CODEWITHSUNDEEP
c++undefined-behaviorprimitive-typesreinterpret-cast
masoud
masoud

Reputation: 56529

Copy bit pattern: float to uint32_t

Copying bit pattern of a float value into a uint32_t or vice versa (not casting them), we can copy bits byte-to-byte using std::copy or memcpy. Another way is to use reinterpret_cast as below (?):

float f = 0.5f;
uint32_t i = *reinterpret_cast<uint32_t*>(&f);

or

uint32_t i;
reinterpret_cast<float&>(i) = 10;

However there is a claim that says two reinterpret_cast used above, invokes undefined behavior. Is that true? How?

Upvotes: 1

Views: 962

Answers (1)

TartanLlama
TartanLlama

Reputation: 65730

Yes, this is undefined behaviour as it breaks the strict aliasing rule:

[basic.lval]/10: If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined — the dynamic type of the object,

— a cv-qualified version of the dynamic type of the object,

— a type similar (as defined in 4.4) to the dynamic type of the object,

— a type that is the signed or unsigned type corresponding to the dynamic type of the object,

— a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,

— an aggregate or union type that includes one of the aforementioned types among its elements or non- static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),

— a type that is a (possibly cv-qualifded) base class type of the dynamic type of the object,

— a char or unsigned char type.

Since uint32_t is none of the above when trying to access an object of type float, the behaviour is undefined.

Upvotes: 6

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