How you can and how you can't use a null pointer in C ++

For some, this banal question has already set the teeth on edge, but we took 7 examples and tried to explain their behavior using the standard:

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

/*1*/ *p;
/*2*/ foo((*p, 5));                     
/*3*/ A a{*p};
/*4*/ p->data_mem;
/*5*/ int b{p->data_mem};
/*6*/ p->non_static_mem_fn();
/*7*/ p->static_mem_fn();

An obvious but important detail: p , initialized with a null pointer, cannot point to an object of type A, because its value is different from the value of any pointer to an object of type A conv.ptr # 1 .

Disclaimer: . dev.to, .

1

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

*p;

(expression statement, stmt.expr#1), *p , , , . * expr.unary.op#1 , (indirection), l-, , . , , . .

, basic.stc#4, , (indirection through an invalid pointer value) . , , basic.compound#3.4, , — .

dcl.ref#5, , «the only way to create such a reference would be to bind it to the “object” obtained by indirection through a null pointer, which causes undefined behavior», .. — «», , . , «» (to bind), , , dcl.ref#5.

, , Core Working Group . , CWG ( drafting), , l- r-. « CWG» , , , 7. CWG.

. (N2176, 6.5.3.2 104), , , .

2

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

foo((*p, 5));  

foo, , «». , , , , 1 (expr.comma#1). , .

3

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

A a{*p};

a , , , const A& , (dcl.ref#5). .

4

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

p->data_mem;

(*(p)).data_mem expr.ref#2, (designate) , (expr.ref#6.2). 1 , , , basic.lookup.qual#1, , to refer to designate , expr.ref. , , (. ).

5

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

int b{p->data_mem};

, int. pr-, (basic.lval#1.2). int, (conv.lval#3.4), , basic.lval#11 .

6

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

p->non_static_mem_fn();

class.mfct.non-static#1 , - , ( ), - . «» — «may be» /, . , .

7

struct A {
    int data_mem;
    void non_static_mem_fn() {}
    static void static_mem_fn() {}
};

void foo(int) {}

A* p{nullptr};

p->static_mem_fn();

1, Core Working Group . , 59, -> , .

constexpr

(expr.const#5), . , . , constexpr , , GCC, MSVC : godbolt. 

:

6 4. , 1.

Thank you for staying with us until the end to follow the adventures of the null pointer in C ++! :-) Usually we share on Habré pieces of code from real projects for the development of embedded software for electronics, but this time we were interested in purely "philosophical" questions, so the examples are synthetic.

If you share our love for controversy in C ++, share your "painful" ones in the comments.