Re: Is it safe in D to cast pointers to structures like this?

[Ola Fosheim Grøstad via Digitalmars-d-learn]

On Tuesday, 14 January 2020 at 12:05:01 UTC, John Burton wrote:
After years of C++ I have become paranoid about any casting of 
pointers being undefined behavior due to aliasing so want to 
see if :-


FWIW, this is safe and portable in C++20:

https://en.cppreference.com/w/cpp/numeric/bit_cast



1) This is safe to do in D.
2) If not is there anything I can do to make it safe.
3) If not, what is the best approach?


What is legal in C, should in theory be legal in D unless the 
documentation states otherwise as the goal for D is to make 
porting C code to D easy. Hence, type punning through union 
should be ok.


I am not sure what the various D compilers do with aliasing, but 
allowing type punning through pointers can inhibit some 
optimizations.

Is it safe in D to cast pointers to structures like this?

[John Burton via Digitalmars-d-learn]

After years of C++ I have become paranoid about any casting of 
pointers being undefined behavior due to aliasing so want to see 
if :-


1) This is safe to do in D.
2) If not is there anything I can do to make it safe.
3) If not, what is the best approach?

I have a void* pointing to a block of allocated memory. In that 
memory I have a header struct at the start, and some of the 
members of that struct are offsets into the memory of other 
structs.


Can I do this? It appears to compile and "work" in dmd 64 bit but 
I need to make sure it's valid and reliable code. (This is a 
minimal example without any error checking etc)


import std.stdio;

//
// getMemory is just an example to make this compile...
//
void* getMemory()
{
static byte[100] someData;
// Something fills in the data here
}

struct Header
{
ulong data1;
ulong data2;
}

struct Data1
{
int a;
}

struct Data2
{
int b;
float[10] d;
}

void main()
{
void* memory = getMemory();
auto header = cast(Header*)memory;
auto data1 = cast(Data1*)(memory + header.data1);
auto data2 = cast(Data2*)(memory + header.data2);

writeln(data1.a, " ", data2.b);

}