小编sms*_*sms的帖子

void foo() try {} catch (...) {}
// OK, function-try-block

[]() try {} catch (...) {} ();
// error: expected ‘{’ before ‘try’

[]() { try {} catch (...) {} } ();
// OK, extra curly braces`

为什么不允许第二种变体？

我想boost::variant用C++ 17 替换s std::variant并摆脱boost::recursive_wrapper,在下面的代码中完全消除对boost的依赖.我该怎么办？

#include <boost/variant.hpp>
#include <type_traits>

using v = boost::variant<int, boost::recursive_wrapper<struct s> >;
struct s
{
    v val;
};

template<template <typename...> class R, typename T, typename ... Ts>
auto reduce(T t, Ts ... /*ts*/)
{
    return R<T, Ts...>{t};
}

template<typename T, typename F>
T adapt(F f)
{
    static_assert(std::is_convertible_v<F, T>, "");
    return f;
}

int main()
{
    int  val1 = 42;
    s    val2;
    auto val3 = adapt<v>(reduce<boost::variant>(val1, val2));
}

有两个通用函数:第一个函数reduce在运行时选择返回哪个参数(这里它只返回第一个参数以便简洁),第二个函数adapt将类型F的值转换为类型T的值. …

我想export（C ++ 20）别名模板。VC ++ 2019编译代码。Clang报告错误。哪一个是正确的，为什么？

// file: m.cppm
export module m;

template<typename T> struct my_template {};

export template<typename T> using my_alias = my_template<T>;

// file: main.cpp
import m;
int main() { my_alias<int> v; }

main.cpp:2:28: error: definition of 'my_template' must be imported from module 'm' before it is required
int main() { my_alias<int> v; }
^
m.cppm:3:29: note: previous definition is here
template<typename T> struct my_template {};

为什么这不编译？(评论r3将编译,但我希望在规则中使用分号)

#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

#include <string>
#include <vector>

struct v
{
    std::string value;
};

BOOST_FUSION_ADAPT_STRUCT
(
    v,
    (std::string, value)
)

using namespace boost::spirit::x3;

auto r1 = rule<struct s1, std::string>{} = lexeme[+alpha];

auto r2 = rule<struct s2, v>{} = r1;

using ast = std::vector<v>;

auto r3 = rule<struct s3, ast>{} = *(r2 >> ';');
//auto r3 = rule<struct s3, ast>{} = *r2;

int main()
{
    std::string script("a;");
    auto begin = script.begin();
    auto   end = script.end();

    ast a;

    phrase_parse(begin, …