Pointer to `void`

Generic interface and type erasure and incomplete type

正如creference Pointers to void中所言：

Pointers to void are used to pass objects of unknown type, which is common in generic interfaces

显然pointer to void是C中实现**generic programming**的方式，这种generic是通过**type erasure**来实现的：

1、void是type-less、是incomplete type，programmer无法直接操作void pointer，必须要先将它static_cast or explicit cast为complete type才能够进行操作；

关于incomplete type，参见 Incomplete-type 章节，在其中讨论了delete void pointer的内容。

2、Pointer to object of any type can be implicitly converted to pointer to void，在这个过程中，就丢失了type，显然这就是type erasure，关于type erasure，参见 Type-Erasure 章节。

cppreference Pointer to `void`

Pointer to object of any type can be implicitly converted to pointer to void (optionally cv-qualified); the pointer value is unchanged. The reverse conversion, which requires static_cast or explicit cast, yields the original pointer value:

NOTE: 上述的conversion，其实就是type erasure，是C中实现generic programming的重要手段。

NOTE: 下面展示的就是static_cast的用法

#include <iostream>

int main()
{
    int n = 1;
    int* p1 = &n;
    void* pv = p1;
    int* p2 = static_cast<int*>(pv);
    std::cout << *p2 << '\n'; // prints 1
}

If the original pointer is pointing to a base class subobject within an object of some polymorphic type, dynamic_cast may be used to obtain a void* that is pointing at the complete object of the most derived type.

NOTE: 这一点就是在cppreference dynamic_cast 中提及的4)

Pointers to void are used to pass objects of unknown type, which is common in C interfaces: std::malloc returns void*, std::qsort expects a user-provided callback that accepts two const void* arguments. pthread_create expects a user-provided callback that accepts and returns void*. In all cases, it is the caller's responsibility to cast the pointer to the correct type before use.

stackoverflow What does void mean in C, C++, and C#?

A

Basically it means "nothing" or "no type"

There are 3 basic ways that void is used:

1、Function argument: int myFunc(void) -- the function takes nothing.

2、Function return value: void myFunc(int) -- the function returns nothing

3、Generic data pointer: void* data -- 'data' is a pointer to data of unknown type, and cannot be dereferenced

NOTE: 所以，it is the caller's responsibility to convert the pointer to the correct type before use.

Note: the void in a function argument is optional in C++, so int myFunc() is exactly the same as int myFunc(void), and it is left out completely in C#. It is always required for a return value.

`void` pointer and alignment

alignment如何保证？

错误观点: void*应该具备最大的alignment，这样才能够保证它能够容纳下所有类型的pointer。

正确理解: void*类型的变量用于保存pointer的值，由于它是incomplete type，所以programmer无法直接对其进行操作（直接+、*一个void*，编译器会报错的，这样的program是无法编译通过的），programmer必须先将它转换为complete type pointer。由于无法直接使用void*，所以考虑考虑它的alignment是没有意义的。需要考虑的是：void*的值是否满足它的destination type的alignment，如果不满足，则是会发生undefined behavior的。

Pointer to void VS pointer to `char`

前面已经描述了pointer to void了。另外一种非常常见的pointer就是pointer to char，两者存在着相似点：

通用性：

pointer type	通用性的体现	目的
`void*`	Pointer to object of any type can be implicitly converted to pointer to `void`	实现generic programming
`char*`	`char`与value representation对应，并且它拥有最小的alignment requirement，所以任何类型的pointer也都可以转换为`char`，从而获得所指向的object的value representation	获得value representation

最能够体现两者之间关联的例子是：std::memcpy、std::memmove，解释如下：

两者的入参类型都是void*，显然这样实现了generic，这些函数内部实现中，都需要执行:

objects are reinterpreted as arrays of unsigned char.

然后按照byte( unsigned char)进行操作。

Examples

Plugin system

int FUNCTION_CALL_MODE HQInit(void* lpInstance, PFOnSetTimeOut pfOnSetTimeOut)
{
    int iRet = I_NONE;
    if (lpInstance)
        iRet = ((CHQImpl*) lpInstance)->OnInit();
    return iRet;
}

redis linked list

typedef struct listNode {
    struct listNode *prev;
    struct listNode *next;
    void *value;
} listNode;

节点的数据类型是pointer，因为redis中的所有的数据都是来自于网络，都是从接收到的数据new出一片空间的；

在networking.c中有如下函数：

/* This function links the client to the global linked list of clients.
 * unlinkClient() does the opposite, among other things. */
void linkClient(client *c) {
    listAddNodeTail(server.clients,c);
    /* Note that we remember the linked list node where the client is stored,
     * this way removing the client in unlinkClient() will not require
     * a linear scan, but just a constant time operation. */
    c->client_list_node = listLast(server.clients);
    uint64_t id = htonu64(c->id);
    raxInsert(server.clients_index,(unsigned char*)&id,sizeof(id),c,NULL);
}

listAddNodeTail函数的原型如下：

list *listAddNodeTail(list *list, void *value)

显然在linkClient函数中，涉及了从client *到void *类型的转换

参见Implicit conversions，其中指出：

A pointer to void can be implicitly converted to and from any pointer to object type with the following semantics:

If a pointer to object is converted to a pointer to void and back, its value compares equal to the original pointer.

No other guarantees are offered