Should we use delete[] or delete on function that return allocated memory pointer?

Question

Should i use delete[] or delete for Register() example below? We hit some memory leak issue and legacy code which is not allow to change the return type and input of the function. I understand that new[] should use delete[]. new should use delete. However for case below, which method is correct?

UINT Register(UINT regaddr, usb_dev_handle* current_handle)
{
    unsigned int val= 0;
    unsigned char* buffer = readregister(current_handle, char((regaddr >> 8) & 0x00FF), char(regaddr & 0x00FF));
    raddrvaluefinal = buffer[2] << 8 | buffer[3];

    delete buffer;
    return val;
}

unsigned char* readregister(usb_dev_handle *current_handle, char one, char two)
{
    unsigned char *buffer = new unsigned char[4];
    char cmd[2];
    cmd[0] = 'D';
    cmd[1] = 'B';
    int rv = 0;
    unsigned char bin_size[2];
    buffer[0] = one + 0x80;
    buffer[1] = two;
    buffer[2] = buffer[3] = 0;
    usb_bulk_write(current_handle, 0x02, cmd, 2, 500);
    bin_size[0] = (5 / 4) & 0x0FF;
    bin_size[1] = ((5 / 4) >> 8) & 0x0FF;
    rv = usb_bulk_write(current_handle, 0x01, (char*)bin_size, 2, 2000);
    unsigned int byteswrite += usb_bulk_write(current_handle, 0x04, (char*)buffer, 4, 2000);
    unsigned int bytesread += usb_bulk_read(current_handle, 0x86, (char*)buffer, 4, 2000);
    buffer[0] = one;
    buffer[1] = two;
    return buffer;
}

Answer 1

I understand that new[] should use delete[]. new should use delete.

unsigned char *buffer = new unsigned char[4];
...
delete buffer;

The program is wrong, and against your understanding. When a dynamic array is allocated dynamically with new[], delete[] must be used to deallocate it. If you use delete, then the behaviour of the program is undefined.

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You shouldn't use owning bare pointers in the first place, and in this case there doesn't even seem to be any need to use a dynamic array. I recommend following instead:

std::array<unsigned char, 4>
readregister(usb_dev_handle *current_handle, char one, char two)
{
    std::array<unsigned char, 4> buffer;
    // ...
    usb_bulk_write(current_handle, 0x04, reinterpret_cast<char*>(buffer.data()), 4, 2000);

---

Sidenote: "new -> delete", "new[] -> delete[]" is a slight oversimplification. delete[] must be used if new is used to create an array:

using T = int[4];
int* ptr = new T;
delete[] ptr; // correct