带宏的外部函数 [英] extern function with macro

问题描述

当我尝试使用extern函数执行marco时，在目标C中存在链接器问题.知道为什么吗?

Im having a linker problem in Objective C when i attempt to do a marco with a extern function. Any idea why?

头文件
为了协助与设备版本进行比较

Header file
To assist in doing comparison with the device version

extern NSString* getOperatingSystemVerisonCode();

#if TARGET_OS_IPHONE // iOS
#define DEVICE_SYSTEM_VERSION                       [[UIDevice currentDevice]      systemVersion]
#else // Mac
#define DEVICE_SYSTEM_VERSION                       getOperatingSystemVerisonCode()
#endif

#define COMPARE_DEVICE_SYSTEM_VERSION(v)            [DEVICE_SYSTEM_VERSION compare:v options:NSNumericSearch]
#define SYSTEM_VERSION_EQUAL_TO(v)                  (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedSame)
#define SYSTEM_VERSION_GREATER_THAN(v)              (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedDescending)
#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)  (COMPARE_DEVICE_SYSTEM_VERSION(v) != NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN(v)                 (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN_OR_EQUAL_TO(v)     (COMPARE_DEVICE_SYSTEM_VERSION(v) != NSOrderedDescending)

.mm文件

NSString* getOperatingSystemVerisonCode()
{
    /*
     [[NSProcessInfo processInfo] operatingSystemVersionString]
     */
    NSDictionary *systemVersionDictionary =
    [NSDictionary dictionaryWithContentsOfFile:
     @"/System/Library/CoreServices/SystemVersion.plist"];

    NSString *systemVersion =
    [systemVersionDictionary objectForKey:@"ProductVersion"];
    return systemVersion;
}

链接器错误:

Undefined symbols for architecture x86_64:
  "_getOperatingSystemVerisonCode", referenced from:
      -[Manager isFeatureAvailable] in Manager.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

推荐答案

问题不是由宏定义引起的.

The problem is not caused by the macro definition.

getOperatingSystemVerisonCode()函数在".mm"文件中定义，因此 编译为Objective- C ++ .特别是，函数名称被改成C ++函数. 但是，从(Objective-)C来源引用时，应该使用不易混淆的名称.

The getOperatingSystemVerisonCode() function is defined in a ".mm" file and therefore compiled as Objective-C++. In particular, the function name is mangled as a C++ function. But when referenced from (Objective-)C sources, the unmangled name is expected.

您可以通过两种方法解决该问题:

You have two options to solve the problem:

• 将".mm"文件重命名为".m"，以便将其编译为Objective-C文件.

• Rename the ".mm" file to ".m", so that it is compiled as an Objective-C file.

在声明函数的头文件中，添加extern "C"声明以强制C链接，即使在(Objective-)C ++文件中也是如此:

In the header file where the function is declared, add the extern "C" declaration to enforce C linkage even in an (Objective-)C++ file:

#ifdef __cplusplus
extern "C" {
#endif

NSString* getOperatingSystemVerisonCode();

#ifdef __cplusplus
}
#endif

有关混合使用C和C ++的更多信息，请参见

For more information about mixing C and C++, see for example

• 结合C ++和C-#ifdef __cplusplus如何工作?

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