重载返回值
06 Nov 2020
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本文是这篇文章的翻译整理
主要是这个场景
// this is OK
std::string to_string(int i);
std::string to_string(bool b);
std::string si = to_string(0);
std::string sb = to_string(true);
// this is not OK
int from_string(std::string_view s);
bool from_string(std::string_view s);
int i = from_string("7");
bool b = from_string("false");
想让返回值更统一
做法是做一个统一的返回类型，然后重载 类型操作符
struct to_string_t
{
std::string_view s;
operator int() const; // int from_string(std::string_view s);
operator bool() const; // bool from_string(std::string_view s);
};
int i = to_string_t{"7"};
bool b = to_string_t{"true"};
每个类型都要写？模版话，给内建类型定义好，自己的类型，自己用sfinae拼
// base template, specialize and provide a static from_string method
template <class T, class = void>
struct to_string_impl
{
};
namespace detail // hide impl detail
{
template <class T>
auto has_from_string(int) -> decltype(
to_string_impl<T>::from_string(std::declval<std::string_view>()),
std::true_type{});
template <class T>
std::false_type has_from_string(char);
}
// check if T has a from_string
template <class T>
constexpr bool has_from_string = decltype(detail::has_from_string<T>(0))::value;
// return-type overload mechanism
struct to_string_t
{
std::string_view s;
template <class T>
operator T() const
{
static_assert(has_from_string<T>, "conversion to T not supported");
return to_string_impl<T>::from_string(s);
}
};
// convenience wrapper to provide a "return-type overloaded function"
to_string_t from_string(std::string_view s) { return to_string_t{s}; }
//各种类型的实现自己拼好就可以了
template <>
struct to_string_impl<int>
{
static int from_string(std::string_view s);
};
template <>
struct to_string_impl<bool>
{
static bool from_string(std::string_view s);
};
//自定义实现
template <class T>
struct my_range { /* ... */ };
template <class T>
struct to_string_impl<my_range<T>, std::enable_if_t<has_from_string<T>>>
{
static my_range<T> from_string(std::string_view s);
};
就是偏特化+sfinae套路
