如何解决具有不同函数类型值的 std::map
我想将 std::map 与某些类型的函数（某些特定的参数类型）一起用作可能的值。但是，以下（最小）示例无法编译。
为什么会这样，我如何允许 FunctionType<bool> 和 FunctionType<int> 作为 Map 的可能值？
#include <functional>
#include <iostream>
#include <map>
#include <string>
#include <variant>
template <typename T>
using FunctionType = std::function<T(T)>;
int main()
{
auto myMap = std::map<std::string,std::variant<FunctionType<bool>,FunctionType<int>>>
{
{"foo",[](bool x){return x;}},{"bar",[](int x){return x;}},};
return 0;
}
错误信息：
$ g++ --std=c++2a -o /tmp/variant-test /tmp/variant-test.cpp && /tmp/variant-test
variant-test.cpp: In function ‘int main()’:
variant-test.cpp:18:5: error: no matching function for call to ‘std::map<std::__cxx11::basic_string<char>,std::variant<std::function<bool(bool)>,std::function<int(int)> > >::map(<brace-enclosed initializer list>)’
18 | };
| ^
In file included from /usr/include/c++/10/map:61,from variant-test.cpp:3:
/usr/include/c++/10/bits/stl_map.h:290:2: note: candidate: ‘template<class _InputIterator> std::map<_Key,_Tp,_Compare,_Alloc>::map(_InputIterator,_InputIterator,const _Compare&,const allocator_type&) [with _InputIterator = _InputIterator; _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> >; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>,std::function<int(int)> > > >]’
290 | map(_InputIterator __first,_InputIterator __last,| ^~~
/usr/include/c++/10/bits/stl_map.h:290:2: note: template argument deduction/substitution failed:
variant-test.cpp:18:5: note: candidate expects 4 arguments,2 provided
18 | };
| ^
In file included from /usr/include/c++/10/map:61,from variant-test.cpp:3:
/usr/include/c++/10/bits/stl_map.h:273:2: note: candidate: ‘template<class _InputIterator> std::map<_Key,_InputIterator) [with _InputIterator = _InputIterator; _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >]’
273 | map(_InputIterator __first,_InputIterator __last)
| ^~~
/usr/include/c++/10/bits/stl_map.h:273:2: note: template argument deduction/substitution failed:
variant-test.cpp:18:5: note: couldn’t deduce template parameter ‘_InputIterator’
18 | };
| ^
In file included from /usr/include/c++/10/map:61,from variant-test.cpp:3:
/usr/include/c++/10/bits/stl_map.h:256:2: note: candidate: ‘template<class _InputIterator> std::map<_Key,std::function<int(int)> > > >]’
256 | map(_InputIterator __first,| ^~~
/usr/include/c++/10/bits/stl_map.h:256:2: note: template argument deduction/substitution failed:
variant-test.cpp:18:5: note: candidate expects 3 arguments,from variant-test.cpp:3:
/usr/include/c++/10/bits/stl_map.h:250:7: note: candidate: ‘std::map<_Key,_Alloc>::map(std::initializer_list<std::pair<const _Key,_Tp> >,const allocator_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >; std::map<_Key,_Alloc>::allocator_type = std::allocator<std::pair<const std::__cxx11::basic_string<char>,std::function<int(int)> > > >]’
250 | map(initializer_list<value_type> __l,const allocator_type& __a)
| ^~~
/usr/include/c++/10/bits/stl_map.h:250:40: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘std::initializer_list<std::pair<const std::__cxx11::basic_string<char>,std::function<int(int)> > > >’
250 | map(initializer_list<value_type> __l,const allocator_type& __a)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10/bits/stl_map.h:244:7: note: candidate: ‘std::map<_Key,_Alloc>::map(std::map<_Key,_Alloc>&&,std::function<int(int)> > > >]’
244 | map(map&& __m,const allocator_type& __a)
| ^~~
/usr/include/c++/10/bits/stl_map.h:244:17: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘std::map<std::__cxx11::basic_string<char>,std::function<int(int)> > >&&’
244 | map(map&& __m,const allocator_type& __a)
| ~~~~~~^~~
/usr/include/c++/10/bits/stl_map.h:240:7: note: candidate: ‘std::map<_Key,_Alloc>::map(const std::map<_Key,_Alloc>&,std::function<int(int)> > > >]’
240 | map(const map& __m,const allocator_type& __a)
| ^~~
/usr/include/c++/10/bits/stl_map.h:240:22: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘const std::map<std::__cxx11::basic_string<char>,std::function<int(int)> > >&’
240 | map(const map& __m,const allocator_type& __a)
| ~~~~~~~~~~~^~~
/usr/include/c++/10/bits/stl_map.h:236:7: note: candidate: ‘std::map<_Key,_Alloc>::map(const allocator_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >]’
236 | map(const allocator_type& __a)
| ^~~
/usr/include/c++/10/bits/stl_map.h:236:7: note: candidate expects 1 argument,2 provided
/usr/include/c++/10/bits/stl_map.h:228:7: note: candidate: ‘std::map<_Key,std::function<int(int)> > > >]’
228 | map(initializer_list<value_type> __l,| ^~~
/usr/include/c++/10/bits/stl_map.h:228:40: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘std::initializer_list<std::pair<const std::__cxx11::basic_string<char>,std::function<int(int)> > > >’
228 | map(initializer_list<value_type> __l,| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10/bits/stl_map.h:215:7: note: candidate: ‘std::map<_Key,_Alloc>&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >]’
215 | map(map&&) = default;
| ^~~
/usr/include/c++/10/bits/stl_map.h:215:7: note: candidate expects 1 argument,2 provided
/usr/include/c++/10/bits/stl_map.h:207:7: note: candidate: ‘std::map<_Key,_Alloc>&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >]’
207 | map(const map&) = default;
| ^~~
/usr/include/c++/10/bits/stl_map.h:207:7: note: candidate expects 1 argument,2 provided
/usr/include/c++/10/bits/stl_map.h:194:7: note: candidate: ‘std::map<_Key,_Alloc>::map(const _Compare&,std::function<int(int)> > > >]’
194 | map(const _Compare& __comp,| ^~~
/usr/include/c++/10/bits/stl_map.h:194:27: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘const std::less<std::__cxx11::basic_string<char> >&’
194 | map(const _Compare& __comp,| ~~~~~~~~~~~~~~~~^~~~~~
/usr/include/c++/10/bits/stl_map.h:185:7: note: candidate: ‘std::map<_Key,_Alloc>::map() [with _Key = std::__cxx11::basic_string<char>; _Tp = std::variant<std::function<bool(bool)>,std::function<int(int)> > > >]’
185 | map() = default;
| ^~~
/usr/include/c++/10/bits/stl_map.h:185:7: note: candidate expects 0 arguments,2 provided
解决方法
这与一个用户定义的转换规则无关。以您的示例并将 FunctionType<int> 替换为 FunctionType<std::string> 很容易看出这一点 - 一切都会编译。
variant<X,Y> 可以从所有转换为 X 或 Y 的东西隐式转换，前提是它可以明确地确定要转换为的类型。附带条件很重要，因为 std::function 不需要完全匹配：std::function<bool(bool)> 接受可以用 bool 调用的所有内容，并返回可以转换为 bool 的内容.显然 [](bool x) { return x; } 满足此要求，但 [](int x) { return x; } 也满足此要求。同样，这两个 lambda 表达式都可以转换为 std::function<int(int)>，因为它们都可以使用 int 调用以生成可转换为 int 的内容。
由于 lambda 表达式可以转换为两种选择，并且两者之间没有排序，variant 将拒绝从中转换。这就是将 FunctionType<int> 替换为 FunctionType<std::string> 使其编译的原因 - 在这种情况下，只有 bool 替代方案是可行的。
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