Hi all,
i want to cast from upper class pointer to lower class i.e from the base class pointer to derived class pointer.
Should i use "Dynamic_cast" or "reinterpret_cast"? please advice me
views:
237answers:
4
+11
A:
Don't use reinterpret_cast - either use static_cast or dynamic_cast. If you're sure that the pointer is to exactly that derived class object use static_cast, otherwise use dynamic_cast (that will require the base class to be polymorhic) and check the result to ensure that the pointer is indeed to the class you want.
sharptooth
2010-05-03 13:01:10
you mean, for upward casting as well as downward casting, static_cast is better? or when do i have to use static_cast or dynamic_cast?
Shadow
2010-05-03 13:29:54
You don't need to use a cast at all for "upward casting". `Derived *dptr = something; Base *bptr = dptr;` works fine.
Steve Jessop
2010-05-03 13:34:02
Ok, fine.. if i am sure that i am going to do upper cast, i will not use any casting, for downward casting always i preffer static_cast..but what is the use of Dynamic_cast? when i have to use "reinterprit_cast too?
Shadow
2010-05-03 13:37:10
`dynamic_cast` is for when you don't know whether the thing pointed to by your `Base*` really is a `Derived` object, and you want to find out (and get a `Derived*` pointer if so). It's rarely needed. `reinterpret_cast` should never be used for casting between related pointer types. In certain circumstances (such as multiple inheritance) it gives the "wrong" answer, for instance gives you a `Derived*` pointer that will crash if you use it as a `Derived*`.
Steve Jessop
2010-05-03 13:45:32
In case of upcast the implicit conversion that "just works" is in fact the static_cast - see this great answer http://stackoverflow.com/questions/2198255/which-kind-of-cast-is-from-type-to-void/2205137#2205137.
sharptooth
2010-05-03 13:48:22
@sharptooth: well, it has the same effect as the static_cast, because static_cast is defined to be able to perform standard conversions: compare 4.10/3 with 5.2.9/2. The reason I say you don't need a cast is just that a cast is by definition an explicit conversion, and this is an implicit conversion. But yes, static_casts between types that can be implicitly converted, and implicit conversions, are in fact the same thing.
Steve Jessop
2010-05-03 13:56:05
+6
A:
It depends on what you know about the pointer.
If you know for sure that the pointer you have is of a child type, you can static_cast<> it safely.
If you don't know for sure, just go with dynamic_cast<> but this has a cost and is slower.
ereOn
2010-05-03 13:01:44
A:
To add to Ereon's answer: And if you do not know the type use dynamic_cast to do a run time type check by catching the bad_cast exception.
RdM
2010-05-03 13:15:02
+2
A:
reinterpret_cast<> is a last resort and should be avoided.
In Qt, if casting between QObjects, use qobject_cast<>. It's semantically like dynamic_cast but has no issues with dll boundaries etc.
Frank
2010-05-03 13:27:44