当使用-Wnullable-to-nonnull-conversion编译时，我们会得到一个正确的警告，代码如下：

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？

目前为止最好的解决方案

我最好的办法就是这个宏：

#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(); \
  } \
}()

它的用法如下：

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

如果赋值给错误类型的变量，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.

如果强制转换为错误的类型，则会产生错误：

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，如果你需要转换为一个带有多个参数的泛型类型，你必须求助于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);
}

我试过的都没用

将maybeFoo直接分配给NSString * _Nonnull不起作用。它会产生与之前相同的错误：

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.

而将maybeFoo转换为NSString * _Nonnull并不安全，因为如果maybeFoo的类型发生了变化，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!

我还尝试在强制转换时使用__typeof__，但__typeof__带有空性说明符，因此当您尝试强制转换为__typeof__(maybeFoo) _Nonnull时，您会遇到空性冲突：

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.

所有内容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