如何在 Objective-C 中安全地将 `_Nullable` 转换为 `_Nonnull`？

Question

当使用-Wnullable-to-nonnull-conversion 进行编译时，我们会收到带有以下代码的适当警告：

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.

如何安全地将 foo 从 NSString * _Nullable 转换为 NSString * _Nonnull？

目前为止我最好的解决方案

我想出的最好的就是这个宏：

#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(); \
  } \
}()

使用如下：

NSString * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
    NSString * _Nonnull foo = ForceUnwrap(NSString *, maybeFoo);
    ^(NSString * _Nonnull bar) {
    }(foo);
}

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.

如果转换为错误的类型会产生错误：

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，如果您需要转换为具有多个参数的泛型类型，则必须求助于preprocessor hacks：

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);
}

我尝试过的方法不起作用

将maybeFoo 直接分配给NSString * _Nonnull 不起作用。它产生与以前相同的错误：

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.

将maybeFoo 转换为NSString * _Nonnull 是不安全的，因为如果maybeFoo 的类型发生变化，编译器不会中断：

NSNumber * _Nullable maybeFoo = @"foo";
if (maybeFoo) {
  NSString * _Nonnull foo = (NSString * _Nonnull) maybeFoo;
  ^(NSString * _Nonnull bar) {  
  }(foo);
}
// no errors!

我也尝试在转换时使用__typeof__，但__typeof__ 带有可空性说明符，因此当您尝试转换为__typeof__(maybeFoo) _Nonnull 时会遇到可空性冲突：

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.

一切都使用深度静态分析器运行，并使用 Xcode 8.2.1 编译，并带有以下标志：

-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

Answer 1

到目前为止，我发现的最好的方法是使用泛型。

本质上，您定义了一个使用泛型的接口，并具有一个将泛型类型返回为nonnull 的方法。然后在您的宏中使用 typeof 但在泛型类型上，这会为您提供正确的类型。

请注意，泛型类永远不会被实例化，它只是用来获取正确的类型。

@interface RBBBox<__covariant Type>

- (nonnull Type)asNonNull;

@end

#define RBBNotNil(V) \
    ({ \
        NSCAssert(V, @"Expected '%@' not to be nil.", @#V); \
        RBBBox<__typeof(V)> *type; \
        (__typeof(type.asNonNull))V; \
    })

不过，这不是我的想法。来源：https://gist.github.com/robb/d55b72d62d32deaee5fa

Answer 2

Michael Ochs' answer 基本上是正确的，但由于缺乏硬性_Nonnull 保证，我后来遇到了一些静态分析器警告。简而言之，如果我们收到nil，我们必须abort，否则当我们执行这样的任务时：

@interface Foo : NSObject
+ (NSString * _Nullable)bar;
@end

int main(int argc, char * argv[]) {
  NSString * _Nonnull bar = RBBNotNil([Foo bar]);
}

在Release 配置中（在我的情况下，归档时），静态分析器会抱怨您试图将_Nullable 值分配给_Nonnull 左值。我收到了这样的警告：

nil assigned to a pointer which is expected to have non-null value

这是我的更新版本：

// We purposefully don't have a matching @implementation.
// We don't want +asNonnull to ever actually be called
// because that will add a lot of overhead to every RBBNotNil
// and we want RBBNotNil to be very cheap.
// If there is no @implementation, then if the +asNonnull is
// actually called, we'll get a linker error complaining about
// the lack of @implementation.
@interface RBBBox <__covariant Type>

// This as a class method so you don't need to
// declare an unused lvalue just for a __typeof
+ (Type _Nonnull)asNonnull;

@end

/*!
 * @define RBBNotNil(V)
 * Converts an Objective-C object expression from _Nullable to _Nonnull. 
 * Crashes if it receives a nil! We must crash or else we'll receive
 * static analyzer warnings when archiving. I think in Release mode,
 * the compiler ignores the _Nonnull cast.
 * @param V a _Nullable Objective-C object expression
 */
#define RBBNotNil(V) \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Wgnu-statement-expression\"") \
({ \
__typeof__(V) __nullableV = V; \
NSCAssert(__nullableV, @"Expected '%@' not to be nil.", @#V); \
if (!__nullableV) { \
    abort(); \
} \
(__typeof([RBBNotNil<__typeof(V)> asNonnull]))__nullableV; \
}) \
_Pragma("clang diagnostic pop")

Answer 3

我使用这个宏：

#define assumeNotNull(_value) \
    ({ if (!_value) abort(); __auto_type const _temp = _value; _temp; })

当然，只有在代码中进行适当的测试后：

if (parameters) {
    [obj processParameters:assumeNotNull(parameters)];
}

忽略宏，编译器会告诉我参数可能是NULL，但processParameters 需要一个非NULL 参数。就我而言，这甚至被配置为错误，而不仅仅是警告。

省略if 检查，代码将编译，但如果我输入NULL，应用程序将当场崩溃。因此，应该只在测试后使用宏，或者如果绝对确定该值由于某种原因不能为NULL，并且您对此非常确定，那么您愿意将您的应用程序稳定性押在它上面。

如果有疑问，请始终测试并记住，如果测试显然是不必要的（例如，之前测试过条件并且如果值为NULL，则永远不会达到代码），编译器将在期间检测到优化阶段并为您删除测试。不必要的测试几乎不会是性能问题，尤其是在廉价的测试中。