将CRTP与SFINAE混合 [英] mixing CRTP with SFINAE
问题描述
我有一个base
,将派生类型作为模板参数.下面的代码按预期方式工作. base<non_default_impl>
的实例化使用non_default_impl::data_t
,并且base<default_impl>
引发编译错误,因为event_data
只是一个前向声明.
template <typename T>
struct event_data;
template<typename T>
struct tovoid {
typedef void type;
};
template <typename T, typename enable = void>
struct get_data{
typedef event_data<T> type;
};
template <typename T>
struct get_data<T, typename tovoid<typename T::data_t>::type >{
typedef typename T::data_t type;
};
template <typename T>
struct base{
typedef typename get_data<T>::type data_type;
base(){
data_type();
}
};
struct non_default_impl{
struct data{};
typedef data data_t;
};
struct default_impl{
};
int main(){
base<non_default_impl> e1;
base<default_impl> e2;
return 0;
}
但是,当non_default_impl
从base<non_default_impl>
继承时,SFINAE将替换为前向声明.
struct non_default_impl: public base<non_default_impl>{
struct data{};
typedef data data_t;
};
int main(){
non_default_impl e1;
// base<default_impl> e2;
return 0;
}
prog.cpp:在'base :: base()的实例中[T = non_default_impl]":
prog.cpp:28:8:从此处开始
prog.cpp:24:3:错误:无效使用不完整类型 'base :: data_type {aka结构 event_data}'data_type();
如何进行这项工作.我想要派生类是否使用data_t
typedef,否则使用event_data<derived_type>
这是CRTP的一个警告:在您的base
模板专门用于non_default_impl
类时,即在其基类列表中,
因此,任何访问其定义中任何内容的尝试(例如data_t
typedef)都将失败.
由于您无法使用non_default_impl
内部的任何内容,因此解决方案是使用外部类型特征来选择您的data_t
:
template <class T>
struct dataType { typedef event_data<T> type; };
template <typename T>
struct base{
typedef typename dataType<T>::type data_type;
// ...
};
// Usage
struct non_default_data {};
template <>
struct dataType<struct non_default_impl> {
typedef non_default_data type;
};
struct non_default_impl: public base<non_default_impl> {
// ...
};
请注意,您不能在non_default_impl
中声明non_default_data
,因为它必须可以从类型特征中进行访问,该特征必须可以从CRTP中进行访问,而CRTP在定义non_default_impl
之前仍必须进行特殊处理. /p>
I have a base
taking derived type as template parameter. The following code works as expected. instantiation of base<non_default_impl>
uses non_default_impl::data_t
and base<default_impl>
throws compilation error because event_data
is only a forward declaration.
template <typename T>
struct event_data;
template<typename T>
struct tovoid {
typedef void type;
};
template <typename T, typename enable = void>
struct get_data{
typedef event_data<T> type;
};
template <typename T>
struct get_data<T, typename tovoid<typename T::data_t>::type >{
typedef typename T::data_t type;
};
template <typename T>
struct base{
typedef typename get_data<T>::type data_type;
base(){
data_type();
}
};
struct non_default_impl{
struct data{};
typedef data data_t;
};
struct default_impl{
};
int main(){
base<non_default_impl> e1;
base<default_impl> e2;
return 0;
}
But when non_default_impl
inherits from base<non_default_impl>
SFINAE substitutes to the forward declaration.
struct non_default_impl: public base<non_default_impl>{
struct data{};
typedef data data_t;
};
int main(){
non_default_impl e1;
// base<default_impl> e2;
return 0;
}
prog.cpp: In instantiation of 'base::base() [with T = non_default_impl]':
prog.cpp:28:8: required from here
prog.cpp:24:3: error: invalid use of incomplete type 'base::data_type {aka struct event_data}' data_type();
How to make this work. I want if the derived class have a data_t
typedef use that otherwise use event_data<derived_type>
This is a caveat of CRTP : at the time your base
template is specialized for your non_default_impl
class, i.e in its base classes list, non_default_impl
itself is not defined yet.
Thus, any attempt to access anything that is part of its definition, for example the data_t
typedef, fails.
Since you cannot use anything that is inside non_default_impl
, a solution is to use an external type trait to choose your data_t
:
template <class T>
struct dataType { typedef event_data<T> type; };
template <typename T>
struct base{
typedef typename dataType<T>::type data_type;
// ...
};
// Usage
struct non_default_data {};
template <>
struct dataType<struct non_default_impl> {
typedef non_default_data type;
};
struct non_default_impl: public base<non_default_impl> {
// ...
};
Note that you can't declare non_default_data
inside non_default_impl
, since it must be accessible from the type trait, which must be accessible from the CRTP, which still has to be specialized before non_default_impl
is defined.
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