Should I worry about the alignment during pointer casting?

Question

In my project we have a piece of code like this:

// raw data consists of 4 ints
unsigned char data[16];
int i1, i2, i3, i4;
i1 = *((int*)data);
i2 = *((int*         


        

Answer 1

The correct way to unpack char buffered data is to use memcpy:

unsigned char data[4 * sizeof(int)];
int i1, i2, i3, i4;
memcpy(&i1, data, sizeof(int));
memcpy(&i2, data + sizeof(int), sizeof(int));
memcpy(&i3, data + 2 * sizeof(int), sizeof(int));
memcpy(&i4, data + 3 * sizeof(int), sizeof(int));

Casting violates aliasing, which means that the compiler and optimiser are free to treat the source object as uninitialised.

Regarding your 3 questions:

1. No, dereferencing a cast pointer is in general unsafe, because of aliasing and alignment.
2. No, in C++, C-style casting is defined in terms of reinterpret_cast.
3. No, C and C++ agree on cast-based aliasing. There is a difference in the treatment of union-based aliasing (C allows it in some cases; C++ does not).