c++類型轉換（英文）

dynamic_cast< T > (arg)

Description

In the expression, dynamic_cast< T > (ptr), T must be a pointer or a reference to a defined class type or void*. The argument ptr must be an expression that resolves to a pointer or reference.

If T is void* then ptr must also be a pointer. In this case, the resulting pointer can access any element of the class that is the most derived element in the hierarchy. Such a class cannot be a base for any other class.

Conversions from a derived class to a base class, or from one derived class to another, are as follows: if T is a pointer and ptr is a pointer to a non-base class that is an element of a class hierarchy, the result is a pointer to the unique subclass. References are treated similarly. If T is a reference and ptr is a reference to a non-base class, the result is a reference to the unique subclass.

A conversion from a base class to a derived class can be performed only if the base is a polymorphic type.

The conversion to a base class is resolved at compile time. A conversion from a base class to a derived class, or a conversion across a hierarchy is resolved at runtime.

If successful, dynamic_cast< T > (ptr) converts ptr to the desired type. If a pointer cast fails, the returned pointer is valued 0. If a cast to a reference type fails, the Bad_cast exception is thrown.

Note: Runtime type identification (RTTI) is required for dynamic_cast.

static_cast< T > (arg)

Description

In the statement, static_cast< T > (arg), T must be a pointer, reference, arithmetic type, or enum type. The arg-type must match the T-type. Both T and arg must be fully known at compile time.

If a complete type can be converted to another type by some conversion method already provided by the language, then making such a conversion by using static_cast achieves exactly the same thing.

Integral types can be converted to enum types. A request to convert arg to a value that is not an element of enum is undefined.

The null pointer is converted to itself.

A pointer to one object type can be converted to a pointer to another object type. Note that merely pointing to similar types can cause access problems if the similar types are not similarly aligned.

You can explicitly convert a pointer to a class X to a pointer to some class Y if X is a base class for Y. A static conversion can be made only under the following conditions:

if an unambiguous conversion exists from Y to X
 if X is not a virtual base class

An object can be explicitly converted to a reference type X& if a pointer to that object can be explicitly converted to an X*. The result of the conversion is an lvalue. No constructors or conversion functions are called as the result of a cast to a reference.

An object or a value can be converted to a class object only if an appropriate constructor or conversion operator has been declared.

A pointer to a member can be explicitly converted into a different pointer-to-member type only if both types are pointers to members of the same class or pointers to members of two classes, one of which is unambiguously derived from the other.

When T is a reference the result of static_cast< T > (arg) is an lvalue. The result of a pointer or reference cast refers to the original expression.

reinterpret_cast< T > (arg)

Description

In the statement, reinterpret_cast< T > (arg), T must be a pointer, reference, arithmetic type, pointer to function, or pointer to member.

A pointer can be explicitly converted to an integral type.

An integral arg can be converted to a pointer. Converting a pointer to an integral type and back to the same pointer type results in the original value.

A yet undefined class can be used in a pointer or reference conversion.

A pointer to a function can be explicitly converted to a pointer to an object type provided the object pointer type has enough bits to hold the function pointer. A pointer to an object type can be explicitly converted to a pointer to a function only if the function pointer type is large enough to hold the object pointer.