C和C++中void指针的区别

Question

Why the following is wrong in C++ (But valid in C)

void*p;
char*s;
p=s;
s=p; //this is wrong ,should do s=(char*)p;

Why do I need the casting,as p now contains address of char pointer and s is also char pointer?

Answer 1

That's valid C, but not C++; they are two different languages, even if they do have many features in common.

In C++, there is no implicit conversion from void* to a typed pointer, so you need a cast. You should prefer a C++ cast, since they restrict which conversions are allowed and so help to prevent mistakes:

s = static_cast<char*>(p);

Better still, you should use polymorphic techniques (such as abstract base classes or templates) to avoid the need to use untyped pointers in the first place; but that's rather beyond the scope of this question.

Answer 2

The value doesn't matter, the type does. Since p is a void pointer and s a char pointer, you have to cast, even if they have the same value. In C it will be ok, void* is the generic pointer, but this is incorrect in C++.

By the way, p doesn't contains char pointer , it's a void pointer and it contains a memory address.

Answer 3

In general, this rule doesn't even have anything to do with pointers. It's just that you can assign values of some type to variables of other types, but not always vice versa. A similar situation would be this:

double d = 0.0;
int i = 0;

d = i;    // Totally OK
i = d;    // Warning!

So that's just something you have to live with.