问题
If you have a C function which returns an integer, you could write a statement like this:
MyInt &= MyFunc();
...where we're using the bitwise-AND assignment operator.
The question is: is MyFunc() guaranteed to be executed, even if MyInt equals zero?
Likwise, if we used the bitwise-OR assignment operator (|=), would MyFunc() always be executed, even if MyInt were set to all ones?
Put another way: is lazy evaluation permitted in C for bitwise operators?
回答1:
MyInt &= MyFunc();
is equivalent to:
MyInt = MyInt & MyFunc();
The language states that the & operator is not short-circuited. However, an optimiser could generate code not to call to the function if MyInt was zero and it was sure that the function had no side effects. I doubt any compilers acrtually do this, as the runtime test probably makes it a pessimisation.
回答2:
No. Bitwise operators are not short-circuited. MyFunc() execution is guaranteed regardless of the value of MyInt.
来源：https://stackoverflow.com/questions/1354138/is-there-a-defined-evaluation-order-for-and