英文:
Splitting consteval from non-consteval expressions in parameter packs
问题
我的目标是使用 C++20 编写一个新的格式库，该库可以写入一个通常如下所示的日志行：
Log(Error,If(result == false),Title("Operation failed"),"Failed to execute task, ID: {}",taskId);
它接受：
- 任意数量的 "日志特性"（
Error、If、Title），在运行时进行评估 - 格式化字符串 + 任意数量的参数作为消息，使用
std::format_to进行格式化
问题是，我想使用 std::format_to 进行其编译时格式检查，这要求字符串 "Failed to execute task, ID: {}"" 必须保持为 consteval，以便将其转换为 std::format_string<...>，但我不希望每个 "日志特性" 也都是 consteval，因为有些需要在运行时评估，比如 If。
我的当前尝试是定义一个 LogTrait 概念，所有 Error、If 和 Title 都遵循：
template<typename T> concept LogTrait = ...
我用它来尝试跳过所有特性，直接到达格式化字符串：
template<typename... Args>consteval auto GetFormatString(std::format_string<Args...> fmt, Args&&... args){return std::make_pair(fmt, std::make_format_args(std::forward<Args>(args)...));}template<typename... Args>constexpr auto GetFormatString(LogTrait auto&&, Args&&... args){return GetFormatString(std::forward<Args>(args)...);}
但如果我将这两个函数都设为 consteval，会出错，因为 If 不能在编译时解析，如果我将任一函数仅设置为 constexpr，那么格式化字符串就无法转换为 std::format_string，因为它只能在 consteval 构造时转换。
有什么想法吗？
英文:
My goal is to use C++20 to write a new format library, which can write a log line that generally looks like this:
Log(Error,If(result == false),Title("Operation failed"),"Failed to execute task, ID: {}",taskId);
Which takes:
- Any number of "Log Traits" (
Error,If,Title), to be evaluated at runtime - A format string + any number of args as the message, to be formatted using
std::format_to
The issue is that I want to use std::format_to for its compile type format checking, which requires that the string "Failed to execute task, ID: {}" must remain consteval for it to be turned into a std::format_string<...>, but I don't want every "Log Trait" to also be consteval, because some need to be evaluated at runtime, such as If.
My current attempt has me defining a LogTrait concept, which Error, If and Title all adhere to:
template<typename T> concept LogTrait = ...
That I use to try to skip all the traits, and get right to the format string:
template<typename... Args>consteval auto GetFormatString(std::format_string<Args...> fmt, Args&&... args){return std::make_pair(fmt, std::make_format_args(std::forward<Args>(args)...));}template<typename... Args>constexpr auto GetFormatString(LogTrait auto&&, Args&&... args){return GetFormatString(std::forward<Args>(args)...);}
But if I make both functions consteval, it'll error because If can't be resolved at compile time, and if I make either function merely constexpr, then the format string can't be converted to std::format_string since it can only be consteval constructed.
Any thoughts?
答案1
得分: 4
以下是翻译好的部分：
最简单的方法是通过更改调用`Log`函数的方式来绕过此问题：```c++Log.Error.If(result == false).Title("Operation failed")("Failed to execute task, ID: {}", taskId);Log(Error,If(result == false),Title("Operation failed"))("Failed to execute task, ID: {}",taskId);// 或者创建一个新的“LogTrait”来保存格式字符串及其参数Log(Error,If(result == false),Title("Operation {} failed", name), // 这也应该是一个格式字符串Body("Failed to execute task, ID: {}", taskId));
尽管保持当前的语法是可能的，但也很困难。一旦您将"Failed to execute task, ID: {}"传递为const char(&)[N]或const char*参数，您就无法构建一个format_string（因为它不能是consteval），并且使用vformat会导致类型安全性丧失，因此您必须采取其他措施。
因此，您必须使Log成为一个模板结构，以便可以推断参数的类型，然后推断Log结构的类型，以便您的format_string可以具有正确的类型。
假设这是您要调用的最终函数：
template<LogTrait... Traits, typename... Args>void do_log(std::tuple<Traits...> traits, std::format_string<Args...> fmt, Args&&... args) {// ...}
您可以像这样实现Log：
template<typename Traits, typename Args>struct Log;template<typename... Traits, typename... Args>struct Log<std::tuple<Traits...>, std::tuple<Args...>>{Log(const Log&) = delete;Log& operator=(const Log&) = delete;~Log() = default;Log(Traits&&... traits, std::format_string<Args...> fmt, Args&&... args) {do_log(std::forward_as_tuple(std::forward<Traits>(traits)...), fmt, std::forward<Args>(args)...);}};template<std::size_t Offset, typename... Args, std::size_t... I>auto tuple_index(std::index_sequence<I...>) -> std::tuple<std::tuple_element_t<Offset + I, std::tuple<Args...>>...>;template<std::size_t N, typename... Args>using tuple_of_first = decltype(tuple_index<0, Args...>(std::make_index_sequence<N>{}));template<std::size_t N, typename... Args>using tuple_of_last = decltype(tuple_index<sizeof...(Args)-N, Args...>(std::make_index_sequence<N>{}));// 第一个非LogTrait参数的索引，即格式字符串template<typename... Args>inline constexpr std::size_t format_string_index = []<std::size_t... I>(std::index_sequence<I...>) {constexpr std::size_t format_string_index = std::min({ LogTrait<Args> ? std::size_t(-1) : I... });static_assert(format_string_index != std::size_t(-1), "Log(...): No format string");return format_string_index;}(std::index_sequence_for<Args...>{});template<typename... Args>struct traits_argument {using type = tuple_of_first<format_string_index<Args...>, Args...>;};template<typename... Args>struct format_argument {using type = tuple_of_last<sizeof...(Args) - 1u - format_string_index<Args...>, Args...>;};template<typename... Args>Log(Args&&... args) -> Log<typename traits_argument<Args...>::type, typename format_argument<Args...>::type>;
https://godbolt.org/z/EbrYheYPW
<details><summary>英文:</summary>The easiest way would be to sidestep this by changing how you call the `Log` function:```c++Log.Error.If(result == false).Title("Operation failed")("Failed to execute task, ID: {}", taskId);Log(Error,If(result == false),Title("Operation failed"))("Failed to execute task, ID: {}",taskId);// Or make a new "LogTrait" to hold the format string with it's argsLog(Error,If(result == false),Title("Operation {} failed", name), // This should also be a format stringBody("Failed to execute task, ID: {}", taskId));
It is possible, albeit difficult, to maintain your current syntax. Once you've passed "Failed to execute task, ID: {}" as a const char(&)[N] or const char* argument, you can't construct a format_string (since it can't be consteval), and using vformat gets rid of type safety, so you have to do something else.
So, you have to make Log a template struct, so you can deduce the type of the arguments then deduce the type of the Log struct so your format_string can have the right type.
Say this is the ultimate function you want to call:
template<LogTrait... Traits, typename... Args>void do_log(std::tuple<Traits...> traits, std::format_string<Args...> fmt, Args&&... args) {// ...}
You can implement Log like this:
template<typename Traits, typename Args>struct Log;template<typename... Traits, typename... Args>struct Log<std::tuple<Traits...>, std::tuple<Args...>>{Log(const Log&) = delete;Log& operator=(const Log&) = delete;~Log() = default;Log(Traits&&... traits, std::format_string<Args...> fmt, Args&&... args) {do_log(std::forward_as_tuple(std::forward<Traits>(traits)...), fmt, std::forward<Args>(args)...);}};template<std::size_t Offset, typename... Args, std::size_t... I>auto tuple_index(std::index_sequence<I...>) -> std::tuple<std::tuple_element_t<Offset + I, std::tuple<Args...>>...>;template<std::size_t N, typename... Args>using tuple_of_first = decltype(tuple_index<0, Args...>(std::make_index_sequence<N>{}));template<std::size_t N, typename... Args>using tuple_of_last = decltype(tuple_index<sizeof...(Args)-N, Args...>(std::make_index_sequence<N>{}));// Index of the first non-LogTrait argument, the format stringtemplate<typename... Args>inline constexpr std::size_t format_string_index = []<std::size_t... I>(std::index_sequence<I...>) {constexpr std::size_t format_string_index = std::min({ LogTrait<Args> ? std::size_t(-1) : I... });static_assert(format_string_index != std::size_t(-1), "Log(...): No format string");return format_string_index;}(std::index_sequence_for<Args...>{});template<typename... Args>struct traits_argument {using type = tuple_of_first<format_string_index<Args...>, Args...>;};template<typename... Args>struct format_argument {using type = tuple_of_last<sizeof...(Args) - 1u - format_string_index<Args...>, Args...>;};template<typename... Args>Log(Args&&... args) -> Log<typename traits_argument<Args...>::type, typename format_argument<Args...>::type>;
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