您不能在Objective-C类中声明类级变量;相反，你需要在实现文件中隐藏它们，通常给它们 static -scope，这样它们就无法从外部访问。

You cannot declare class-level variables in Objective-C classes; instead you need to "hide" them in the implementation file, often giving them static-scope so they cannot be accessed externally.

Connections.m：

Connections.m:

#import "Connections.h"

static Connections *_sharedInstance = nil;

@implementation Connections

...

@end

如果这是一个单例，你通常会定义一个类级访问器来在第一次使用时创建单例：

And if this is a singleton, you typically define a class-level accessor to create the singleton upon first use:

+ (Connections *)sharedInstance
{
    if (_sharedInstance == nil)
    {
        _sharedInstance = [[Connections alloc] init];
    }
    return _sharedInstance;
}

（你需要在.h文件中添加声明） ：

(and you'll need to add the declaration in the .h file):

+ (Connections *)sharedInstance;