static_cast and dynamic_cast?

[ajoo]

what is the difference between
static_cast<A*> (p)
and
(A*)p
?
does C++ put some type info in the memory for dynamic_cast to work?

[Kaye]

gz

[ajoo]

hey, Kaye(菜菜狗)
what does "gz" mean?
I'm a newbie of csdn.

[ajoo]

faint!
I should have searched csdn first.
seems static_cast supports downcast, but not stupidcast.
if A is not super type of B, B is not super type of A,
then it fails.
but does anybody know how dynamic_cast get the type info?

[saucer]

run-time type information (RTTI):
typeid, type_info

[ajoo]

well. conceptually, yes.
But how? if the pointer is pointing to the memory address, how can it know the real type of the object?

[saucer]

I could guess some additional bytes were allocated at that memory address which are RTTI related, for example, these bytes could point to some global type table

[ajoo]

the problem is, if you have a
const char* p = "hello"+1;
how can typeid(p) work? how does it know where to get the type info? p should definitely points to 'e' in this case.

[北极猩猩]

static_cast是在编译其进行类型校验的，因此只支持上溯造型。
dynamic_cast在运行时通过RTTI进行类型判断，可以进行下塑造性。
如：
class A{};
class B:public A{};

A* a=new A();
A* b=new B();
B* c;
c=static_cast<B*>(b) //error
c=dynamic_cast<B*>(b) //OK

如果dynamic_cast发现非法转换会返回NULL

[canco]

看《C++语言的设计和演化》关于“强制”的这一章，里面有完整的阐述。

[ajoo]

polarislee(北极星):
c=static_cast<B*>(b) should work.

[mylove0618]

dynamic_cast的目的是在类层次结构中提升一个对象，别提升的对象必须是被强制转换的类型的直接或间接继承。之所以称为提升，是因为在层次结构中，基类比较接近层次的根，因此强制转换成基类就是其在结构中向上移动。如果dynamic_cast使用引用而不是指针，eg:dynamic_cast<R&>(p).并且引用不是类型p的直接或间接类，那么dynamic_cast就会发生一个bad_cast异常。

[greening]

你可以看一下effective c++,四种转换。我想你就清楚很多了。
static_cast是最普通的转换，比如：int转换成double。
const_cast最普通的用途就是转换掉对象的const属性，详见第21条款。
dynamic_cast把指向基类的指针或引用转换成其派生类或兄弟类的指针或引用，详见第39条款。
reinterpret_cast最普通的用途就是在函数类型指针之间进行转换。

[ajoo]

I think i'm pretty clear about their usage now.
But what I'm wondering is that how dynamic_cast can know the type info at runtime. what is it's runtime scheme?

[SlaSk]

static_cast是在编译其进行类型校验的，因此只支持上溯造型。

补充：对于无法确定的转化就用它。

dynamic_cast在运行时通过RTTI进行类型判断，可以进行下塑造性。

[sbg]

static_cast是最普通的转换，之能用于内建类型。比如：int转换成double。
const_cast最普通的用途就是转换掉对象的const属性
dynamic_cast把指向基类的指针或引用转换成其派生类或兄弟类的指针或引用
reinterpret_cast最普通的用途就是在函数类型指针之间进行转换 最不常用

[caoxin]

how dynamic_cast can know the type info at runtime：

看《Inside C++ Object Model》书上有