为什么没有开关指针?

问题描述

例如:

#include <stdio.h>

void why_cant_we_switch_him(void *ptr)
{
    switch (ptr) {
        case NULL:
            printf("NULL!
");
            break;
        default:
            printf("%p!
", ptr);
            break;
    }
}

int main(void)
{
    void *foo = "toast";
    why_cant_we_switch_him(foo);
    return 0;
}

gcc test.c -o test
test.c: In function 'why_cant_we_switch_him':
test.c:5: error: switch quantity not an integer
test.c:6: error: pointers are not permitted as case values

只是好奇.这是技术限制吗?

Just curious. Is this a technical limitation?

人们似乎认为只有一个常量指针表达式.但这真的是真的吗?例如，这是 Objective-C 中的一个常见范例(除了 NSString、id 和 nil 之外，它实际上只有 C，它们只是一个指针，所以它仍然是相关的——我只是想指出 是，事实上，它的一个常见用途，尽管这只是一个技术问题):

People seem to think there is only one constant pointer expression. Is that is really true, though? For instance, here is a common paradigm in Objective-C (it is really only C aside from NSString, id and nil, which are merely a pointers, so it is still relevant — I just wanted to point out that there is, in fact, a common use for it, despite this being only a technical question):

#include <stdio.h>
#include <Foundation/Foundation.h>

static NSString * const kMyConstantObject = @"Foo";

void why_cant_we_switch_him(id ptr)
{
    switch (ptr) {
        case kMyConstantObject: // (Note that we are comparing pointers, not string values.)
            printf("We found him!
");
            break;
        case nil:
            printf("He appears to be nil (or NULL, whichever you prefer).
");
            break;
        default:
            printf("%p!
", ptr);
            break;
    }
}

int main(void)
{
    NSString *foo = @"toast";
    why_cant_we_switch_him(foo);
    foo = kMyConstantObject;
    why_cant_we_switch_him(foo);

    return 0;
}

gcc test.c -o test -framework Foundation
test.c: In function 'why_cant_we_switch_him':
test.c:5: error: switch quantity not an integer
test.c:6: error: pointers are not permitted as case values

原因似乎是 switch 只允许整数值(正如编译器警告所说).所以我想一个更好的问题是问为什么会这样?(虽然现在可能为时已晚.)

It appears that the reason is that switch only allows integral values (as the compiler warning said). So I suppose a better question would be to ask why this is the case? (though it is probably too late now.)

推荐答案

Switch 语句只对整数值进行操作.这就是错误消息是开关量不是整数"的原因.我不认为这是一个技术限制，因为它超出了语言语法.

Switch statements operate on integral values only. That's why the error message is "switch quantity not an integer." I don't think it's a technical limitation so much as it's outside the language syntax.

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