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A comparison of the C++ casting operators

In addition to the C-style casting operator (provided for backwards compatibility) the C++ standard defines four additional casting operators:

The purpose of these new casting operators is to provide better type checking. Their use is encouraged over the old C-style casting operator.

Deficiencies of the old C-style cast

Two forms of the C-style cast are supported in C++:

  int age = (int) sqrt( foo / 3.25 );
  int age = int( sqrt(foo/3.25 ) );

However, using the same syntax for a variety of different casting operations can make the intent of the programmer unclear.

Furthermore, it can be difficult to find a specific type of cast in a large codebase.

Finally, the generality of the C-style cast can be overkill for situations where all that is needed is a simple conversion. The ability to select between several different casting operators of differing degrees of power can prevent programmers from inadvertently casting to an incorrect type.

Different operators for different uses

The four casting operators in C++ can be used in different cases, where each is most appropriate:

static_cast is the most useful cast. It can be used to perform any implicit cast. When an implicit conversion loses some information, some compilers will produce warnings, and static_cast will eliminate these warnings. Making implicit conversion through static_cast is also useful to resolve ambiguity or to clarify the conversion presence. It also can be used to call a unary constructor, declared as explicit. It also can be used to cast up and down a class hierarchy, like dynamic_cast, except that no runtime checking is performed.

const_cast is used to apply or remove const or volatile qualifier from a variable.

dynamic_cast is used on polymorphic pointers or references to move up or down a class hierarchy. Note that dynamic_cast performs runtime-checks: if the object's type is not the one expected, it will return NULL during a pointer-cast and throw a std::bad_cast exception during a reference-cast.

reinterpret_cast is used to perform conversions between unrelated types, like conversion between unrelated pointers and references or conversion between an integer and a pointer.

Old-style cast may correspond to static_cast, reinterpret_cast or const_cast, or even a combination of them. This means that none of these casting operators is as powerful as old-style cast.

Related links: http://www.acm.org/crossroads/xrds3-1/ovp3-1.html

 
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