Correct cast syntax

Question

I have code like this:

const quint8* data[2];    
const float *p0 = (float*)data[0]

In QtCreator, I am getting a warning:

"use of old-style cast".

I tried to write it like this:

const float *p0 = const_cast<float*>(data[0])

But I'm getting another error that between the types cannot be casted.

What should be the right syntax?

Related, maybe a duplicate: https://stackoverflow.com/q/332030/1896169 — Justin, Aug 16 '18 at 16:50
`reinterpret_cast` is for this, but why??? And why are you assuming `sizeof(float)` is `2 * sizeof(quint8)`? Just by the name, I expect `sizeof(data)` to be 2 and `sizeof(float)` 4. — LogicStuff, Aug 16 '18 at 16:51
@LogicStuff - he isn't. `data` is a pair of pointers. He is assuming the first is a pointer to float. — Martin Bonner supports Monica, Aug 16 '18 at 17:08

bolov · Answer 1 · 2018-08-16T17:16:25.013

Ok so this is the answer:

const float *p0 = reinterpret_cast<const float*>(data[0]);

BUT be extremely careful of the C++ strict aliasing rules. Their implication for your example is that you can legally access via p0 if and only if data[0] points to a float object. For instance

This is legal

const float f = 24.11f;

const quint8* data[2] {};

data[0] = reinterpret_cast<const quint8*>(&f);
// for the above line:
// data[0] can legally alias an object of type float
//    if quint8 is typedef for a char type (which is highly likely it is)
// data[0] now points to a float object

const float *p0 = reinterpret_cast<const float*>(data[0]);
// p0 points to the float object f. Legal

float r = *p0; // legal because of the above

return r;

And this is illegal:

const quint8 memory[4]{};
memory[0] = /* ... */;
memory[1] = /* ... */;
memory[2] = /* ... */;
memory[3] = /* ... */;

const quint8* data[2] {};

data[0] = memory;
// data[0] doesn't point to a float object

const float *p0 = reinterpret_cast<const float*>(data[0]);
// no matter what trickery you use to get the cast
// it is illegal for a `float*` pointer to alias a `quint8[]` object

float r = *p0; // *p0 is illegal and causes your program to have Undefined Behavior

Correct cast syntax

1 Answers1