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boost::static_assert

原文地址：
http://www.smth.edu.cn/bbsanc.php?path=%2Fgroups%2Fdevelop.faq%2FProgramming%2Flanguages%2Fc_cpp%2Fboost%2FM.1021441404.O0

發信人: flier (小海 (:好日子不多了:)), 信區: Programming
標題: boost::static_assert
發信站: BBS 水木清華站 (Wed May 15 13:43:35 2002)

boost::static_assert

BOOST_STATIC_ASSERT的使用

BOOST_STATIC_ASSERT是一個簡單但常用的宏，顧名思義起到編譯期
斷言的功效，可以通過它，在編譯時對開發環境以及類型定義進行檢查。
此類型檢測對程序運行時無任何效率和空間上的影響。

  例如在namespace中加入

namespace my_conditions {
  BOOST_STATIC_ASSERT(sizeof(int) * CHAR_BIT >= 32);
  BOOST_STATIC_ASSERT(WCHAR_MIN >= 0);
} // namespace my_conditions

  確保int類型至少32位，wchar_t類型為unsigned。

  也可以在模板函數中檢測入口參數的類型是否符合要求

template <class RandomAccessIterator >
RandomAccessIterator foo(RandomAccessIterator from, RandomAccessIterator to)
{
   // this template can only be used with
   // random access iterators...
   typedef typename std::iterator_traits
     < RandomAccessIterator >::iterator_category cat;
   BOOST_STATIC_ASSERT((boost::is_convertible
     <cat, const std::random_access_iterator_tag&>::value));
   //
   // detail goes here...
   return from;
}

  確保foo函數的輸入迭代子符合random access iterator的concept

  此外還可以在模板類里面驗證模板參數

template <class UnsignedInt>
class myclass
{
private:
   BOOST_STATIC_ASSERT(sizeof(UnsignedInt) * CHAR_BIT >= 16);
   BOOST_STATIC_ASSERT(std::numeric_limits<UnsignedInt>::is_specialized
                        && std::numeric_limits<UnsignedInt>::is_integer
                        && !std::numeric_limits<UnsignedInt>::is_signed);
public:
   /* details here */
};

  通過以上示例，我們可以總結出BOOST_STATIC_ASSERT的基本使用規律

首先是全局上對編譯環境進行檢測，看是否符合設計開發時的假定，這對程序的
可移植性有很大幫助。其次是在模板類、模板函數中，對作為模板參數的類型
進行檢測，驗證其是否符合設計時假設。最后是能夠將gp中的concept概念貫徹
到代碼中，使代碼強制性與設計同步，并增強代碼可讀性。

BOOST_STATIC_ASSERT的實現

  在實現上，BOOST_STATIC_ASSERT宏利用c++規范中，對不完整類型
即不可實例化的類型，在對其進行sizeof運算時編譯錯誤的特性，完成功能
大概的簡化代碼如下

template <bool x> struct STATIC_ASSERTION_FAILURE;
template <> struct STATIC_ASSERTION_FAILURE<true>{};
template<int x> struct static_assert_test{};

#define BOOST_STATIC_ASSERT(B) \
  typedef static_assert_test<sizeof(STATIC_ASSERTION_FAILURE<B> > \
    boost_static_assert_typedef_ ## __LINE__

  注意為了能夠使用多個BOOST_STATIC_ASSERT，在類型命名時加入了
行號以區別。對namespace而言，因為同一namespace可能分布在多個不同的
頭文件中，而不同頭文件中可能在同一行使用BOOST_STATIC_ASSERT檢測，
所以必須用一個namespace把斷言檢測隔離開，如上面例子所示。

  此外，對于VC來說，在使用/ZI參數時，__LINE__宏會發生錯誤，
(參見MSDN中Q199057錯誤)，好在VC會忽略typedef重復定義。
對某些不支持typedef此類特性的編譯器，boost提供了使用enum的替代方案

附：boost/static_assert.hpp

//  (C) Copyright John Maddock 2000.
//  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:
   02 August 2000
      Initial version.
*/

#ifndef BOOST_STATIC_ASSERT_HPP
#define BOOST_STATIC_ASSERT_HPP

#include <boost/config.hpp>

#ifdef __BORLANDC__
//
// workaround for buggy integral-constant expression support:
#define BOOST_BUGGY_INTEGRAL_CONSTANT_EXPRESSIONS
#endif

namespace boost{

// HP aCC cannot deal with missing names for template value parameters
template <bool x> struct STATIC_ASSERTION_FAILURE;

template <> struct STATIC_ASSERTION_FAILURE<true>{};

// HP aCC cannot deal with missing names for template value parameters
template<int x> struct static_assert_test{};

}

//
// Implicit instantiation requires that all member declarations be
// instantiated, but that the definitions are *not* instantiated.
//
// It's not particularly clear how this applies to enum's or typedefs;
// both are described as declarations [7.1.3] and [7.2] in the standard,
// however some compilers use "delayed evaluation" of one or more of
// these when implicitly instantiating templates.  We use typedef declarations
// by default, but try defining BOOST_USE_ENUM_STATIC_ASSERT if the enum
// version gets better results from your compiler...
//
// Implementation:
// Both of these versions rely on sizeof(incomplete_type) generating an error
// message containing the name of the incomplete type.  We use
// "STATIC_ASSERTION_FAILURE" as the type name here to generate
// an eye catching error message.  The result of the sizeof expression is either
// used as an enum initialiser, or as a template argument depending which version
// is in use...
// Note that the argument to the assert is explicitly cast to bool using old-
// style casts: too many compilers currently have problems with static_cast
// when used inside integral constant expressions.
//
#if !defined(BOOST_BUGGY_INTEGRAL_CONSTANT_EXPRESSIONS) && !defined(__MWERKS__)
#ifndef BOOST_MSVC
#define BOOST_STATIC_ASSERT( B ) \
   typedef ::boost::static_assert_test<\
      sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >)>\
         BOOST_JOIN(boost_static_assert_typedef_, __LINE__)
#else
// __LINE__ macro broken when -ZI is used see Q199057
// fortunately MSVC ignores duplicate typedef's.
#define BOOST_STATIC_ASSERT( B ) \
   typedef ::boost::static_assert_test<\
      sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >)\
      > boost_static_assert_typedef_
#endif
#else
// alternative enum based implementation:
#define BOOST_STATIC_ASSERT( B ) \
   enum { BOOST_JOIN(boost_static_assert_enum_, __LINE__) \
      = sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >) }
#endif

#endif // BOOST_STATIC_ASSERT_HPP

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※ 來源:·BBS 水木清華站 bbs.edu.cn·[FROM: 202.114.32.225]

posted on 2007-12-14 10:02 金慶閱讀(1044) 評論(1) 編輯收藏引用所屬分類: 1. C/C++

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boost::static_assert

BOOST_STATIC_ASSERT的使用

BOOST_STATIC_ASSERT的實現

附：boost/static_assert.hpp

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