Regular cast vs. static_cast vs. dynamic_cast

static_cast

static_cast is used for cases where you basically want to reverse an implicit conversion, with a few restrictions and additions. static_cast performs no runtime checks. This should be used if you know that you refer to an object of a specific type, and thus a check would be unnecessary. Example:

void func(void *data) {
  // Conversion from MyClass* -> void* is implicit
  MyClass *c = static_cast<MyClass*>(data);
  ...
}

int main() {
  MyClass c;
  start_thread(&func, &c)  // func(&c) will be called
      .join();
}

In this example, you know that you passed a MyClass object, and thus there isn't any need for a runtime check to ensure this.

dynamic_cast

dynamic_cast is useful when you don't know what the dynamic type of the object is. It returns a null pointer if the object referred to doesn't contain the type casted to as a base class (when you cast to a reference, a bad_cast exception is thrown in that case).

if (JumpStm *j = dynamic_cast<JumpStm*>(&stm)) {
  ...
} else if (ExprStm *e = dynamic_cast<ExprStm*>(&stm)) {
  ...
}

You can not use dynamic_cast for downcast (casting to a derived class) if the argument type is not polymorphic. For example, the following code is not valid, because Base doesn't contain any virtual function:

struct Base { };
struct Derived : Base { };
int main() {
  Derived d; Base *b = &d;
  dynamic_cast<Derived*>(b); // Invalid
}

An "up-cast" (cast to the base class) is always valid with both static_cast and dynamic_cast, and also without any cast, as an "up-cast" is an implicit conversion (assuming the base class is accessible, i.e. it's a public inheritance).

Regular Cast

These casts are also called C-style cast. A C-style cast is basically identical to trying out a range of sequences of C++ casts, and taking the first C++ cast that works, without ever considering dynamic_cast. Needless to say, this is much more powerful as it combines all of const_cast, static_cast and reinterpret_cast, but it's also unsafe, because it does not use dynamic_cast.

In addition, C-style casts not only allow you to do this, but they also allow you to safely cast to a private base-class, while the "equivalent" static_cast sequence would give you a compile-time error for that.

Some people prefer C-style casts because of their brevity. I use them for numeric casts only, and use the appropriate C++ casts when user defined types are involved, as they provide stricter checking.

You should look at the article C++ Programming/Type Casting.

It contains a good description of all of the different cast types. The following taken from the above link:

const_cast

const_cast(expression) The const_cast<>() is used to add/remove const(ness) (or volatile-ness) of a variable.

static_cast

static_cast(expression) The static_cast<>() is used to cast between the integer types. 'e.g.' char->long, int->short etc.

Static cast is also used to cast pointers to related types, for example casting void* to the appropriate type.

dynamic_cast

Dynamic cast is used to convert pointers and references at run-time, generally for the purpose of casting a pointer or reference up or down an inheritance chain (inheritance hierarchy).

dynamic_cast(expression)

The target type must be a pointer or reference type, and the expression must evaluate to a pointer or reference. Dynamic cast works only when the type of object to which the expression refers is compatible with the target type and the base class has at least one virtual member function. If not, and the type of expression being cast is a pointer, NULL is returned, if a dynamic cast on a reference fails, a bad_cast exception is thrown. When it doesn't fail, dynamic cast returns a pointer or reference of the target type to the object to which expression referred.

reinterpret_cast

Reinterpret cast simply casts one type bitwise to another. Any pointer or integral type can be casted to any other with reinterpret cast, easily allowing for misuse. For instance, with reinterpret cast one might, unsafely, cast an integer pointer to a string pointer.

FYI, I believe Bjarne Stroustrup is quoted as saying that C-style casts are to be avoided and that you should use static_cast or dynamic_cast if at all possible.

Barne Stroustrup's C++ style FAQ

Take that advice for what you will. I'm far from being a C++ guru.

Avoid using C-Style casts.

C-style casts are a mix of const and reinterpret cast, and it's difficult to find-and-replace in your code. A C++ application programmer should avoid C-style cast.

C-style casts conflate const_cast, static_cast, and reinterpret_cast.

I wish C++ didn't have C-style casts. C++ casts stand out properly (as they should; casts are normally indicative of doing something bad) and properly distinguish between the different kinds of conversion that casts perform. They also permit similar-looking functions to be written, e.g. boost::lexical_cast, which is quite nice from a consistency perspective.

dynamic_cast has runtime type checking and only works with references and pointers, whereas static_cast does not offer runtime type checking. For complete information, see the MSDN article static_cast Operator.