From 23e5f7cca38eb3487b0a527abe017c09456f4782 Mon Sep 17 00:00:00 2001 From: Darin Adler Date: Wed, 6 Feb 2002 19:42:04 +0000 Subject: [PATCH] New casts for smart pointers. [SVN r12743] --- cast.htm | 29 +++++++++++++++++------------ 1 file changed, 17 insertions(+), 12 deletions(-) diff --git a/cast.htm b/cast.htm index d64473b..9122c01 100644 --- a/cast.htm +++ b/cast.htm @@ -1,3 +1,5 @@ + + @@ -9,46 +11,49 @@ -

c++boost.gif (8819 bytes)Header +

c++boost.gif (8819 bytes)Header boost/cast.hpp

+

Cast Functions

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The header boost/cast.hpp + +

The header boost/cast.hpp provides polymorphic_cast, polymorphic_downcast, and numeric_cast function templates designed to complement the C++ built-in casts.

-

The program cast_test.cpp can be used to + +

The program cast_test.cpp can be used to verify these function templates work as expected.

Polymorphic casts

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 +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 +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.  

+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, +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( +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 (ODR) if NDEBUG is inconsistently -defined across translation units.  [See ISO Std 3.2]

+defined across translation units. [See ISO Std 3.2]

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 +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.

polymorphic_cast and polymorphic_downcast synopsis

@@ -132,7 +137,7 @@ Abrahams.
numeric_cast was contributed by Kevlin Henney.

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Revised  06 January, 2001

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