C/C++中指针类型声明: `*`的位置

引入

我们用两个简单(?)的问题作为引入，当然我们整篇文章都会围绕这两个问题：

下面四种类型声明等价吗？
- int* i;
- int *i;
- int * i;
- int*i;
下面四种类型声明中，j拥有哪一个数据类型？是int或者int的指针?
- int* i, j;
- int *i, j;
- int * i, j;
- int*i, j;

当然看到这里，如果你已经有非常自信的答案，你当然可以无视这一篇文章后面的部分。

第一个问题

第一个问题的答案是:　四种指针的类型声明的等价的。

很好，既然我们知道它们等价，或许已经足够了。但是我们数据库教授总是在课上提醒大家，知道区别并不足够，还需要问自己其中的哪一个更好，什么情况下用哪一种方式。

哪一种写法更好呢？

C++之父在他的一篇文章：Bjarne Stroustrup's C++ Style and Technique FAQ中提到：

The choice between int* p; and int *p; is not about right and wrong, but about style and emphasis. C emphasized expressions; declarations were often considered little more than a necessary evil. C++, on the other hand, has a heavy emphasis on types.
A typical C programmer writes int *p; and explains it *p is what is the int emphasizing syntax, and may point to the C (and C++) declaration grammar to argue for the correctness of the style. Indeed, the * binds to the name p in the grammar.
A typical C++ programmer writes int* p; and explains it p is a pointer to an int emphasizing type. Indeed the type of p is int*. I clearly prefer that emphasis and see it as important for using the more advanced parts of C++ well.

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Linux kernel coding style中的习惯规则是：

When declaring pointer data or a function that returns a pointer type, the
preferred use of * is adjacent to the data name or function name and not
adjacent to the type name. Examples:

char *linux_banner;
unsigned long long memparse(char *ptr, char **retptr);
char *match_strdup(substring_t *s);

某种程度上它也拟合了C++之父Bjarne Stroustrup的想法。

第二个问题

第二个问题的答案是:　j的类型是int。

当然这问题是编译器的行为，或者语言自身的规定，很难去阐述行为或规定背后的为什么。

我们继续看Bjarne Stroustrup's C++ Style and Technique FAQ，这部分紧接问题一中对应的引用：

The critical confusion comes (only) when people try to declare several pointers with a single declaration:
int* p, p1; // probable error: p1 is not an int*
Placing the * closer to the name does not make this kind of error significantly less likely.
int *p, p1; // probable error?
Declaring one name per declaration minimizes the problem - in particular when we initialize the variables. People are far less likely to write:
int* p = &i;
int p1 = p; // error: int initialized by int*
And if they do, the compiler will complain.

从他的这一部分文字中，很容易可以看出我们这文章中的第一个问题和第二个问题完全是紧密结合。第二个问题中的四种写法的确是等价的，但是无一能让人轻松理解。从他的这一个观点中：Declaring one name per declaration minimizes the problem，我们可以对这篇文章下一个主观的结论。