如何解决在可变参数模板上生成变量列表
在C ++ 17和更高版本中,可以这样将结构分解为变量:
struct MyStruct
{
int f1;
int f2;
int f3;
};
auto& [f1,f2,f3] = my_struct;
我正试图在可变参数包函数中做到这一点,
template <class ... T,std::size_t...I>
constexpr std::size_t CountFields(std::index_sequence<I...>)
{
T t;
auto& [] = t;
return 0;
}
但是我不完全确定如何填充方括号之间的空间以获取变量。最终目标是使用这些变量创建一个元组(即,我正在尝试创建一个通用的元组函数结构)。我发现与此最接近的是:struct to/from std::tuple conversion。
但是出于多种原因,我需要避免使用BOOST。
解决方法
不幸的是,很显然,没有办法在结构化绑定中使用参数包。另一方面,如果没有结构化绑定,就无法绑定数据成员。
但是,如果您提供数据成员的数量,则可以通过传统方式将其绑定:
template<size_t>
struct structToTupleHelper;
template<>
struct structToTupleHelper<0>; // ISO C++17 does not allow a decomposition group to be empty.
template<>
struct structToTupleHelper<1>{
template<typename X,size_t... Is>
static auto convert(X&& x,std::index_sequence<Is...>){
auto&& [a1] = std::forward<X>(x); // bound variables are always thought as lvalue.
auto temp = std::forward_as_tuple(a1);
return std::forward_as_tuple(std::get<Is>(temp)...);
}
};
template<>
struct structToTupleHelper<2>{
template<typename X,std::index_sequence<Is...>){
auto&& [a1,a2] = std::forward<X>(x); // bound variables are always thought as lvalue.
auto temp = std::forward_as_tuple(a1,a2);
return std::forward_as_tuple(std::get<Is>(temp)...);
}
};
// ...
// maybe 16 is enough?
template<typename X,size_t... Is>
auto structToTuple(X&& x,std::index_sequence<Is...> _1){
return structToTupleHelper<sizeof...(Is)>::convert(std::forward<X>(x),_1);
}
template<size_t N,typename X,std::index_sequence<Is...> _1){
return structToTupleHelper<N>::convert(std::forward<X>(x),typename X>
auto structToTuple(X&& x){
return structToTupleHelper<N>::convert(std::forward<X>(x),std::make_index_sequence<N>());
}
,然后您可以使用它,例如:
int ii;
struct A{
int& a;
} a{ii};
struct B{
int a;
} b;
auto t1 = structToTuple<1>(A{ii}); // tuple<int&>,valid.
auto t2 = structToTuple<1>(a); // tuple<int&>,valid.
auto t3 = structToTuple<1>(B{}); // tuple<int&>,invalid: a lvalue reference is bound to a temporary object.
auto t4 = structToTuple<1>(b); // tuple<int&>,valid.
somefunc(structToTuple<1>(B{})); // valid. the temporary object is alive inside 'somefunc'.
structToTuple<1>(A{ii}) = std::tuple(1); // valid. assign 1 to 'ii'.
structToTuple<1>(a) = std::tuple(1); // valid. assign 1 to 'ii'.
structToTuple<1>(B{}) = std::tuple(1); // unexpected. assign 1 to the member of a temporary object.
structToTuple<1>(b) = std::tuple(1); // valid. assign 1 to 'b.a'.
// struct C{
// int a : 8;
// } c;
// auto t5 = structToTuple<1>(C{}); // invalid: bitfields can not be treated as non-const lvalue reference
// auto t6 = structToTuple<1>(c); // invalid: bitfields can not be treated as non-const lvalue reference
也许您认为进行专业化很麻烦。幸运的是,我们可以使用宏来简化它:(这正是BOOST始终所做的。)
#define XXX_CONCAT_HELPER(a,b) a##b
#define XXX_CONCAT(a,b) XXX_CONCAT_HELPER(a,b)
#define XXX_COMMA,#define XXX_COMMA_FUNC(a),#define XXX_EMPTY
#define XXX_EMPTY_FUNC(a)
#define XXX_REPEAT_0(func,join)
#define XXX_REPEAT_1(func,join) func(1)
#define XXX_REPEAT_2(func,join) XXX_REPEAT_1(func,join) join(2) func(2)
// ...
#define XXX_REPEAT_256(func,join) XXX_REPEAT_255(func,join) join(256) func(256)
#define XXX_REPEAT(func,times,join) XXX_CONCAT(XXX_REPEAT_,times)(func,join)
// macro is not allowed to be recursive,so we need another repeat function.
#define XXX_ALIAS_REPEAT_0(func,join)
#define XXX_ALIAS_REPEAT_1(func,join) func(1)
#define XXX_ALIAS_REPEAT_2(func,join) XXX_ALIAS_REPEAT_1(func,join) join(2) func(2)
// ...
#define XXX_ALIAS_REPEAT_256(func,join) XXX_ALIAS_REPEAT_255(func,join) join(256) func(256)
#define XXX_ALIAS_REPEAT(func,join) XXX_CONCAT(XXX_ALIAS_REPEAT_,join)
#define STRUCT_TO_TUPLE_TOKEN_FUNC(n) XXX_CONCAT(a,n)
#define STRUCT_TO_TUPLE_FUNC(n) \
template<> \
struct structToTupleHelper<n>{ \
template<typename X,size_t... Is> \
static auto convert(X&& x,std::index_sequence<Is...>){ \
auto&& [XXX_REPEAT(STRUCT_TO_TUPLE_TOKEN_FUNC,n,XXX_COMMA_FUNC)] = std::forward<X>(x); \
auto temp = std::forward_as_tuple(XXX_REPEAT(STRUCT_TO_TUPLE_TOKEN_FUNC,XXX_COMMA_FUNC)); \
return std::forward_as_tuple(std::get<Is>(temp)...); \
} \
}; \
XXX_ALIAS_REPEAT(STRUCT_TO_TUPLE_FUNC,128,XXX_EMPTY_FUNC)
#undef STRUCT_TO_TUPLE_TOKEN_FUNC
#undef STRUCT_TO_TUPLE_FUNC
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