Conversion of false to object via const char * constructor

Question

I have built the following minimal example:

class A
{
    public:
        A(const char *s);

    private:
        const char *p;
};

A::A(const char *s)
  :          


        

Answer 1

The OP 's program fails in compilation in g++ & clang++ when use -std=c++11, -std=c++14.

See live demo here on g++ 6.1.0. It gives following error

prog.cc: In function 'A foo()':
prog.cc:17:12: error: could not convert 'false' from 'bool' to 'A'
     return false;
            ^~~~~ 

See live demo here on clang++. It gives following diagnosis.

main.cpp:17:12: error: no viable conversion from returned value of type 'bool' to function return type 'A'
    return false;
           ^~~~~
main.cpp:1:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'bool' to 'const A &' for 1st argument
class A
      ^
main.cpp:1:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'bool' to 'A &&' for 1st argument
class A
      ^
main.cpp:10:4: note: candidate constructor not viable: no known conversion from 'bool' to 'const char *' for 1st argument
A::A(const char *s)
   ^
main.cpp:22:12: error: no viable conversion from returned value of type 'bool' to function return type 'A'
    return true;
           ^~~~
main.cpp:1:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'bool' to 'const A &' for 1st argument
class A
      ^
main.cpp:1:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'bool' to 'A &&' for 1st argument
class A
      ^
main.cpp:10:4: note: candidate constructor not viable: no known conversion from 'bool' to 'const char *' for 1st argument
A::A(const char *s)
   ^
main.cpp:7:21: warning: private field 'p' is not used [-Wunused-private-field]
        const char *p;
                    ^

false should not convert to pointer type since C++11. See similar question here which I've asked sometimes ago.

Answer 2

As published, C++ 11 has the rule "an integral constant expression prvalue of integer type which evaluates to 0 can be converted to any pointer type, yielding the null pointer value of that type." (C++98/03 had a similarly worded rule with the same net effect).

bool is an integer type, and false evaluates to 0. So false is a valid null pointer constant.

Apart from this extra rule, C++ has no implicit conversions from integral types to pointers. Which is why true cannot implicitly be converted to a pointer.

However, C++14 changed the definition of a null pointer constant so that only integer literals (and not integral constant expressions) qualify. false is a boolean literal, not an integer one, so under C++14, the code will not compile.

Furthermore, since the issue was recognised by the standard committee as a defect in C++11, newer C++11 compilers are likely to obey the C++14 rules in this regard and not treat false as a null pointer constant. Thanks to @Destructor for tracking down the issue status.

---

As to why two implicit conversions seem to be allowed here: the rule is not "at most one implicit conversion is allowed." The rule is "at most one user-defined conversion is allowed." Pointer conversions (such as converting a null pointer constant to a null pointer value) are not classified as user-defined conversions. So the conversion sequence in your case is a pointer conversion (bool to const char *) followed by a user-defined conversion (const char * to A).

Answer 3

For 1), false is of type bool, and it can be promoted to an int implicitely:

• the type bool can be converted to int with the value false becoming ​0​ and true becoming 1.

So, false is basically promoted to NULL/0 (or nullptr), which can be assigned to a pointer.

For 2), §4.10 states that:

A null pointer constant is an integer literal (2.14.2) with value zero or a prvalue of type std::nullptr_t.

A null pointer constant can be converted to a pointer type; [...]

Only a Null pointer constant can be converted to a pointer, and a null pointer constant is either an intergral with value 0 or std::nullptr_t. true (or 1 for that matter) aren't specified, and thus they can't be converted to a pointer.