[英]typedef'ng a pointer and const
I was looking at an example which showed that why typedef'ng a pointer is a bad practice. 我正在看一个例子,它表明为什么typedef'ng指针是一种不好的做法。 The part I didn't understand about the example is that why the compiler wasn't able to catch the problem.
我对该示例不了解的部分是为什么编译器无法捕获问题。 I elaborated the example into the following code:
我在下面的代码中详细说明了这个例子:
#include <stdio.h>
typedef int *TYPE;
void set_type(TYPE t) {
*t = 12;
}
void foo(const TYPE mytype) {
set_type(mytype); // Error expected, but in fact compiles
}
int main() {
TYPE a;
int b = 10;
a = &b;
printf("A is %d\n",*a);
foo(a);
printf("A is %d\n",*a);
return 0;
}
So, I was able to change the value of a
. 所以,我能够改变的值
a
。 Even though, the example explained that you would have to do, typedef const int *TYPE
, to solve the problem, I dont understand why the compiler was not able to catch the error when I am setting TYPE to be const
in the function argument itself. 即便如此,该示例解释说,你将不得不这样做,
typedef const int *TYPE
,要解决这个问题,我不明白为什么编译器没能赶上,当我设置错误TYPE to be const
在函数参数本身。 I am sure I am missing something very basic. 我确信我遗漏了一些非常基本的东西。
The issue here is confusion about what const
is being applied to: is it applied to the pointer value itself, or the value being pointed to? 这里的问题是关于
const
应用于什么的混淆:它是应用于指针值本身,还是应用于指向的值?
const TYPE x
is saying that the pointer value should be constant. const TYPE x
表示指针值应该是常量。 This is roughly equivalent to int * const x
. 这大致相当于
int * const x
。 Note that the following code does result in an error: 请注意,以下代码确实会导致错误:
int b = 10;
const TYPE p = NULL;
p = &b; // error here (assign to const)
What you were expecting is const int * x
, which makes the value pointed to by x a constant. 您期望的是
const int * x
,它使x指向的值为常量。
Since the typedef hides the fact that TYPE
is a pointer type, there's no way to specify that the thing pointed to by TYPE
should be const
, aside from declaring another typedef: 由于typedef隐藏了
TYPE
是指针类型的事实,除了声明另一个typedef之外,没有办法指定TYPE
指向的东西应该是const
:
typedef const int * CONST_TYPE;
However, at this point the typedef seems to be causing more trouble than it solves... Probably not the best idea. 然而,在这一点上,typedef似乎造成了比它解决的更多麻烦......可能不是最好的主意。
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