2-0. 編寫一個一元元函數add_const_ref，如果T是一個引用類型，就返回T，否則返回T
const&。編寫一個程序來測試你的元函數。提示：可以使用boost::is_same來測試結果。
2-1. 編寫一個三元元函數replace_type
，它接收一個任意的復合類型c作為其第一個參數，
並將c中出現的所有type x替換為y：
typedef replace_type< void*, void, int >::type t1; // int*
typedef replace_type<
int const*[10]
, int const
, long
>::type t2; // long* [10]
typedef replace_type<
char& (*)(char&)
, char&
, long&
>::type t3; // long& (*)(long&)
你可以將所操作的函數類型限制為具有少於兩個參數的函數。
這個比較復雜：
分為4步解決
1. 首先判斷c中是否含有類型x
2.如果有替換
3沒有就返回元類型
4考慮一些模板特化
template
struct is_same: boost::mpl::bool_
{
};//#1
template
struct is_same : boost::mpl::bool_
{
};//#1
template
struct replace_type_imp;//#2
template
struct replace_type
{
static bool const value = is_same::value;//#3
typedef typename replace_type_imp::type type;//#4
};
#1判斷兩個類型時候為同一個類型 如果是返回true，否則返回false
#2replace_type的具體實現，包含一個特化情況
#3返回時候相同value記錄返回結果
#4根據vlaue的值返回類型
下面是一般類型的實現：
////特化void
//TC:void const*, TX:void const
template
struct replace_type_imp
{
typedef typename replace_type::type type();
};
//特化TC*
//TC:int const*, TX:int const
template
struct replace_type_imp
{
typedef typename replace_type::type* type;
};
//特化TC&
//TC::int const&, TX:int const
template
struct replace_type_imp
{
typedef typename replace_type::type& type;
};
//特化TC[]
//TC::int const[], TX:int const
template
struct replace_type_imp
{
typedef typename replace_type::type type[];
};
//特化TC[N]
//TC::int const[N], TX:int const
template
struct replace_type_imp
{
typedef typename replace_type::type type[N];
};
函數的特化
//接受一個參數
//TC:: char* (*)(char*), TX: char*, TY: int
template
struct replace_type_imp
{
typedef typename replace_type::type type(typename replace_type::type);//#1
//#1處怎麼能定義成一個 函數指針
};
//接受兩個參數
//TC:: char* (*)(char*, const char* ), TX: char*, TY:: int
template
struct replace_type_imp
{
typedef typename replace_type::type type(typename replace_type::type,
typename replace_type::type);
};
//接受三個函數
template
struct replace_type_imp
{
typedef typename replace_type::type type(typename replace_type::type,
typename replace_type::type,
typename replace_type::type);
};
//...接受任一多參數
value 為true：
template
struct replace_type_imp
{
typedef TY type;
};
最後測試
void fun_is_same()
{
typedef char& (*FunPoint[])(char&);
typedef char& Rchar;
typedef const int v1;
typedef const int v2;
typedef int* v3;
typedef int* (*IntPoint[])(int*);
if (is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
if (boost::is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
typedef replace_type::type v4;
if (is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
if (boost::is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
typedef replace_type::type v5;
if (is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
if (boost::is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
typedef replace_type::type v6;
if (is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
if (boost::is_same::value) std::cout << "same\n\n";
else std::cout << "false\n\n" << std::endl;
}
2-2. boost::polymorphic_downcast
函數模板
實現一個帶檢查版本的static_cast，用於將指向多態
對象的指針向下轉型：
template inline Target polymorphic_downcast(Source* x)
{
assert( dynamic_cast(x) == x );
return static_cast(x);
}
在發行版的軟件中，assertion消失並且polymorphic_downcast可以和簡單的static_cast一樣高效。使用該type traits設施來編寫一個模板實現品，使其既可接收指針參數也可接收引用參數：
struct A { virtual ~A() {} };
struct B : A {};
B b;
A* a_ptr = &b;B* b_ptr = polymorphic_downcast(a_ptr);A& a_ref = b;
B& b_ref = polymorphic_downcast(a_ref);
這個題沒有理解題意，不會做，囧~~~~~~
2-3. 使用type traits
設施實現一個type_descriptor類模板，當被流化（streamed）時，其實例打印
其模板參數的類型：
std::cout<()；//打印“char*”；
std::cout<()；//打印“longconst*&”；
你可以假定type_descriptor的模板參數局限於
根據以下四種整型構建的復合類型：char、short int、int以及long int。
這個題比較簡單，不說了直接貼代碼：
template
struct get_description
{
static std::string value;
operator const char*()
{
return value.c_str();
}
};
template
std::string get_description::value = "can not deduce the type";
template<>
std::string get_description::value = "int";
template<>
std::string get_description::value = "char";
template<>
std::string get_description::value = "short";
template<>
std::string get_description::value = "long";
template<>
std::string get_description::value = "float";
template<>
std::string get_description::value = "double";
template
struct get_description
{
operator const char*()
{
static std::string ret = get_description();
ret += " const";
return ret.c_str();
}
};
template
struct get_description
{
operator const char*()
{
static std::string ret = get_description();
ret += " *";
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
static std::string ret = get_description();
ret += " &";
return ret.c_str();
}
};
測試代碼：
void fun_type_descriptor()
{
std::cout << get_description() << std::endl;
std::cout << get_description()<< std::endl;
//std::cout << get_description()<< std::endl;
}
2-5. 修改練習2-3中的type_descriptor
模板，使其輸出type的偽英語描述，就像cdecl程序的
explain命令所做的那樣:
//打印“array of pointer to function returning pointer to char”
std::cout << type_descriptor< char *(*[])() >();
這個題也比較簡單，直接上代碼：
template
struct get_description
{
static std::string value;
operator const char*()
{
return value.c_str();
}
};
template
std::string get_description::value = "can not deduce the type";
template<>
std::string get_description::value = "int";
template<>
std::string get_description::value = "char";
template<>
std::string get_description::value = "short";
template<>
std::string get_description::value = "long";
template<>
std::string get_description::value = "float";
template<>
std::string get_description::value = "double";
template
struct get_description
{
operator const char*()
{
static std::string ret = get_description();
ret += " const";
return ret.c_str();
}
};
template
struct get_description
{
operator const char*()
{
static std::string ret = get_description();
ret += " volatile";
return ret.c_str();
}
};
template
struct get_description
{
operator const char*()
{
static std::string ret = "pointer to ";
ret += get_description();
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
static std::string ret = "reference to ";
ret += get_description();
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
std::cout << typeid(T).name() << "\n";
static std::string ret = "array of ";
ret += get_description();
std::cout << typeid(T).name() << "\n";
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
std::cout << typeid(T).name() << "\n";
static std::string ret = "array of ";
ret += get_description();
std::cout << typeid(T).name() << "\n";
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
static std::string ret = "pointer to function returning ";
ret += get_description();
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
static std::string ret = "this is a pointer function with ";
ret += get_description();
ret += " and it pointer to function returning ";
ret += get_description();
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
std::cout << typeid(T).name() << "\n";
static std::string ret = "pointer to function with ";
ret += get_description();
ret += " returning ";
ret += get_description();
std::cout << typeid(T).name() << "\n";
return ret.c_str();
}
};
template
struct get_description
{
operator const char* ()
{
std::cout << typeid(T).name() << "\n";
static std::string ret = "this is a pointer function with ";
ret += get_description();
ret += " and it pointer to function with ";
ret += get_description();
ret += " returning ";
ret += get_description();
std::cout << typeid(T).name() << "\n";
return ret.c_str();
}
};
測試代碼：
void fun_text_descriptor()
{
//std::cout << get_description() << std::endl;
typedef int (*FunPoint[10])(char*);
std::cout << get_description()<< std::endl;
std::cout << get_description() << std::endl;
}
好了就做了這幾個題！！！！
