Static cast
The static cast performs conversions between compatible types. It is similar to the C-style cast, but is more restrictive. For example, the C-style cast would allow an integer pointer to point to a char.
char c = 10; // 1 byte
int *p = (int*)&c; // 4 bytes
Since this results in a 4-byte pointer pointing to 1 byte of allocated memory, writing to this pointer will either cause a run-time error or will overwrite some adjacent memory.
*p = 5; // run-time error: stack corruption
In contrast to the C-style cast, the static cast will allow the compiler to check that the pointer and pointee data types are compatible, which allows the programmer to catch this incorrect pointer assignment during compilation.
int *q = static_cast(&c); // compile-time error
Reinterpret cast
To force the pointer conversion, in the same way as the C-style cast does in the background, the reinterpret cast would be used instead.
int *r = reinterpret_cast(&c); // forced conversion
This cast handles conversions between certain unrelated types, such as from one pointer type to another incompatible pointer type. It will simply perform a binary copy of the data without altering the underlying bit pattern. Note that the result of such a low-level operation is system-specific and therefore not portable. It should be used with caution if it cannot be avoided altogether.
Dynamic cast
This one is only used to convert object pointers and object references into other pointer or reference types in the inheritance hierarchy. It is the only cast that makes sure that the object pointed to can be converted, by performing a run-time check that the pointer refers to a complete object of the destination type. For this run-time check to be possible the object must be polymorphic. That is, the class must define or inherit at least one virtual function. This is because the compiler will only generate the needed run-time type information for such objects.
Dynamic cast examples
In the example below, a MyChild pointer is converted into a MyBase pointer using a dynamic cast. This derived-to-base conversion succeeds, because the Child object includes a complete Base object.
class MyBase
{
public:
virtual void test() {}
};
class MyChild : public MyBase {};
int main()
{
MyChild *child = new MyChild();
MyBase *base = dynamic_cast(child); // ok
}
The next example attempts to convert a MyBase pointer to a MyChild pointer. Since the Base object does not contain a complete Child object this pointer conversion will fail. To indicate this, the dynamic cast returns a null pointer. This gives a convenient way to check whether or not a conversion has succeeded during run-time.
MyBase *base = new MyBase();
MyChild *child = dynamic_cast(base);
if (child == 0)
std::cout << "Null pointer returned";
If a reference is converted instead of a pointer, the dynamic cast will then fail by throwing a bad_cast exception. This needs to be handled using a try-catch statement.
#include
// …
try
{
MyChild &child = dynamic_cast(*base);
}
catch(std::bad_cast &e)
{
std::cout << e.what(); // bad dynamic_cast
}
Dynamic or static cast
The advantage of using a dynamic cast is that it allows the programmer to check whether or not a conversion has succeeded during run-time. The disadvantage is that there is a performance overhead associated with doing this check. For this reason using a static cast would have been preferable in the first example, because a derived-to-base conversion will never fail.
MyBase *base = static_cast(child); // ok
However, in the second example the conversion may either succeed or fail. It will fail if the MyBase object contains a MyBase instance and it will succeed if it contains a MyChild instance. In some situations this may not be known until run-time. When this is the case dynamic cast is a better choice than static cast.
// Succeeds for a MyChild object
MyChild *child = dynamic_cast(base);
If the base-to-derived conversion had been performed using a static cast instead of a dynamic cast the conversion would not have failed. It would have returned a pointer that referred to an incomplete object. Dereferencing such a pointer can lead to run-time errors.
// Allowed, but invalid
MyChild *child = static_cast(base);
// Incomplete MyChild object dereferenced
(*child);
Const cast
This one is primarily used to add or remove the const modifier of a variable.
const int myConst = 5;
int *nonConst = const_cast(&myConst); // removes const
Although const cast allows the value of a constant to be changed, doing so is still invalid code that may cause a run-time error. This could occur for example if the constant was located in a section of read-only memory.
*nonConst = 10; // potential run-time error
Const cast is instead used mainly when there is a function that takes a non-constant pointer argument, even though it does not modify the pointee.
void print(int *p)
{
std::cout << *p;
}
The function can then be passed a constant variable by using a const cast.
print(&myConst); // error: cannot convert
// const int* to int*
print(nonConst); // allowed
Source and More Explanations
