该布尔表达式如何求值？

Question

In C++, let the following construct :

template<typename ValueType>
ValueType * func(Foo * foo)
{
    Bar bar;
    return foo && typeid(foo) == typeid(ValueType) ? &static_cast<ValueType*>bar : 0;
}

How is the return statement evaluated? Like so?

if ((bar && typeid(bar)) == typeid(ValueType))
    return &static_cast<ValueType*>bar
return false;

Answer 1

foo && typeid(foo) == typeid(ValueType) ? &static_cast<ValueType*>bar : 0;

...corrected with parenthesis after the static_cast<> , is evaluated as...

(foo && (typeid(foo) == typeid(ValueType))) ? (&(static_cast<ValueType*>(bar))) : 0;

The precedence rules are listed here . Notice that the ?: ternary operator is at precedence level 15 in this list - lower than the other operators you've used, so it defines the outer structure of the evaluation. && is at 13 - way below == at 9. (I don't think these numbers are used anywhere in the Standard, but they're convenient references for pointing out things in the cppreference table).