static NSString * const testString = @"google";
//表示testString这个指针不能被修改,如若对testString赋值则会报错:testString = @"hello";编译器会报错
static NSString const *testString = @"google";
//表示testString指向对对象不能被修改,testString指针本身可以被修改:此时 testString = @"hello"; 编译器不会报错
http://stackoverflow.com/questions/6828831/sending-const-nsstring-to-parameter-of-type-nsstring-discards-qualifier