问题描述
如果我理解正确，我们可以将 void * 归类为"C可保留指针类型".因此，将其分配给Objective-C对象将被隐式桥接.但是，编译器会引发错误，要求进行显式桥接.
const void * somePtr = (void *)0x12345678;
- (void)someMethod:(id)sender
{
NSObject *obj = (NSObject *)somePtr;
}
此外，我检查了空指针常量，并且在编译时没有显式桥接.
NSObject *obj = (void *)0;
我正在使用XCode 4.5(Clang 4.1(标签/Apple/clang-421.11.66)(基于 LLVM 3.1svn )).
问题:我知道为NSObject分配一些任意/不相关的指针有点奇怪，但是我想确保是否正确理解了规则.我对"C可保留指针类型" 有点怀疑.尤其是关于(可能合格)和(可能合格)的意图.我们可以将哪些指针类型归类为"C可保留指针类型" ?
还有，它实际上是指系统全局变量" 语句中来自系统的全局变量吗?
http://clang.llvm.org/docs/AutomaticReferenceCounting.html
似乎const系统全局变量实际上不需要显式桥接.即kCFBooleanTrue(CFBoolean实例)，kCFNumberNaN或kABPersonPhoneMobileLabel.
NSObject *obj = (NSObject *)kCFBooleanTrue;
请注意，CFBoolean不是免费的桥接器，但编译器仍可以隐式桥接它.我定义了全局常量，但无法使用隐式桥接对其进行编译.因此，我想知道编译器如何确定变量是否来自系统?(或者可能正在检查类型是否来自CoreFoundation.framework，这不是一个很好的解决方案...)
---编辑---
参考rob mayoff的答案，我尝试了隐式桥接，但是它仍然没有用.可能有一个编译器标志，用于将该文件确定为Core Foundation文件.
NSObject *obj = (NSObject *)myGlobal;
"mytest.h"文件
#ifndef mytest_h
#define mytest_h
#pragma clang arc_cf_code_audited begin
typedef const struct MyStruct * MyStructPtr;
CF_EXPORT
const MyStructPtr myGlobal;
#pragma clang arc_cf_code_audited end
#endif
"mytest.c"文件
#include "mytest.h"
struct MyStruct {
int a;
};
static struct MyStruct __myglobal = { 123 };
const MyStructPtr myGlobal = &__myglobal;
---编辑---
我还修改了CoreFoundation.framework中的CFNumber.h头文件，并删除了 CF_IMPLICIT_BRIDGING_ENABLED/CF_IMPLICIT_BRIDGING_DISABLED ，然后清理/构建了该项目，但并未禁用这些常量的隐式桥接. >
If i understood correctly we could classify void * as a "C retainable pointer type".So, assigning it to an Objective-C object will be implicitly bridged. However, compiler raises error that explicit bridging is required.
const void * somePtr = (void *)0x12345678;
- (void)someMethod:(id)sender
{
NSObject *obj = (NSObject *)somePtr;
}
Also, i checked the null pointer constant and it compiles without explicit bridging.
NSObject *obj = (void *)0;
I am using XCode 4.5(Clang 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn)).
Question:I know it's a little bit weird to assign some arbitrary/irrelevant pointer to NSObject but i want to make sure if am correctly understanding the rules. I am little bit suspicious about the "C retainable pointer type". description; especially about the intention of (possibly qualified) and (possibly qualifier). What pointer types could we classify as "C retainable pointer type"?
Also, does it actually mean a global variable from system by the "system global variable" statement?
http://clang.llvm.org/docs/AutomaticReferenceCounting.html
It seems that const system global variables actually do not require explicit bridging.i.e. kCFBooleanTrue(CFBoolean instance), kCFNumberNaN or kABPersonPhoneMobileLabel.
NSObject *obj = (NSObject *)kCFBooleanTrue;
Note that CFBoolean is not toll free bridged but it could be still implicitly bridged by compiler.I define global constants but could not get them compiled with implicit bridging.So, i wonder how could compiler determine if a variable comes from system or not?(Or may be it is checking if type comes from CoreFoundation.framework which is not a neat solution...)
--- EDIT ---
Referring to rob mayoff's answer, i tried implicit bridging but it still did not work. May be there is a compiler flag to determine the file as a Core Foundation file.
NSObject *obj = (NSObject *)myGlobal;
"mytest.h" File
#ifndef mytest_h
#define mytest_h
#pragma clang arc_cf_code_audited begin
typedef const struct MyStruct * MyStructPtr;
CF_EXPORT
const MyStructPtr myGlobal;
#pragma clang arc_cf_code_audited end
#endif
"mytest.c" File
#include "mytest.h"
struct MyStruct {
int a;
};
static struct MyStruct __myglobal = { 123 };
const MyStructPtr myGlobal = &__myglobal;
--- EDIT ---
I also modified the CFNumber.h header file in CoreFoundation.framework and removed CF_IMPLICIT_BRIDGING_ENABLED/CF_IMPLICIT_BRIDGING_DISABLED, then clean/build the project and yet it did not disable implicit bridging for those constants.
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