在某些情况下，我们需要动态的向一个类插入一个实例方法（也可以是一个类方法）；这时我们可以用class_addMethod函数来完成:
BOOL class_addMethod ( Class cls, SEL name, IMP imp, const char *types ); 在Objective-C Runtime Reference 中可以看到各个参数的含义：
Parameters
**cls**
The class to which to add a method.
**name**
A selector that specifies the name of the method being added.
**imp**
A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
**types**
An array of characters that describe the types of the arguments to the method. For possible values, see Objective-C Runtime Programming Guide > Type Encodings. Since the function must take at least two arguments—self and _cmd, the second and third characters must be “@:” (the first character is the return type).前面几个参数比较好理解，我们来看看最后一个：这是一个字符串按照imp的签名依次排列返回值和各个参数。apple开发者官网有对应的编码：
Type Encodings
我们也可以用@encode来查看实际的编码，比如:
NSLog(@"%s",@encode(int *));