当-Wnullable-to-nonnull-conversion
编译,我们可以得到下面的代码一个适当的警告:如何在Objective-C中安全地将`_Nullable`转换为`_Nonnull`?
NSString * _Nullable maybeFoo = @"foo";
^(NSString * _Nonnull bar) {
}(maybeFoo);
Tests.m:32:7: error: implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull' [-Werror,-Wnullable-to-nonnull-conversion]
}(maybeFoo);
^
1 error generated.
如何安全地转换foo
从NSString * _Nullable
到NSString * _Nonnull
?
我有最好的解决方案至今
我想出的最好的是这个宏:
#define ForceUnwrap(type, nullableExpression) ^type _Nonnull() { \
type _Nullable maybeValue___ = nullableExpression; \
if (maybeValue___) { \
return (type _Nonnull) maybeValue___; \
} else { \
NSLog(@"Attempted to force unwrap a null: " #nullableExpression); \
abort(); \
} \
}()
这是用这样的:
NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSString * _Nonnull foo = ForceUnwrap(NSString *, maybeFoo);
^(NSString * _Nonnull bar) {
}(foo);
}
和如果分配给错误类型的变量会产生错误:
NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSNumber * _Nonnull foo = ForceUnwrap(NSString *, maybeFoo);
^(NSNumber * _Nonnull bar) {
}(foo);
}
Tests.m:40:29: error: incompatible pointer types initializing 'NSNumber * _Nonnull' with an expression of type 'NSString * _Nonnull' [-Werror,-Wincompatible-pointer-types]
NSNumber * _Nonnull foo = ForceUnwrap(NSString *, maybeFoo);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
并产生一个错误,如果投给了错误的类型:
NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSNumber * _Nonnull foo = ForceUnwrap(NSNumber *, maybeFoo);
^(NSNumber * _Nonnull bar) {
}(foo);
}
Tests.m:40:35: error: incompatible pointer types initializing 'NSNumber * _Nullable' with an expression of type 'NSString * _Nullable' [-Werror,-Wincompatible-pointer-types]
NSNumber * _Nonnull foo = ForceUnwrap(NSNumber *, maybeFoo);
^ ~~~~~~~~
Tests.m:27:16: note: expanded from macro 'ForceUnwrap'
type _Nullable maybeValue___ = nullableExpression; \
^ ~~~~~~~~~~~~~~~~~~
1 error generated.
不幸的是,如果你需要转换为通用型多参数,你不得不求助于preprocessorhacks:
NSDictionary<NSString *, NSString *> * _Nullable maybeFoo =
[NSDictionary<NSString *, NSString *> new];
if (maybeFoo) {
NSDictionary<NSString *, NSString *> * _Nonnull foo =
#define COMMA ,
ForceUnwrap(NSDictionary<NSString * COMMMA NSString *>, maybeFoo);
#undef COMMA
^(NSDictionary<NSString *, NSString *> * _Nonnull bar) {
}(foo);
}
事情我已经试过,不工作
分配maybeFoo
直接到NSString * _Nonnull
不起作用。它产生相同的错误面前:
NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSString * _Nonnull foo = maybeFoo;
^(NSString * _Nonnull bar) {
}(foo);
}
Tests.m:30:35: error: implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull' [-Werror,-Wnullable-to-nonnull-conversion]
NSString * _Nonnull foo = maybeFoo;
^
1 error generated.
而且铸造maybeFoo
到NSString * _Nonnull
是不是安全的,因为如果maybeFoo
的类型变化,编译器将不会打破:
NSNumber * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSString * _Nonnull foo = (NSString * _Nonnull) maybeFoo;
^(NSString * _Nonnull bar) {
}(foo);
}
// no errors!
我也尝试过使用__typeof__
,铸造时,但__typeof__
携带空性说明符,所以当你尝试转换为__typeof__(maybeFoo) _Nonnull
你得到一个非空冲突:
NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
NSString * _Nonnull foo = (__typeof__(maybeFoo) _Nonnull) maybeFoo;
^(NSString * _Nonnull bar) {
}(foo);
}
Tests.m:30:57: error: nullability specifier '_Nonnull' conflicts with existing specifier '_Nullable'
NSString * _Nonnull foo = (__typeof__(maybeFoo) _Nonnull) maybeFoo;
^
Tests.m:30:35: error: implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull' [-Werror,-Wnullable-to-nonnull-conversion]
NSString * _Nonnull foo = (__typeof__(maybeFoo) _Nonnull) maybeFoo;
^
2 errors generated.
一切都与深静态分析仪运行并与Xcode 8.2.1
有以下标志编译:
-Wnon-modular-include-in-framework-module
-Werror=non-modular-include-in-framework-module
-Wno-trigraphs
-Werror
-Wno-missing-field-initializers
-Wno-missing-prototypes
-Wunreachable-code
-Wno-implicit-atomic-properties
-Wno-arc-repeated-use-of-weak
-Wduplicate-method-match
-Wno-missing-braces
-Wparentheses
-Wswitch
-Wunused-function
-Wno-unused-label
-Wno-unused-parameter
-Wunused-variable
-Wunused-value
-Wempty-body
-Wuninitialized
-Wno-unknown-pragmas
-Wno-shadow
-Wno-four-char-constants
-Wno-conversion
-Wconstant-conversion
-Wint-conversion
-Wbool-conversion
-Wenum-conversion
-Wshorten-64-to-32
-Wpointer-sign
-Wno-newline-eof
-Wno-selector
-Wno-strict-selector-match
-Wundeclared-selector
-Wno-deprecated-implementations
-Wno-sign-conversion
-Wno-infinite-recursion
-Weverything
-Wno-auto-import
-Wno-objc-missing-property-synthesis
-Wno-cstring-format-directive
-Wno-direct-ivar-access
-Wno-double-promotion
你不知道。不要为Objective-C使用该属性。他们是Swift的。 Objective-C有一个明确定义的消息来消除行为。 –
在将'_Nullable'右值传递给'_Nonnull'左值时,clang中会有警告。因此,它们不仅仅适用于Swift。这个问题是关于管理该警告。 –
他们只是为了Swift。编译器不会更改生成的代码的位,这取决于引用的可空性。 –