这是我要执行的操作的最低版本:
template<typename D>
struct Base {
void common() {
// ... do something ...
static_cast<D *>(this)->impl();
// ... do something ...
}
void common_with_arg(typename D::Arg arg) {
// ... do something ...
static_cast<D *>(this)->impl_with_arg(arg);
// ... do something more ...
}
};
struct Derived : Base<Derived> {
void impl() { }
using Arg = int;
void impl_with_arg(Arg arg) { }
};
Base::common()
和Derived::impl()
工作正常(如预期)。Base::common_with_arg()
和Derived::impl_with_arg()
却没有。以gcc为例,出现以下错误:
1.cc: In instantiation of ‘struct Base<Derived>’:
1.cc:18:18: required from here
1.cc:11:7: error: invalid use of incomplete type ‘struct Derived’
void common_with_arg(typename D::Arg arg) {
^~~~~~~~~~~~~~~
1.cc:18:8: note: forward declaration of ‘struct Derived’
struct Derived : Base<Derived> {
凭直觉(不了解有关模板实例化的所有详细信息),这似乎是一个明智的错误。还有另一种方法可以实现相同的功能吗?
最佳答案
void common_with_arg(typename D::Arg arg)
// ^^^^^^
您无法在此处访问
D::Arg
,因为需要Derived
的定义。但是定义永远不可用,因为Base
模板正在此处实例化...struct Derived : Base<Derived> {
// ^^^^^^^^^^^^^
...
Derived
尚未完全定义。一种可能的解决方法是使
common_with_arg
为功能模板:template <typename T>
void common_with_arg(T&& arg) {
// ... do something ...
static_cast<D *>(this)->impl_with_arg(std::forward<T>(arg));
// ... do something more ...
}
example on wandbox
如果您确实需要
Arg
类型别名,请阅读以下问题:"C++ static polymorphism (CRTP) and using typedefs from derived classes"。关于c++ - CRTP:具有基于派生参数的函数,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/41551030/