问题描述
是否允许将 false 隐式转换为指针在clang ++和g ++之间是不同的：
Whether false is allowed to be implicitly converted to pointer is different between clang++ and g++:
g ++ - 4.8：总是带或不带-std = c ++ 11的警告
g++-4.8: always a warning with or without -std=c++11
clang ++（trunk）：如果没有-std = c ++ 11， if with -std = c ++ 11
clang++ (trunk): a warning if without -std=c++11, and an error if with -std=c++11
所以任何人都知道为什么g ++和clang ++的行为不同，谁是正确的？ C ++标准（C ++ 03和C ++ 11）中的哪些段落谈论这种情况。
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.
感谢。
[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.
推荐答案
右侧：
6 bool类型的值为 true 或 false 。 [注意：没有有符号，无符号，短或长bool类型或值。 - end
note]类型bool的值参与积分促销（4.5）。
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 没有值零，因此无法转换为空指针：
bool does not have value zero, so can not be converted to null pointer:
1空指针常数是一个整数常数表达式（5.19）整数类型的prvalue strong>或
的prvalue类型std :: nullptr_t。空指针常量可以转换为
指针类型;
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;
可能建议由积分提升code> bool 到 int ）和空指针转换，但它不会有效：
One might suggest conversion sequence consisting of integral promotion (bool to int) and null pointer conversion, but it would not be valid:
在意义。
第4条列举了这些转换的完整集合。标准
转换序列是
中顺序之后的标准转换序列：
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:
- 零个或一个转换以下集合：lvalue到rvalue的转换，数组到指针的转换和函数到指针
的转换。 - 零或一个转换来自以下集合：整数促销，浮点促销，积分转换，浮动
点转化，浮动积分转换，指针
转化 ，指向成员转换的指针和布尔转换。 - 零或一个资格转换。
- 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.
[注意：标准转换序列可以为空，即它可以
由无转换组成。 - end note]如果需要，标准转换序列
将应用于表达式，以将其转换为
所需的目标类型。
[ 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.
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