如何在使用NSString的方法中使用NSMutableString？

Question

NSScanner採用指向一個NSString：

NSMutableString *myString; 
... 
[scanner scanUpToCharactersFromSet:charSet intoString:&myString]; 
[myString appendString:@"hello"]; 

但MyString的是可變的，所以我得到一個錯誤，當我嘗試做以後追加，說我試圖變異的未mutatable。 我必須創建臨時副本並返回&，還是有更有效的方法來執行此操作？

Answer 1

要麼做

NSString *myString; //this is just a variable identifier, no object was created and assigned to it yet 
... 
//scanner will create a NSString and write it to myString. It won't know it is passed to a variable typed NSMutableString or NSString 
[scanner scanUpToCharactersFromSet:charSet intoString:&myString]; 

//instead of creating mutable string you can create a new immutable by appending a string 
myString = [myString stringByAppendingString:@"hello"]; 

或_{好：看下面的代碼片段}

NSMutableString *myString; //this is just a variable identifier, no object was created and assigned to it yet 
... 

//scanner will create a NSString and write it to myString. It won't know it is passed to a variable typed NSMutableString 
[scanner scanUpToCharactersFromSet:charSet intoString:&myString]; 
//myString contains a NSString now, not NSMutableString 

myString = [myString mutableCopy]; // make a mutable copy and assign it to the variable 
[myString appendString:@"hello"]; 

---

在第二種方法中，你將有一個短暫而NSMutable類型不一致變量將保存一個不可變的字符串。這絕對是一個缺陷。解決方案之一更清潔。如果你喜歡兩個，你應該引入第二個NSString變量，並將其傳遞給掃描器，並將其傳遞給你的NSMutableString。

NSString *tempString; 
NSMutableString *myString; 
... 

[scanner scanUpToCharactersFromSet:charSet intoString:&tempString]; 

myString = [tempString mutableCopy]; 
[myString appendString:@"hello"]; 

Answer 2

如果您需要重複即多次，只有一個可變的字符串做：

NSMutableString *mutableString = [NSMutableString string]; 
NSString *string = nil; 

[scanner scanUpToCharactersFromSet:charSet1 intoString:&string]; 
[mutableString appendString:string]; 

[scanner scanUpToCharactersFromSet:charSet2 intoString:&string]; 
[mutableString appendString:string]; 

//... 

無需將它們複製或做其他任何轉換。