(temp, cipherBlock);
memcpy(outBlock, temp, 16);
}


so would I put a delete method in my AES128 class (see edit)? or do I put it at the end of me using it in the one method?

or should I put in a "outbuffer" in AES128 then in my destructor delete my outbuffer?

Sorry for a seemingyl newb question but C++ is not my strength (i hope it will be one day though :) )

EDIT1 ok I put in a few deletes and my program crashes saying something AccessViolationException

grar!! i hate that exception. I found out where it crashes, but i don't know why it would crash. Any tips on that? do i need to use delete on pre-allocated memories?

-

## closed as off topic by Corbin, svick, James Khoury, Brian Reichle, Glenn RogersFeb 5 '13 at 10:17

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Your crash is due to trying to delete[] an array allocated on the stack. –  Richard J. Ross III Feb 4 '13 at 16:33
@RichardJ.RossIII I found that if I change all my arrays to BYTE* = new BYTE[size] that the problem goes away. Was that the correct move? –  Robert Snyder Feb 4 '13 at 16:37
The rule is: use new[] to allocate and delete[] to deallocate. You can't deallocate a fixed-size array. You can only use delete[] to deallocate memory allocated with new[] as in your previous comment :) –  Morwenn Feb 4 '13 at 21:13

Note:

x = new X;             =>  delete x;
// or
x = new X[Size];       =>  delete [] x;
// or
x = (X)malloc(exp);    =>  free(x);


These are you three ways to allocate memory.
Each has a specific way to be released. The de-allocation MUST match the allocation.

Variables that are not allocated dynamcially should not be deleted (ie if you don't use new then don't call delete/free on them).

char   Buffer[100] = "Stuff"; // Not dynamically allocated
// No call to new.
// Therefore no call to delete


So:

These deletes are broken:

delete[] buffer;
delete[] IV;
delete[] divkey;


All of these buffers are declared locally.

// No new so these should not be deleted.
BYTE buffer[33] = {0x00};
BYTE IV[16] = {0x00};
BYTE divkey[16];


The othere delete here is done correctly.
But the design is bad for C++

delete[] RandB;  // This is OK as you allocated this with new
// though you hide the fact. and don't make the ownership
// very noticable.


The trouble here is the code is not exception safe (probably not a big deal for this code). But you should learn to write your code in a way that is exception safe for future C++.

A better idea would have been to use a dynamically sizeable container:

std::vector<BYTE>    RandB;

// Then change the function header:
// Notice we are passing by reference
void AES128::DecryptData(std::vector<BYTE>& outBlock, const BYTE* inBlock, size_t length)

// Then you can resize it when you know the size:
outBlock.resize(newLength);        //(BYTE*)malloc(newLength);
memcpy(&outBlock[0], temp, newLength);


Notice that container contains the new/delete inside its constructor/destructor so they will always be called (even if there is an exception).

Similarly:

BYTE* temp = new BYTE[newLength];//(BYTE*)malloc(newLength);

// should replace with:
std::vector<BYTE> temp(newLength);