Does reinterpret_cast with uint8_t break the Strict Aliasing Rule?

Question

Is the following code legal in terms of the Strict Aliasing Rule (and in general)?

const int size = 1024;
uint8_t* buffer = allocateBuffer(size);
float* float_pointer = reinterpret_cast<float*>(buffer);
std::fill(float_pointer, float_pointer + size / sizeof(float), 1.5f);

As far as I understand, generally speaking SAR says we cannot access (either read or write) data through a pointer of a different type - unless we use a pointer of character type.

However, in the above code we use a pointer of a non-character type (float*) to read or write data of (probably) a character type (uint8_t), which I think is illegal.

Am I correct?

According to [this answer](https://stackoverflow.com/a/98702/3476954), yes it does. — Mansoor, Sep 08 '20 at 15:52
Which version of C++? C++20 has relaxed a lot of the rules around this for basic types. Not sure if makes this legal or not, but it does probably change the answer from previously. — NathanOliver, Sep 08 '20 at 15:53
Recap of type aliasing: https://en.cppreference.com/w/cpp/language/reinterpret_cast#Type_aliasing — Eljay, Sep 08 '20 at 17:09

score 5 · Accepted Answer · answered Sep 08 '20 at 15:57

However, in the above code we use a pointer of a non-character type (float*) to read or write data of (probably) a character type (uint8_t), which I think is illegal.

Am I correct?

Yes.

Is the following code legal in terms of the Strict Aliasing Rule (and in general)?

No.

Besides pointer aliasing, another consideration is that alignment requirement of float is stricter than that of uint8_t. There is doubt whether uint8_t* allocateBuffer(arg) returns a pointer that satisfies the alignment of float.

Does reinterpret_cast with uint8_t break the Strict Aliasing Rule?

1 Answers1