Operands of sizeof, noexcept and decltype are unevaluated. So side effects in these operands (which are all
the effects an expression can have in addition to producing a value), will not be applied. And that may be surprising to the reader.
Additionally, the operand of typeid may or may not be evaluated, depending on its type: it will be evaluated if it is a function call
that returns reference to a polymorphic type, but it will not be evaluated in all the other cases. This difference of behavior is tricky to apprehend
and that is why both cases are reported here.
This rules reports an issue when operands of such operators have side-effects.
Noncompliant code example
class A {
public:
virtual ~A();
/*...*/
};
class B : public A { /* ....*/ };
A& create(string const &xml);
void test(string const &xml) {
int i = 0;
cout << noexcept(++i); // Noncompliant, "i" is not incremented
cout << typeid(++i).name(); // Noncompliant, "i" is not incremented
auto p1 = malloc(sizeof(i = 5)); // Noncompliant, "i" is not changed
cout << typeid(create(xml)).name(); // Noncompliant, even if the side-effects will be evaluated in this case
}
Compliant solution
class A {
public:
virtual ~A();
/*...*/
};
class B : public A { /* ....*/ };
A& create(string const &xml);
void test(string const &xml) {
int i = 0;
++i;
cout << noexcept(i); // Compliant
++i;
cout << typeid(i).name(); // Compliant
i = 5;
auto p1 = malloc(sizeof(i)); // Compliant
auto a = create(xml);
cout << typeid(a).name(); // Compliant
}
