boost is_arrary 源码分析

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最近再看boost的源代码，看到type_traits部分，现在拿出boost::is_array的源代码进行分析。boost的版本为1.42

boost::is_array的源码如下

// (C) Copyright Dave Abrahams, Steve Cleary, Beman Dawes, Howard// Hinnant & John Maddock 2000. // Use, modification and distribution are subject to the Boost Software License,// Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at// http://www.boost.org/LICENSE_1_0.txt).//// See http://www.boost.org/libs/type_traits for most recent version including documentation.// Some fixes for is_array are based on a newgroup posting by Jonathan Lundquist.#ifndef BOOST_TT_IS_ARRAY_HPP_INCLUDED#define BOOST_TT_IS_ARRAY_HPP_INCLUDED#include <boost/type_traits/config.hpp>#ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION# include <boost/type_traits/detail/yes_no_type.hpp># include <boost/type_traits/detail/wrap.hpp>#endif#include <cstddef>// should be the last #include#include <boost/type_traits/detail/bool_trait_def.hpp>namespace boost {#if defined( __CODEGEARC__ )BOOST_TT_AUX_BOOL_TRAIT_DEF1(is_array,T,__is_array(T))#elif !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)BOOST_TT_AUX_BOOL_TRAIT_DEF1(is_array,T,false)#if !defined(BOOST_NO_ARRAY_TYPE_SPECIALIZATIONS)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,is_array,T[N],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,is_array,T const[N],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,is_array,T volatile[N],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,is_array,T const volatile[N],true)#if !BOOST_WORKAROUND(__BORLANDC__, < 0x600) && !defined(__IBMCPP__) && !BOOST_WORKAROUND(__DMC__, BOOST_TESTED_AT(0x840))BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T[],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T const[],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T volatile[],true)BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T const volatile[],true)#endif#endif#else // BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATIONnamespace detail {using ::boost::type_traits::yes_type;using ::boost::type_traits::no_type;using ::boost::type_traits::wrap;template< typename T > T(* is_array_tester1(wrap<T>) )(wrap<T>);char BOOST_TT_DECL is_array_tester1(...);template< typename T> no_type is_array_tester2(T(*)(wrap<T>));yes_type BOOST_TT_DECL is_array_tester2(...);template< typename T >struct is_array_impl{ BOOST_STATIC_CONSTANT(bool, value = sizeof(::boost::detail::is_array_tester2( ::boost::detail::is_array_tester1( ::boost::type_traits::wrap<T>() ) )) == 1 );};#ifndef BOOST_NO_CV_VOID_SPECIALIZATIONSBOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_array,void,false)BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_array,void const,false)BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_array,void volatile,false)BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_array,void const volatile,false)#endif} // namespace detailBOOST_TT_AUX_BOOL_TRAIT_DEF1(is_array,T,::boost::detail::is_array_impl<T>::value)#endif // BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION} // namespace boost#include <boost/type_traits/detail/bool_trait_undef.hpp>#endif // BOOST_TT_IS_ARRAY_HPP_INCLUDED

现在一行一行的分析

14 #ifndef BOOST_TT_IS_ARRAY_HPP_INCLUDED
15 #define BOOST_TT_IS_ARRAY_HPP_INCLUDED
是防止此文件被多次include的－－可以现不关心

19 #ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
20 # include <boost/type_traits/detail/yes_no_type.hpp>
21 # include <boost/type_traits/detail/wrap.hpp>
22 #endif
23
24 #include <cstddef>
25
26 // should be the last #include
27 #include <boost/type_traits/detail/bool_trait_def.hpp>

BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION 是看你的编译器是不是支持偏特化的，现在的编译器都支持，所以不对它的实现作分析。

__CODEGEARC__ 是看编译器是否是borland的

最后看下来，也就

41 BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T[],true)
42 BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T const[],true)
43 BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T volatile[],true)
44 BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T const volatile[],true)
这四个宏是分析的重点

现在对41行进行分析。

41行的代码为 BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(typename T,is_array,T[],true)

122 #define BOOST_TT_AUX_BOOL_TRAIT_PARTIAL_SPEC1_1(param,trait,sp,C) /
123 template< param > struct trait< sp > /
124 BOOST_TT_AUX_BOOL_C_BASE(C) /
125 { /
126 BOOST_TT_AUX_BOOL_TRAIT_VALUE_DECL(C) /
127 }; /

有用到一下两个宏，分别如下

62 #ifndef BOOST_TT_AUX_BOOL_C_BASE
63 # define BOOST_TT_AUX_BOOL_C_BASE(C) : ::boost::integral_constant<bool,C>
64 #endif

和

51 #   define BOOST_TT_AUX_BOOL_TRAIT_VALUE_DECL(C) /
52     typedef ::boost::integral_constant<bool,C> base_; /
53     using base_::value; /

最后展开为

template<typename T> struct isarray<T[]>: ::boost::integral_constant<bool,true>

{

typedef ::boost::integral_constant<bool,true> base_;

using base_::value;

}

:boost::integral_constant的定义是

18 template <class T, T val>

20 struct integral_constant : public mpl::integral_c<T, val>
21 {
22 typedef integral_constant<T,val> type;
23 };
mpl::integral_c<T, val>的定义为

39 template< bool C >
40 struct integral_c<bool, C>
41 {
42     BOOST_STATIC_CONSTANT(bool, value = C);
43     typedef integral_c_tag tag;
44     typedef integral_c type;
45     typedef bool value_type;
46     operator bool() const { return this->value; }
47 };

BOOST_STATIC_CONSTANT(bool, value = C);的定义为

377 # ifdef BOOST_NO_INCLASS_MEMBER_INITIALIZATION
378 # define BOOST_STATIC_CONSTANT(type, assignment) enum { assignment }
379 # else
380 # define BOOST_STATIC_CONSTANT(type, assignment) static const type assignment
381 # endif

22 struct integral_c_tag { BOOST_STATIC_CONSTANT(int, value = 0); };

现在又对boost::integral_constant<bool,true>展开为

struct integral_constant: public mpc::integral_<bool,true>

{

typedef integral_constant<bool,true> type;

}

integral_constant<bool,true> 展开为

struct integral_c<bool, true>
{

    static const bool value = true;
    typedef static const int value = 0 tag;
    typedef integral_c type;
    typedef bool value_type;
     operator bool() const { return this->value; }
};

最后结合isarray<T[]>的展开，可以发现，boost判定是否为array的方法是用到了模板偏特化。

但是boost实现isarrary的判断更加严谨，具有夸平台的功能，并且对多种编译器进行实现，真是好东西呀。