在objective c中什么地方声明常数?

I declared a constant in a header file const double EARTH_RADIUS=6353; that is imported to various other headers and I got a linker error.

我在头文件const双EARTH_RADIUS=6353中声明了一个常量;它被导入到不同的头文件中，我得到了一个链接器错误。

Ld /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator/BadgerNew.app/BadgerNew normal i386
    cd /Users/Teguh/Dropbox/badgers/BadgerNew
    setenv MACOSX_DEPLOYMENT_TARGET 10.6
    setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/llvm-g++-4.2 -arch i386 -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk -L/Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator -F/Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator -filelist /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/BadgerNew.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -framework CoreLocation -framework UIKit -framework Foundation -framework CoreGraphics -framework CoreData -o /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator/BadgerNew.app/BadgerNew

ld: duplicate symbol _EARTH_RADIUS in /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/NearbyIsiKota.o and /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/FrontPageofBadger.o for architecture i386
collect2: ld returned 1 exit status

Basically I want the constant to be available for all classes on my project. Where should I declare it?

基本上，我希望常数对我项目中的所有类都可用。我应该在哪里申报?

4 个解决方案

#1

You can declare in the header, define it in a code file. Simply declare it as

您可以在头中声明，在代码文件中定义它。简单的声明这是

extern const double EARTH_RADIUS;

then in a .m file somewhere (usually the .m for the .h you declared it in)

然后在某处的。m文件中(通常是。m代表你声明的。h)

const double EARTH_RADIUS = 6353;

#2

There are two ways to accomplish this:

有两种方法可以做到这一点:

1st option- As previous replies pointed, in the .h file:

第一个选项-正如之前的回复所指出的，在.h文件中:

myfile.h
extern const int MY_CONSTANT_VARIABLE;

and in myfile.m define them

myfile。m定义它们

myfile.m    
const int MY_CONSTANT_VARIABLE = 5;

2nd option- My favourite:

第二选项-我最喜欢的:

myfile.h
static const int MY_CONSTANT_VARIABLE = 5 ;

#3

Declare it in a source file and have external linkage to it( using extern keyword ) so as to use in all other source files.

在源文件中声明它，并与它有外部链接(使用extern关键字)，以便在所有其他源文件中使用。

#4

Best practice would be to declare it in your .h and .m files. See Constants in Objective-C for a very detailed set of answers regarding this same question.

最佳实践是在.h和.m文件中声明它。在Objective-C中看到关于这个相同问题的非常详细的答案集。

#1