C++ Explicit type conversions
Introduction
An expression can be explicitly converted or cast to type T using dynamic_cast<T>, static_cast<T>, reinterpret_cast<T>, or const_cast<T>, depending on what type of cast is intended.
C++ also supports function-style cast notation, T(expr), and C-style cast notation, (T)expr.
Syntax
- simple-type-specifier
() - simple-type-specifier
(expression-list) - simple-type-specifier braced-init-list
- typename-specifier
() - typename-specifier
(expression-list) - typename-specifier braced-init-list
dynamic_cast<type-id>(expression)static_cast<type-id>(expression)reinterpret_cast<type-id>(expression)const_cast<type-id>(expression)(type-id)cast-expression
Remarks
All six cast notations have one thing in common:
- Casting to an lvalue reference type, as in
dynamic_cast<Derived&>(base), yields an lvalue. Therefore, when you want to do something with the same object but treat it as a different type, you would cast to an lvalue reference type. - Casting to an rvalue reference type, as in
static_cast<string&&>(s), yields an rvalue. - Casting to a non-reference type, as in
(int)x, yields a prvalue, which may be thought of as a copy of the value being cast, but with a different type from the original.
The reinterpret_cast keyword is responsible for performing two different kinds of "unsafe" conversions:
- The "type punning" conversions, which can be used to access memory of one type as though it is of a different type.
- Conversions between integer types and pointer types, in either direction.
The static_cast keyword can perform a variety of different conversions:
-
Base to derived conversions
-
Any conversion that can be done by a direct initialization, including both implicit conversions and conversions that call an explicit constructor or conversion function. See here and here for more details.
-
To
void, which discards the value of the expression.// on some compilers, suppresses warning about x being unused static_cast<void>(x); -
Between arithmetic and enumeration types, and between different enumeration types. See enum conversions
-
From pointer to member of derived class, to pointer to member of base class. The types pointed to must match. See derived to base conversion for pointers to members
C++11
- From an lvalue to an xvalue, as in
std::move. See move semantics.
