c++11 - c++ access static members using null pointer

Question

Recently tried the following program and it compiles, runs fine and produces expected output instead of any runtime error.

#include <iostream>
class demo
{
    public:
        static void fun()
        {
            std::cout<<"fun() is called
";
        }
        static int a;
};
int demo::a=9;
int main()
{
    demo* d=nullptr;
    d->fun();
    std::cout<<d->a;
    return 0;
}

If an uninitialized pointer is used to access class and/or struct members behaviour is undefined, but why it is allowed to access static members using null pointers also. Is there any harm in my program?

Answer 1

TL;DR: Your example is well-defined. Merely dereferencing a null pointer is not invoking UB.

There is a lot of debate over this topic, which basically boils down to whether indirection through a null pointer is itself UB.
The only questionable thing that happens in your example is the evaluation of the object expression. In particular, d->a is equivalent to (*d).a according to [expr.ref]/2:

The expression E1->E2 is converted to the equivalent form (*(E1)).E2; the remainder of 5.2.5 will address only the first option (dot).

*d is just evaluated:

The postfix expression before the dot or arrow is evaluated;⁶⁵ the result of that evaluation, together with the id-expression, determines the result of the entire postfix expression.

^{65) If the class member access expression is evaluated, the subexpression evaluation happens even if the result is unnecessary to determine the value of the entire postfix expression, for example if the id-expression denotes a static member.}

Let's extract the critical part of the code. Consider the expression statement

*d;

In this statement, *d is a discarded value expression according to [stmt.expr]. So *d is solely evaluated¹, just as in d->a.
Hence if *d; is valid, or in other words the evaluation of the expression *d, so is your example.

Does indirection through null pointers inherently result in undefined behavior?

There is the open CWG issue #232, created over fifteen years ago, which concerns this exact question. A very important argument is raised. The report starts with

At least a couple of places in the IS state that indirection through a null pointer produces undefined behavior: 1.9 [intro.execution] paragraph 4 gives "dereferencing the null pointer" as an example of undefined behavior, and 8.3.2 [dcl.ref] paragraph 4 (in a note) uses this supposedly undefined behavior as justification for the nonexistence of "null references."

Note that the example mentioned was changed to cover modifications of const objects instead, and the note in [dcl.ref] - while still existing - is not normative. The normative passage was removed to avoid commitment.

However, 5.3.1 [expr.unary.op] paragraph 1, which describes the unary "*" operator, does not say that the behavior is undefined if the operand is a null pointer, as one might expect. Furthermore, at least one passage gives dereferencing a null pointer well-defined behavior: 5.2.8 [expr.typeid] paragraph 2 says

If the lvalue expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value (4.10 [conv.ptr]), the typeid expression throws the bad_typeid exception (18.7.3 [bad.typeid]).

This is inconsistent and should be cleaned up.

The last point is especially important. The quote in [expr.typeid] still exists and appertains to glvalues of polymorphic class type, which is the case in the following example:

int main() try {

    // Polymorphic type
    class A
    {
        virtual ~A(){}
    };

    typeid( *((A*)0) );

}
catch (std::bad_typeid)
{
    std::cerr << "bad_exception
";
}

The behavior of this program is well-defined (an exception will be thrown and catched), and the expression *((A*)0) is evaluated as it isn't part of an unevaluated operand. Now if indirection through null pointers induced UB, then the expression written as

*((A*)0);

would be doing just that, inducing UB, which seems nonsensical when compared to the typeid scenario. If the above expression is merely evaluated as every discarded-value expression is¹, where is the crucial difference that makes the evaluation in the second snippet UB? There is no existing implementation that analyzes the typeid-operand, finds the innermost, corresponding dereference and surrounds its operand with a check - there would be a performance loss, too.

A note in that issue then ends the short discussion with:

We agreed that the approach in the standard seems okay: p = 0; *p; is not inherently an error. An lvalue-to-rvalue conversion would give it undefined behavior.

I.e. the committee agreed upon this. Although the proposed resolution of this report, which introduced so-called "empty lvalues", was never adopted…

However, “not modifiable” is a compile-time concept, while in fact this deals with runtime values and thus should produce undefined behavior instead. Also, there are other contexts in which lvalues can occur, such as the left operand of . or .*, which should also be restricted. Additional drafting is required.

…that does not affect the rationale. Then again, it should be noted that this issue even precedes C++03, which makes it less convincing while we approach C++17.

---

CWG-issue #315 seems to cover your case as well:

Another instance to consider is that of invoking a member function from a null pointer:

  struct A { void f () { } };
  int main ()
  {
    A* ap = 0;
    ap->f ();
  }

[…]

Rationale (October 2003):

We agreed the example should be allowed. p->f() is rewritten as (*p).f() according to 5.2.5 [expr.ref]. *p is not an error when p is null unless the lvalue is converted to an rvalue (4.1 [conv.lval]), which it isn't here.

According to this rationale, indirection through a null pointer per se does not invoke UB without further lvalue-to-rvalue conversions (=accesses to stored value), reference bindings, value computations or the like. (Nota bene: Calling a non-static member function with a null pointer should invoke UB, albeit merely hazily disallowed by [class.mfct.non-static]/2. The rationale is outdated in this respect.)

I.e. a mere evaluation of *d does not suffice to invoke UB. The identity of the object is not required, and neither is its previously stored value. On the other hand, e.g.

*p = 123;

is undefined since there is a value computation of the left operand, [expr.ass]/1:

In all cases, the assignment is sequenced after the value computation of the right and left operands

Because the left operand is expected to be a glvalue, the identity of the object referred to by that glvalue must be determined as mentioned by the definition of evaluation of an expression in [intro.execution]/12, which is impossible (and thus leads to UB).

---

¹ [expr]/11:

In some contexts, an expression only appears for its side effects. Such an expression is called a discarded-value expression. The expression is evaluated and its value is discarded. […]. The lvalue-to-rvalue conversion (4.1) is applied if and only if the expression is a glvalue of volatile-qualified type and […]