提问人：anni 提问时间：7/1/2022 最后编辑：anni 更新时间：7/1/2022 访问量：105
如何使用初始化器列表初始化包含 boost：：array<> 的 Boost.Fusion 向量
How to initialize Boost.Fusion vector containing boost::array<> with initializer list
问：
我需要使用初始化器列表初始化一个 boost：：fusion：：vector，将 boost：：array 作为元素。这可能吗？
boost::array<bool, 2> ary{true, false}; // works
boost::fusion::vector<int, bool> vec1{ 5, false}; // works
boost::fusion::vector<bool, std::array<bool, 2>> vec2{ false, {true, false} }; // doesn't work :(
错误：
<source>: In function 'int main()':
<source>:116:55: error: converting to 'boost::fusion::vector<int, bool>' from initializer list would use explicit constructor 'boost::fusion::vector<T>::vector(U&& ...) [with U = {int, bool}; <template-parameter-2-2> = void; T = {int, bool}]'
boost::fusion::vector<int, bool> vec1 = { 5, false};
^
<source>:117:81: error: no matching function for call to 'boost::fusion::vector<bool, std::array<bool, 2> >::vector(<brace-enclosed initializer list>)'
boost::fusion::vector<bool, std::array<bool, 2>> vec2{ false, {true, false} };
^
In file included from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector.hpp:12,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/include/vector.hpp:11,
from <source>:14:
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:302:9: note: candidate: 'template<class Sequence, class> constexpr boost::fusion::vector<T>::vector(Sequence&&)'
vector(Sequence&& seq)
^~~~~~
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:302:9: note: template argument deduction/substitution failed:
<source>:117:81: note: candidate expects 1 argument, 2 provided
boost::fusion::vector<bool, std::array<bool, 2>> vec2{ false, {true, false} };
^
In file included from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector.hpp:12,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/include/vector.hpp:11,
from <source>:14:
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:289:18: note: candidate: 'template<class ... U, class> boost::fusion::vector<T>::vector(U&& ...)'
explicit vector(U&&... u)
^~~~~~
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:289:18: note: template argument deduction/substitution failed:
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:279:35: error: mismatched argument pack lengths while expanding 'boost::is_convertible<U, T>'
sizeof...(U) >= 1 &&
~~~~~~~~~~~~~~~~~~^~
fusion::detail::and_<is_convertible<U, T>...>::value &&
~~~~~~
In file included from /opt/compiler-explorer/libs/boost_1_70_0/boost/config.hpp:61,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/aux_/config/msvc.hpp:19,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/aux_/config/adl.hpp:17,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/aux_/adl_barrier.hpp:17,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/bool_fwd.hpp:17,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/bool.hpp:17,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/aux_/na.hpp:17,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/mpl/vector.hpp:19,
from <source>:5:
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:274:9: note: candidate: 'constexpr boost::fusion::vector<T>::vector() [with T = {bool, std::array<bool, 2>}]'
BOOST_DEFAULTED_FUNCTION(vector(), {})
^~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:274:9: note: candidate expects 0 arguments, 2 provided
In file included from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector.hpp:12,
from /opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/include/vector.hpp:11,
from <source>:14:
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:262:12: note: candidate: 'constexpr boost::fusion::vector<bool, std::array<bool, 2> >::vector(const boost::fusion::vector<bool, std::array<bool, 2> >&)'
struct vector
^~~~~~
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:262:12: note: candidate expects 1 argument, 2 provided
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:262:12: note: candidate: 'constexpr boost::fusion::vector<bool, std::array<bool, 2> >::vector(boost::fusion::vector<bool, std::array<bool, 2> >&&)'
/opt/compiler-explorer/libs/boost_1_70_0/boost/fusion/container/vector/vector.hpp:262:12: note: candidate expects 1 argument, 2 provided
<source>:115:27: warning: unused variable 'ary' [-Wunused-variable]
boost::array<bool, 2> ary{true, false};
^~~
使用 GCC 8.3，提升 1.70
感觉我缺少一些关于初始值设定项列表如何工作的基本理解。任何帮助将不胜感激。
答：
1赞
sehe
7/1/2022
#1
这是对推导嵌套大括号初始值设定项的语言限制。
我可以使用 C++17 CTAD 想出的最佳版本是
#include <array>
#include <boost/fusion/include/make_vector.hpp>
#include <boost/fusion/include/vector.hpp>
int main() {
auto vec = boost::fusion::make_vector(false, std::array{true, false});
}
评论
0赞
anni
7/2/2022
感谢您的回复！它似乎适用于 boost 1.53，但不适用于 boost 1.70，所以感觉它与 boost 库有关，而不是语言限制。
2赞
sehe
7/2/2022
这大大改变了问题，我真的帮不上忙。最有可能的变化是 Fusion 开始支持更多的东西，这使得构造函数不可推导/模棱两可。这并不是真正的倒退，更像是新功能/权衡
评论