在 switch 语句中使用字符串――我们在 C++17 中的立场是什么?
我们每个人(可能)都有儿时写作的梦想:
Every one of us has (probably) had the childhood dream of writing:
switch(my_std_string) {
case "foo": do_stuff(); break;
case "bar": do_other_stuff(); break;
default: just_give_up();
}
但这是不可能的，正如过去(2009 年)对这个问题的回答中所解释的那样:
but this is not possible, as is explained in the answers to this question from the olden days (2009):
为什么switch语句不能应用于字符串?
从那时起，我们见证了 C++11 的出现，它让我们走得更远:
Since then we've seen the advent of C++11, which lets us go as far as:
switch (my_hash::hash(my_std_string)) {
case "foo"_hash: do_stuff(); break;
case "bar"_hash: do_other_stuff(); break;
default: just_give_up();
}
如 answer 中所述，编译时间字符串散列 - 这还不错，虽然它实际上并没有完全按照我们的意愿做 - 有可能发生冲突.
as described in an answer to Compile time string hashing - which is not so bad, although it doesn't actually do exactly what we wanted - there's a chance of collision.
我的问题是:从那时起该语言的发展(我想主要是 C++14)是否影响了人们编写某种字符串 case 语句的方式?还是简化了实现上述目标的具体细节?
My question is: Has the development of the language since then (mostly C++14 I suppose) affected the way one would write a sort-of-a string case statement? Or simplified the nuts-and-bolts for achieving the above?
具体来说，以 结尾C++17 标准 指日可待 - 我在鉴于我们可以假设标准将包含的内容，对答案感兴趣.
Specifically, with the conclusion of the C++17 standard being just around the corner - I'm interested in the answer given what we can assume the standard will contain.
推荐答案
我的建议可以使用 C++14，但使用 if constexpr 和 std::string_view写起来有点麻烦.
My proposal is possible with C++14, but with if constexpr and std::string_view it is a little esier to write.
首先 - 我们需要 constexpr 字符串 - 像这样:
First - we need constexpr string - like this one:
template <char... c>
using ConstString = std::integer_sequence<char, c...>;
template <char ...c>
constexpr auto operator ""_cstr ()
{
return ConstString<c...>{};
}
operator == 也更容易使用 tuple 的无模板构造以及 tuple 现在具有 constexpr 运营商==:
operator == is also easier to write with template-less construction of tuple and with the fact that tuple has now constexpr operator ==:
template <char... c1, char ...c2>
constexpr bool operator == (ConstString<c1...>, ConstString<c2...>)
{
if constexpr (sizeof...(c1) == sizeof...(c2)) // c++17 only
{
return tuple{c1...} == tuple{c2...}; // c++17 only
}
else
{
return false;
}
}
接下来 - 定义 switch-case 代码:
Next thing - define switch-case code:
template <typename Callable, typename Key>
class StringSwitchCase;
template <typename Callable, char ...c>
struct StringSwitchCase<Callable, ConstString<c...>>
{
constexpr bool operator == (const std::string_view& str) // c++17 only
{
constexpr char val[] = {c..., ''};
return val == str;
}
Callable call;
static constexpr ConstString<c...> key{};
};
template <typename Callable, char ...c>
constexpr auto makeStringSwitchCase(CString<c...>, Callable call)
{
return StringSwitchCase<Callable, ConstString<c...>>{call};
}
还需要默认情况:
template <typename Callable>
struct StringSwitchDefaultCase
{
constexpr bool operator == (const std::string_view&)
{
return true;
}
Callable call;
};
template <typename Callable>
constexpr auto makeStringSwitchDefaultCase(Callable call)
{
return StringSwitchDefaultCase<Callable>{call};
}
所以，StringSwitch - 实际上，它是 if () {} else if () {} ... else {} 构造:
So, the StringSwitch - actually, it is if () {} else if () {} ... else {} construction:
template <typename ...Cases>
class StringSwitch
{
public:
StringSwitch(Cases&&... cases) : cases(std::forward<Cases>(cases)...) {}
constexpr auto call(const std::string_view& str)
{
return call<0u>(str);
}
private:
template <std::size_t idx>
constexpr auto call(const std::string_view& str)
{
if constexpr (idx < sizeof...(Cases))
{
if (std::get<idx>(cases) == str)
{
return std::get<idx>(cases).call();
}
return call<idx + 1>(str);
}
else
{
return;
}
}
std::tuple<Cases...> cases;
};
以及可能的用法:
StringSwitch cstrSwitch(
makeStringSwitchCase(234_cstr,
[] {
cout << "234
";
}),
makeStringSwitchCase(ConstString<'a', 'b', 'c'>{}, // only C++ standard committee know why I cannot write "abc"_cstr
[] {
cout << "abc
";
}),
makeStringSwitchDefaultCase([] {
cout << "Default
";
}));
cstrSwitch.call("abc"s);
工作演示.
基于此 ，我设法以更简单的方式执行 ConstString发布.工作 demo2.
添加部分如下:
#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/comma_if.hpp>
#define ELEMENT_OR_NULL(z, n, text) BOOST_PP_COMMA_IF(n) (n < sizeof(text)) ? text[n] : 0
#define CONST_STRING(value) typename ExpandConstString<ConstString<BOOST_PP_REPEAT(20, ELEMENT_OR_NULL, #value)>,
ConstString<>, sizeof(#value) - 1>::type
template <typename S, typename R, int N>
struct ExpandConstString;
template <char S1, char ...S, char ...R, int N>
struct ExpandConstString<ConstString<S1, S...>, ConstString<R...>, N> :
ExpandConstString<ConstString<S...>, ConstString<R..., S1>, N - 1>
{};
template <char S1, char ...S, char ...R>
struct ExpandConstString<ConstString<S1, S...>, ConstString<R...>, 0>
{
using type = ConstString<R...>;
};
通过更改 BOOST_PP_REPEAT(20, ELEMENT_OR_NULL, #value) 中的第一个参数 (20) 我们可以控制 ConstString 的最大可能大小- 用法如下:
By changing first parameter (20) in BOOST_PP_REPEAT(20, ELEMENT_OR_NULL, #value) we can control the maximum possible size of ConstString - and the usage is as follows:
int main() {
StringSwitch cstrSwitch(
makeStringSwitchCase(CONST_STRING(234){},
[] {
cout << "234
";
}),
makeStringSwitchCase(CONST_STRING(abc){},
[] {
cout << "abc
";
}),
makeStringSwitchDefaultCase([] {
cout << "Default
";
}));
cstrSwitch.call("abc"s);
}
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