C/C++中指針類型聲明: `*`的位置

時間 2020-02-20

標籤 c++ 指針類型聲明位置欄目 C&C++ 简体版

原文原文鏈接

引入

咱們用兩個簡單(?)的問題做爲引入，固然咱們整篇文章都會圍繞這兩個問題：html

下面四種類型聲明等價嗎？linux
- int* i;
- int *i;
- int * i;
- int*i;
下面四種類型聲明中，j擁有哪個數據類型？是int或者int的指針?數據庫
- int* i, j;
- int *i, j;
- int * i, j;
- int*i, j;

固然看到這裏，若是你已經有很是自信的答案，你固然能夠無視這一篇文章後面的部分。express

第一個問題

第一個問題的答案是:　四種指針的類型聲明的等價的。編程

很好，既然咱們知道它們等價，或許已經足夠了。可是咱們數據庫教授老是在課上提醒你們，知道區別並不足夠，還須要問本身其中的哪個更好，什麼狀況下用哪種方式。less

哪種寫法更好呢？

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

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.

Linux kernel coding style中的習慣規則是：this

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);

某種程度上它也擬合了C++之父Bjarne Stroustrup的想法。spa

第二個問題

第二個問題的答案是:　j的類型是int。指針

固然這問題是編譯器的行爲，或者語言自身的規定，很難去闡述行爲或規定背後的爲何。

咱們繼續看Bjarne Stroustrup's C++ Style and Technique FAQ，這部分緊接問題一中對應的引用：

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.

從他的這一部分文字中，很容易能夠看出咱們這文章中的第一個問題和第二個問題徹底是緊密結合。第二個問題中的四種寫法的確是等價的，可是無一能讓人輕鬆理解。從他的這一個觀點中：Declaring one name per declaration minimizes the problem，咱們能夠對這篇文章下一個主觀的結論。