C++-style Casting ================= In C, you can cast in one of two ways: (type)var type(var) The problem with C-style casting is that it allows a programmer to get away with too much, and is also not designed to handle C++ classes. C++ has 4 types of casting in addition to allowing C-style casting. They are: static_cast const_cast dynamic_cast reinterpret_cast The syntax is usually *_cast(var). static_cast ----------- From my experience, this cast is closest to C-style casting for non-pointer types as well as between some (but not all) pointer types. This type of cast, like C-style casting, is performed at compile-time. static_cast can also do a downcast of a derived class to a base class, but only if the base class is not a virtual base class. Sometimes the result of this cast can become undefined. static_cast is a bit more strict that C-style casting, though. It disallows certain class conversions that would've been allowed with a C-style cast. static_cast also doesn't allow you to cast to an incomplete type. In these cases, I would try either dynamic_cast or reinterpret_cast. const_cast ---------- This cast is mainly to add or remove const-ness or volatile-ness from a variable. This is safer than using a C-style cast to change the const-ness of a variable. In most cases if you try to use one of the other casts and it complains about const-ness, you will want to either use this cast instead or wrap the other cast around this cast. An example: const int *a; static_cast(a); <-- This will fail. To remedy the above, you would might try this: const int *a; const_cast(a); <-- But this will still fail. The real solution is this: const int *a; static_cast(const_cast(a)); It is not recommended to use const_cast on the this variable within a member function of a class that is declared const. Instead you should use the mutable keyword on the variable in the class's definition. dynamic_cast ------------ This cast can only be used on pointers or references to classes. It can cast a derived class to a base class, a derived class to another derived class (provided that both are children of the same base class), or a base class to a derived class. You can also use this to cast a class to void *. This cast is done at run-time as opposed to the other casts, and relies on C++'s RTTI to be enabled. It is meant to be used on polymorphic classes, so use static_cast on non-polymorphic classes. derived-to-base conversions are actually done statically, so you use either dynamic_cast or static_cast on them, regardless of if the classes are polymorphic or not. derived-to-derived or base-to-derived conversions, however, rely on run-time type information, and this cast is used on those classes that are polymorphic. This is safer than C-style casting in that an invalid pointer conversion will return a NULL pointer, and an invalid reference conversion will throw a Bad_cast exception. Note that in Anope we prefer if Anope::debug_cast is used. This uses dynamic_cast (and checks for a NULL pointer return) on debug builds and static_cast on release builds, to speed up the program because of dynamic_cast's reliance on RTTI. reinterpret_cast ---------------- This cast I would use only as a last resort if static_cast isn't allowed on a conversion. It allows for conversions between two unrelated types, such as going from char * to int *. It can also be used to convert a pointer to an integral type and vica versa. The sites I've read mention how the result is non-portable, which I assume means the resulting object code is non-portable, so since the code is compiled on many systems anyways, I don't see this as being a huge issue. It is recommended to only use this if necessary, though. Links ===== The following links are web sites I've used to get this information, and might describe some of the above a bit better than I have. :P https://www.acm.org/crossroads/xrds3-1/ovp3-1.html http://www.cplusplus.com/doc/tutorial/typecasting.html http://www.codeguru.com/forum/showthread.php?t=312456 https://web.archive.org/web/20170810222238/http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/BitOp/cast.html https://www.microsoft.com/en-us/download/details.aspx?id=55984 https://en.wikibooks.org/wiki/C%2B%2B_Programming/Type_Casting https://web.archive.org/web/20160510114447/http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=134 -- CyberBotX, Nov 23, 2008