把pair的源码拷贝出来，起个新名字叫MyPair
template<class _Ty1,
class _Ty2>
struct MyPair
{ // store a MyPair of values
using first_type = _Ty1;
using second_type = _Ty2;
//
// 本文主要是改造这一行
//
MyPair& operator=(const volatile MyPair&) = delete;
template<class _Uty1 = _Ty1,
class _Uty2 = _Ty2,
enable_if_t<conjunction_v<
is_default_constructible<_Uty1>,
is_default_constructible<_Uty2>,
_Is_implicitly_default_constructible<_Uty1>,
_Is_implicitly_default_constructible<_Uty2>
>, int> = 0>
constexpr MyPair()
_NOEXCEPT_COND(is_nothrow_default_constructible_v<_Uty1>
&& is_nothrow_default_constructible_v<_Uty2>) // strengthened
: first(), second()
{
}
template<class _Uty1 = _Ty1,
class _Uty2 = _Ty2,
enable_if_t<conjunction_v<
is_default_constructible<_Uty1>,
is_default_constructible<_Uty2>,
negation<conjunction<
_Is_implicitly_default_constructible<_Uty1>,
_Is_implicitly_default_constructible<_Uty2>>>
>, int> = 0>
constexpr explicit MyPair()
_NOEXCEPT_COND(is_nothrow_default_constructible_v<_Uty1>
&& is_nothrow_default_constructible_v<_Uty2>) // strengthened
: first(), second()
{
}
template<class _Uty1 = _Ty1,
class _Uty2 = _Ty2,
enable_if_t<conjunction_v<
is_copy_constructible<_Uty1>,
is_copy_constructible<_Uty2>,
is_convertible<const _Uty1&, _Uty1>,
is_convertible<const _Uty2&, _Uty2>
>, int> = 0>
constexpr MyPair(const _Ty1& _Val1, const _Ty2& _Val2)
_NOEXCEPT_COND(is_nothrow_copy_constructible_v<_Uty1>
&& is_nothrow_copy_constructible_v<_Uty2>) // strengthened
: first(_Val1), second(_Val2)
{
}
template<class _Uty1 = _Ty1,
class _Uty2 = _Ty2,
enable_if_t<conjunction_v<
is_copy_constructible<_Uty1>,
is_copy_constructible<_Uty2>,
negation<conjunction<
is_convertible<const _Uty1&, _Uty1>,
is_convertible<const _Uty2&, _Uty2>>>
>, int> = 0>
constexpr explicit MyPair(const _Ty1& _Val1, const _Ty2& _Val2)
_NOEXCEPT_COND(is_nothrow_copy_constructible_v<_Uty1>
&& is_nothrow_copy_constructible_v<_Uty2>) // strengthened
: first(_Val1), second(_Val2)
{
}
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, _Other1>,
is_constructible<_Ty2, _Other2>,
is_convertible<_Other1, _Ty1>,
is_convertible<_Other2, _Ty2>
>, int> = 0>
constexpr MyPair(_Other1&& _Val1, _Other2&& _Val2)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>
&& is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
: first(_STD forward<_Other1>(_Val1)),
second(_STD forward<_Other2>(_Val2))
{
}
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, _Other1>,
is_constructible<_Ty2, _Other2>,
negation<conjunction<
is_convertible<_Other1, _Ty1>,
is_convertible<_Other2, _Ty2>>>
>, int> = 0>
constexpr explicit MyPair(_Other1&& _Val1, _Other2&& _Val2)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>
&& is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
: first(_STD forward<_Other1>(_Val1)),
second(_STD forward<_Other2>(_Val2))
{
}
MyPair(const MyPair&) = default;
MyPair(MyPair&&) = default;
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, const _Other1&>,
is_constructible<_Ty2, const _Other2&>,
is_convertible<const _Other1&, _Ty1>,
is_convertible<const _Other2&, _Ty2>
>, int> = 0>
constexpr MyPair(const MyPair<_Other1, _Other2>& _Right)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, const _Other1&>
&& is_nothrow_constructible_v<_Ty2, const _Other2&>) // strengthened
: first(_Right.first), second(_Right.second)
{
}
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, const _Other1&>,
is_constructible<_Ty2, const _Other2&>,
negation<conjunction<
is_convertible<const _Other1&, _Ty1>,
is_convertible<const _Other2&, _Ty2>>>
>, int> = 0>
constexpr explicit MyPair(const MyPair<_Other1, _Other2>& _Right)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, const _Other1&>
&& is_nothrow_constructible_v<_Ty2, const _Other2&>) // strengthened
: first(_Right.first), second(_Right.second)
{
}
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, _Other1>,
is_constructible<_Ty2, _Other2>,
is_convertible<_Other1, _Ty1>,
is_convertible<_Other2, _Ty2>
>, int> = 0>
constexpr MyPair(MyPair<_Other1, _Other2>&& _Right)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>
&& is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
: first(_STD forward<_Other1>(_Right.first)),
second(_STD forward<_Other2>(_Right.second))
{
}
template<class _Other1,
class _Other2,
enable_if_t<conjunction_v<
is_constructible<_Ty1, _Other1>,
is_constructible<_Ty2, _Other2>,
negation<conjunction<
is_convertible<_Other1, _Ty1>,
is_convertible<_Other2, _Ty2>>>
>, int> = 0>
constexpr explicit MyPair(MyPair<_Other1, _Other2>&& _Right)
_NOEXCEPT_COND(is_nothrow_constructible_v<_Ty1, _Other1>
&& is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
: first(_STD forward<_Other1>(_Right.first)),
second(_STD forward<_Other2>(_Right.second))
{
}
template<class _Tuple1,
class _Tuple2,
size_t... _Indexes1,
size_t... _Indexes2> inline
MyPair(_Tuple1& _Val1,
_Tuple2& _Val2,
index_sequence<_Indexes1...>,
index_sequence<_Indexes2...>);
template<class... _Types1,
class... _Types2> inline
MyPair(piecewise_construct_t,
tuple<_Types1...> _Val1,
tuple<_Types2...> _Val2);
template<class _Other1 = _Ty1,
class _Other2 = _Ty2,
enable_if_t<conjunction_v<
is_assignable<_Ty1&, const _Other1&>,
is_assignable<_Ty2&, const _Other2&>
>, int> = 0>
MyPair& operator=(const MyPair<_Other1, _Other2>& _Right)
_NOEXCEPT_COND(is_nothrow_assignable_v<_Ty1&, const _Other1&>
&& is_nothrow_assignable_v<_Ty2&, const _Other2&>) // strengthened
{
first = _Right.first;
second = _Right.second;
return (*this);
}
template<class _Other1 = _Ty1,
class _Other2 = _Ty2,
enable_if_t<conjunction_v<
is_assignable<_Ty1&, _Other1>,
is_assignable<_Ty2&, _Other2>
>, int> = 0>
MyPair& operator=(MyPair<_Other1, _Other2>&& _Right)
_NOEXCEPT_COND(is_nothrow_assignable_v<_Ty1&, _Other1>
&& is_nothrow_assignable_v<_Ty2&, _Other2>) // strengthened
{
first = _STD forward<_Other1>(_Right.first);
second = _STD forward<_Other2>(_Right.second);
return (*this);
}
void swap(MyPair& _Right)
_NOEXCEPT_COND(_Is_nothrow_swappable<_Ty1>::value
&& _Is_nothrow_swappable<_Ty2>::value)
{
if (this != _STD addressof(_Right))
{ // different, worth swapping
_Swap_adl(first, _Right.first);
_Swap_adl(second, _Right.second);
}
}
_Ty1 first; // the first stored value
_Ty2 second; // the second stored value
};测试代码：
MyPair<const int, int> t0;
MyPair<int, int> t1;
t0 = t1;
编译错误：
error C2679: 二进制“=”: 没有找到接受“MyPair<int,int>”类型的右操作数的运算符(或没有可接受的转换)
note: 可能是“MyPair<const int,int> &MyPair<const int,int>::operator =(volatile const MyPair<const int,int> &)”
note: 尝试匹配参数列表“(MyPair<const int,int>, MyPair<int,int>)”时尝试把delete的MyPair的operator=给个实现呢：
把MyPair& operator=(const volatile MyPair&)=delete;
改为：
MyPair& operator=(const volatile MyPair&)
{
return (*this);
}
编译错误为：
error C2679: 二进制“=”: 没有找到接受“MyPair<int,int>”类型的右操作数的运算符(或没有可接受的转换)
note: 可能是“MyPair<const int,int> &MyPair<const int,int>::operator =(volatile const MyPair<const int,int> &)”
note: 尝试匹配参数列表“(MyPair<const int,int>, MyPair<int,int>)”时
因为左边的MyPair实例化的是<const int,int>，而
MyPair& operator=(const volatile MyPair&)里括号里的MyPair里实例化的是<int,int>，
按照函数的写法，这两者应该一样。但此时不一样，所以编译错误。
下面编译成功。也就是把operator=采用模板函数，括号里的形参进行单独推导
把MyPair& operator=(const volatile MyPair&) = delete;
改为：
template<typename T, typename U>
MyPair& operator=(const volatile MyPair<T, U>& _Right)
{
//first = _Right.first;
//second = _Right.second;
return (*this);
}template<typename T, typename U>
MyPair& operator=(const volatile MyPair<T, U>& _Right)
{
//强行把this的first的const给去掉，就可以被参数给赋值了
using decayType = std::decay_t<_Ty1>;
decayType& _first = const_cast<decayType&>(first);
_first = _Right.first;
second = _Right.second;
return (*this);
}
测试代码：
编译通过，t0的内容就是(3,4)，被成功赋值
MyPair<const int, int> t0{1,2};
MyPair<int, int> t1{ 3,4 };
t0 = t1;
return 1;