Is the following undefined behaviour?
union {
int foo;
float bar;
} baz;
baz.foo = 3.14 * baz.bar;
I remember that writing and reading from the same underlying memory between two sequence points is UB, but I am not certain.
I remember that writing and reading from the same underlying memory between two sequence points is UB, but I am not certain.
Reading and writing to the same memory location in the same expression does not invoke undefined behavior until and unless that location is modified more than once between two sequence points or the side effect is unsequenced relative to the value computation using the value at the same location.
If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. [...]
The expression
baz.foo = 3.14 * baz.bar;
has well defined behaviour if bar is initialized before. The reason is that the side effect to baz.foo is sequenced relative to the value computations of the objects baz.foo and baz.bar.
[...] The side effect of updating the stored value of the left operand is sequenced after the value computations of the left and right operands. The evaluations of the operands are unsequenced.