在 C++ 的模板化函数中使用正确的字符串文字 [英] Use proper string literal in templated function in C++

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问题描述

我有一个函数,它被模板化以匹配每个 std::basic_string 实例化:

I have a function, that is templated to match every std::basic_string instantiation:

template <typename _valueType>
void addFoo(std::basic_string<_valueType>& string_)
{
    string_ += "foo";
}

我希望 "foo"_valueType 依赖的文字.我不想使用专业化,因为在实际项目中,我对整个类进行了模板化,这将是很多工作.
我目前在函数体中使用 if constexpr,但问题开始了,当我有这样的函数采用默认参数时:

I want "foo" to be _valueType-dependent literal. I would like not to use specialization, because in the actual project I have whole class templated and it would be a lot of work.
I am currently using if constexpr in function body, but problems start, when I have function taking default parameter like this:

template <typename _valueType>
void foo(_valueType const delimiter_ = '.');

推荐答案

我假设你只支持 charwchar_t.如果您想支持更长的枚举类型列表,您也可以这样做.这不适用于非本地枚举的类型列表.

I'll assume you only support char and wchar_t. If you want to support a longer enumerated list of types, you can also do that. This doesn't work for a non-locally enumerated list of types.

template<class Index, class...Args>
decltype(auto) dispatch( Index, Args&&... args ) {
  return std::get<Index{}>( std::forward_as_tuple( std::forward<Args>(args)... ) );
}

这是一个简洁的小助手,可让您在任意数量的参数之间进行编译时分派.

This is a neat little helper that lets you compile-time dispatch between any number of arguments.

现在在你的类中定义:

template<class Char, class WChar>
static decltype(auto) pick(Char&& c, WChar&& w) {
  using is_wchar_t = std::is_same<_valueType, wchar_t>;
  return dispatch( is_wchar_t{}, std::forward<Char>(c), std::forward<WChar>(w) );
}

您现在可以这样做:

template <typename _valueType>
void addFoo(std::basic_string<_valueType>& string_)
{
  string_ += pick( "foo", L"foo" );
}

如果你不喜欢 DRY 失败

if you dislike the DRY failure

#define BOTH_CHARTYPE(...) __VA_ARGS__, L __VA_ARGS__

template <typename _valueType>
void addFoo(std::basic_string<_valueType>& string_)
{
  string_ += pick( BOTH_CHARTYPE("foo") );
}

或类似的(可能需要更多的宏魔法才能使 L 正确附加到 "").

or somesuch (might need more macro magic to make the L attach to the "" properly).

这是写在 ,但它可以适用于 相对容易;将 Index{} 替换为 Index::value 并将 decltype(auto) 替换为 ->decltype()> 子句.

This is written in c++14, but it can be adapted for c++11 relatively easily; replace Index{} with Index::value and replace decltype(auto) with a ->decltype() clause.

template<class...Ts>
struct types { using type=types; };
template<std::size_t I>
using index_t=std::integral_constant<std::size_t, I>;

template<class T, class Types>
struct type_index;
template<class T, class...Ts>
struct type_index<T, types<T, Ts...>>:index_t<0> {};
template<class T, class T0, class...Ts>
struct type_index<T, types<T0, Ts...>>:index_t<
  type_index<T, types<Ts...>>{}+1
>{};

using char_types = types<char, wchar_t, char16_t, char32_t>;

template<class T>
using char_index = type_index<T, char_types >;

#define ALL_CHAR_TYPES(...) __VA_ARGS__, L __VA_ARGS__, u __VA_ARGS__, U __VA_ARGS__

template<class T, class...Chars>
decltype(auto) pick(Chars&&...chars) {
  return dispatch( char_index< T >{}, std::forward<Chars>(chars) );
}

void addFoo( std::basic_string<_valueType>& string_ ) {
  string_ += pick<_valueType>( ALL_CHAR_TYPES("foo") );
}

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