false implicitly convert to null pointer
c++, c++11
Solution
I'd say clang with C++11 is right:
3.9.1 Fundamental types [basic.fundamental]
6 Values of type bool are either `true` or `false`. [ Note: There are no signed, unsigned, short, or long bool types or values. — end note ] Values of type bool participate in integral promotions (4.5).
`bool` does not have value zero, so can not be converted to null pointer:
4.10 Pointer conversions [conv.ptr]
1 A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type;
One might suggest conversion sequence consisting of integral promotion (`bool` to `int`) and null pointer conversion, but it would not be valid:
4 Standard conversions [conv]
1 Standard conversions are implicit conversions with built-in meaning. Clause 4 enumerates the full set of such conversions. A standard conversion sequence is a sequence of standard conversions in the following order:
- Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.
- Zero or one conversion from the following set: integral promotions, floating point promotion, integral conversions, floating point conversions, floating-integral conversions, pointer conversions, pointer to member conversions, and boolean conversions.
- Zero or one qualification conversion.
[ Note: A standard conversion sequence can be empty, i.e., it can consist of no conversions. — end note ] A standard conversion sequence will be applied to an expression if necessary to convert it to a required destination type.
Problem
Whether `false` is allowed to be implicitly converted to pointer is different between clang++ and g++: g++-4.8: always a warning with or without -std=c++11 clang++ (trunk): a warning if without -std=c++11, and an error if with -std=c++11 So anyone knows why g++ and clang++ behaves differently, and who is correct? What paragraphs in C++ standard (both C++03 and C++11) talks about the situation. Thanks. ``` [hidden ~]$ cat b.cpp const char* f() { return false; } [hidden ~]$ g++ -c b.cpp b.cpp: In function ‘const char* f()’: b.cpp:1:26: warning: converting ‘false’ to pointer type ‘const char*’ [-Wconversion-null] const char* f() { return false; } ^ [hidden ~]$ g++ -std=c++11 -c b.cpp b.cpp: In function ‘const char* f()’: b.cpp:1:26: warning: converting ‘false’ to pointer type ‘const char*’ [-Wconversion-null] const char* f() { return false; } ^ [hidden ~]$ clang++ -c b.cpp b.cpp:1:26: warning: initialization of pointer of type 'const char *' to null from a constant boolean expression [-Wbool-conversion] const char* f() { return false; } ^~~~~ 1 warning generated. [hidden ~]$ clang++ -std=c++11 -c b.cpp b.cpp:1:26: error: cannot initialize return object of type 'const char *' with an rvalue of type 'bool' const char* f() { return false; } ^~~~~ 1 error generated. ```