ios - 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，我们会通过以下代码收到正确的警告:

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？

我目前为止最好的解决方案

我想到的最好的是这个宏:

#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(); \
  } \
}()

使用方式如下:

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，则会产生错误:

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通用类型，您必须求助于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);
}

我尝试过但行不通的方法

将 maybeFoo 直接分配给 NSString * _Nonnull 是行不通的。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.

并且将 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!

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

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，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，我发现的最好的是泛型技巧。

本质上，您定义了一个使用泛型的接口(interface)，并具有一个将泛型类型返回为 nonnull 的方法。然后在您的宏中您使用 typeof 但在通用类型上，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; \
    })

不过这不是我的主意。来源:https://gist.github.com/robb/d55b72d62d32deaee5fa