C + + Primer learning notes and thinking _7 void and void* pointers usage

(i) the meaning of void
Void literally means "no type", and void is almost only "annotated" and restricts the function of the program, since no one will ever define a void variable, let's try to define:
void A;
This line of statement compiles with an error, prompting "illegal use of type ' void '". However, even if the compilation of void A does not go wrong, it does not have any practical significance.
Void really plays a role in:
(1) The qualification of function return;

(2) The qualification of function parameters.

int f (void); equal to   int f (); void f ();

(ii) void* hands
C + + provides a special pointer type void*, which can hold the address of any type of object, void* indicates that the pointer is related to an address value, but it is unclear what type of object is on this address. The void* pointer supports only a few limited operations:
1. Compare with another pointer
2. Passing a void* pointer to a function or returning a void* pointer from a function
3. Assign a value to another void* pointer
4. Do not allow to manipulate the object it points to with the void* pointer
Because other pointers contain address information, assigning the values of other pointers to null type pointers is legal, whereas assigning null-type pointers to other pointers is not allowed unless explicitly converted.
Thus, the role of void* is broadly as follows:
1. Reference: Common Type
Can be used as a function template, linked list parameters such as the general parameters. When you use it, you only need to force the type conversion.
In WinSocket programming, the parameter of the _beginthread () function is an instance of using void*.
2. Forcing type conversions
Sometimes due to overloading and other disturbances, resulting in the need to convert to void *, to take the address.
For example, (void *) Obj.member, the address of member can be taken, and the direct & (Obj.member) is actually the starting address of obj.
3. Point to the 0 address
(void *) 0, pointing to an address that is all 0, equivalent to NULL.
A non-void type is explicitly converted to a void type expression, and is used to avoid warnings from some code static checker tools.

#include <iostream> #include <string>using namespace Std;int main () {double Obj=3.14;int a=7;double *pd= &obj;void *pv=&obj;  OK pv=&a;  OK     (*PV) + +;//error, do not manipulate object int *ptr=&a;void *p=ptr;//okptr=p;  

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C + + Primer learning notes and thinking _7 void and void* pointers usage