Header
-boost/cast.hppHeader
-boost/cast.hppThe header boost/cast.hpp
-provides polymorphic_cast, polymorphic_downcast,
-and numeric_cast template functions designed
-to complement the C++ Standard's built-in casts.
The program cast_test.cpp can be used to -verify these function templates work as expected.
-polymorphic_cast was suggested by Bjarne Stroustrup in "The C++
-Programming Language".
-polymorphic_downcast was contributed by Dave
-Abrahams.
-numeric_cast was contributed by Kevlin
-Henney.
- namespace boost {
- namespace cast {
- // all synopsis below included here
- }
- using ::boost::cast::polymorphic_cast;
- using ::boost::cast::polymorphic_downcast;
- using ::boost::cast::bad_numeric_cast;
- using ::boost::cast::numeric_cast;
-}
-
-Pointers to polymorphic objects (objects of classes which define at least one -virtual function) are sometimes downcast or crosscast. Downcasting means -casting from a base class to a derived class. Crosscasting means casting -across an inheritance hierarchy diagram, such as from one base to the other in a -Y diagram hierarchy.
-Such casts can be done with old-style casts, but this approach is never to be -recommended. Old-style casts are sorely lacking in type safety, suffer -poor readability, and are difficult to locate with search tools.
-The C++ built-in static_cast can be used for efficiently downcasting -pointers to polymorphic objects, but provides no error detection for the case -where the pointer being cast actually points to the wrong derived class. The polymorphic_downcast -template retains the efficiency of static_cast for non-debug -compilations, but for debug compilations adds safety via an assert() that a dynamic_cast -succeeds.
-The C++ built-in dynamic_cast can be used for downcasts and crosscasts -of pointers to polymorphic objects, but error notification in the form of a -returned value of 0 is inconvenient to test, or worse yet, easy to forget to -test. The polymorphic_cast template performs a dynamic_cast, -and throws an exception if the dynamic_cast returns 0.
-A polymorphic_downcast is preferred when debug-mode tests will cover -100% of the object types possibly cast and when non-debug-mode efficiency is an -issue. If these two conditions are not present, polymorphic_cast is -preferred. It must also be used for crosscasts. It does an assert( -dynamic_cast<Derived>(x) == x ) where x is the base pointer, ensuring that -not only is a non-zero pointer returned, but also that it correct in the -presence of multiple inheritance. . Warning:: Because polymorphic_downcast -uses assert(), it violates the One Definition Rule if NDEBUG is inconsistently -defined across translation units.
-The C++ built-in dynamic_cast must be used to cast references rather -than pointers. It is also the only cast that can be used to check whether -a given interface is supported; in that case a return of 0 isn't an error -condition.
---template <class Derived, class Base> -inline Derived polymorphic_cast(Base* x); -// Throws: std::bad_cast if ( dynamic_cast<Derived>(x) == 0 ) -// Returns: dynamic_cast<Derived>(x) - -template <class Derived, class Base> -inline Derived polymorphic_downcast(Base* x); -// Effects: assert( dynamic_cast<Derived>(x) == x ); -// Returns: static_cast<Derived>(x)-
- #include <boost/cast.hpp>
-...
-class Fruit { public: virtual ~Fruit(){}; ... };
-class Banana : public Fruit { ... };
-...
-void f( Fruit * fruit ) {
-// ... logic which leads us to believe it is a Banana
- Banana * banana = boost::polymorphic_downcast<Banana*>(fruit);
- ...
-
-A static_cast, implicit_cast or implicit conversion will not -detect failure to preserve range for numeric casts. The numeric_cast -template function are similar to static_cast and certain (dubious) -implicit conversions in this respect, except that they detect loss of numeric -range. An exception is thrown when a runtime value preservation check fails.
-The requirements on the argument and result types are:
----
-- Both argument and result types are CopyConstructible [20.1.3].
-- Both argument and result types are Numeric, defined by
-std::numeric_limits<>::is_specialized- being true.- The argument can be converted to the result type using static_cast.
-
- class bad_numeric_cast : public std::bad_cast {...};
-
-template<typename Target, typename Source>
- inline Target numeric_cast(Source arg);
- // Throws: bad_numeric_cast unless, in converting arg from Source to Target,
- // there is no loss of negative range, and no underflow, and no
- // overflow, as determined by std::numeric_limits
- // Returns: static_cast<Target>(arg)
-
-
- #include <boost/cast.hpp>
-using namespace boost::cast;
-
-void ariane(double vx)
-{
- ...
- unsigned short dx = numeric_cast<unsigned short>(vx);
- ...
-}
-
-The form of the throws condition is specified so that != is not a required -operation.
-Revised 28 June, 2000
-© Copyright boost.org 1999. Permission to copy, use, modify, sell and -distribute this document is granted provided this copyright notice appears in -all copies. This document is provided "as is" without express or -implied warranty, and with no claim as to its suitability for any purpose.
- - - - diff --git a/cast_test.cpp b/cast_test.cpp deleted file mode 100644 index 1602f16..0000000 --- a/cast_test.cpp +++ /dev/null @@ -1,153 +0,0 @@ -// boost utility cast test program -----------------------------------------// - -// (C) Copyright boost.org 1999. Permission to copy, use, modify, sell -// and distribute this software is granted provided this copyright -// notice appears in all copies. This software is provided "as is" without -// express or implied warranty, and with no claim as to its suitability for -// any purpose. - -// See http://www.boost.org for most recent version including documentation. - -// Revision History -// 28 Jun 00 implicit_cast removed (Beman Dawes) -// 30 Aug 99 value_cast replaced by numeric_cast -// 3 Aug 99 Initial Version - -#include