Suppose you want to look at the bits of a floating point number, as though it were an integer. The type punning way to do that would be to access a double in memory through an integer pointer:
double d = 3.14;
int64_t *p = (int64_t *) &d; // !!!
int64_t n = *p;
The union aliasing way would be to makeset the double field in a union and read the integer field back:
typedef union {
double d;
int64_t n;
} int_or_double;
int_or_double u;
u.d = 3.14
int64_t n = u.n
According to the standard, the union aliasing approach is allowed, but the type punning one is undefined behavior.
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The standard team decided to forbid type punning to allow compilers to implement more optimizations. They still allow union aliasing because they felt there should still be a way to do low-level type casts if for those that absolutely want to.
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The standard team decided to forbid type punning to allow compilers to implement more optimizations. They still allow union aliasing because they felt there should still be a way to do low-level type casts if for those that absolutely want to.