如何将NSString转换为char [英] How to convert an NSString to char

问题描述

我试图将 NSString 转换为 ResType ，如下面 MacTypes.h 。

I am trying to convert an NSString to a ResType, as defined below in MacTypes.h.

FourCharCode // A 32-bit value made by packing four 1 byte characters together
typedef FourCharCode ResType;

我想我可以使用 [aString getCharacters：range：]

I think I could use [aString getCharacters:range:], but is there a more straight forward way to make this conversion?

在尝试过大卫的建议之后，这里有一些更详细的信息。

After trying suggestions from David, here is some further information.

我使用GTResourceFork，一个Cocoa包装器访问资源分叉。我调用的方法是：
- （NSArray *）usedResourcesOfType：（ResType）type;

I am using GTResourceFork, a Cocoa wrapper for accessing resource forks. The method I am calling is: - (NSArray *)usedResourcesOfType: (ResType)type;

如果我硬编码'RTF'得到我期望的结果。我不知道如何将包含RTF的NSString转换为硬编码的值。我创建了一个测试用例使用NSString的getCharacters和getBytes，他们都给我不同的整数值。如何转换NSString给我一个与硬编码的整数值相同的整数值？

If I hard code a value of 'RTF ', I get the results I expect. I cannot figure out how to convert an NSString containing "RTF " to the hard-coded value. I created a test case using NSString's getCharacters and getBytes, and they all give me different integer values. How can I convert the NSString to give me the same integer value as the hard-coded one?

 Method used:       Value:  Casted int value:
 Hard Coded(Works): 'RTF ' '1381254688'
 getCharacters:     'RTF ' '5505106'
 getBytes(ASCII):   'RTF ' '541480018'
 getBytes(UTF8):    'RTF ' '541480018'

提前感谢
Lance

Thanks in advance, Lance

推荐答案

getCharacters：range：的问题是它给你的UTF -16个字符（ unichar s），而您想要ASCII。

The problem with getCharacters:range: is that it gives you UTF-16 characters (unichars), whereas you want ASCII.

* *）[aString UTF8String] 会将字符串转换为UTF-8（这相当于ASCII，只要字符串中的所有字符都符合ASCII范围），然后给你前四个字节作为 ResType 值。

*(ResType*)[aString UTF8String] will convert the string to UTF-8 (which is equivalent to ASCII as long as all the characters in the string fit within the ASCII range) and then give you the first four bytes as a ResType value. Whether this is efficient enough depends on how often you want to do these conversions.

另一个选择是使用 getBytes：maxLength：usedLength：encoding：选项：range：remainingRange：，编码设置为 NSASCIIStringEncoding 或 NSUTF8StringEncoding 缓冲区设置为指向现有 ResType 变量的指针，最大长度设置为 4 （或 sizeof（ResType））。

Another option is to use getBytes:maxLength:usedLength:encoding:options:range:remainingRange: with the encoding set to NSASCIIStringEncoding or NSUTF8StringEncoding, the destination buffer set to a pointer to an existing ResType variable, and the maximum length set to 4 (or sizeof (ResType)).

我想出了为什么你没有得到我的建议的正确结果。事实证明，在四字符整数文字中，字节存储的方式与它们的写法相反。这里有一个例子：

I've figured out why you're not getting the correct result with my suggestion. It turns out that in four-character integer literals, the bytes are stored in the opposite order to how they're written. Here's an example:

#include <Foundation/Foundation.h>

int main() {
    int code = 'RTF ';
    printf("'%c%c%c%c' = %d\n", ((char*)&code)[0], ((char*)&code)[1],
                                ((char*)&code)[2], ((char*)&code)[3],
                                code);
}

输出'FTR'= 1381254688 。所以，如果你想从 NSString s转换为这些值，这里有几个选项：

The output is ' FTR' = 1381254688. So, if you want to convert from NSStrings to these values, here are a few options:

• 将字符串复制到一个四字节缓冲区（使用我建议的方法之一），然后通过交换字节 0 与字节 3 ，以及字节 1 与字节 2 $ b
• 同样，但是使用标准的endianness-swapping算法（如 c>
使用 characterAtIndex：迭代第一个并将它们反向插入一个四字节缓冲区。请记住， characterAtIndex：返回一个UTF-16字符，但这可以很容易地转换为ASCII字符，假设它在ASCII范围内。

• Copy the string into a four-byte buffer (using one of the methods I suggested) and then reverse it by swapping byte 0 with byte 3, and byte 1 with byte 2.
• The same, but do the reversing using a standard "endianness-swapping" algorithm like the one on this page.
• Iterate through first (and only) four characters using characterAtIndex:, and insert them into a four-byte buffer in reverse. Remember that characterAtIndex: returns a UTF-16 character, but this can be easily cast to as ASCII character assuming it is within the ASCII range.

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