I've been confused recently by a few code examples -- sometimes it seems that inheriting typedefs exposed by a base class works, and sometimes it seems that it doesn't.
My questions are
- Why doesn't it always work?
- What are the situations that it will / won't work?
- What are the good workarounds when it doesn't work?
Here's some specific code:
// First example: Inheriting `static const int ...`
// Basic TypeList object
template<typename... Ts>
struct TypeList {
static const int size = sizeof...(Ts);
};
// Repeat metafunction
template<typename T>
struct repeat;
template<typename... Ts>
struct repeat<TypeList<Ts...>> : TypeList<Ts..., Ts...> {};
// Checks
typedef TypeList<int, float, char> MyList;
static_assert(MyList::size == 3, "D:");
static_assert(repeat<MyList>::size == 6, "D:");
// Second example: Inheriting typedefs
// Meta function to compute a bundle of types
template <typename T>
struct FuncPtrTypes {
typedef int result_type;
typedef T input_type;
typedef result_type(*func_ptr_type)(input_type);
};
// template <typename T, typename FuncPtrTypes<T>::func_ptr_type me>
// struct FuncPtr : FuncPtrTypes<T> {
// static result_type apply(input_type i) {
// return me(i);
// }
// };
//
// Doesn't compile (?): clang 3.6:
// main.cpp:34:9: error: unknown type name 'result_type'
// static result_type apply(input_type i) {
// ^
// main.cpp:34:27: error: unknown type name 'input_type'
// static result_type apply(input_type i) {
// ^
//
// g++ 4.8.4:
// main.cpp:34:9: error: ‘result_type’ does not name a type
// static result_type apply(input_type i) {
// ^
// main.cpp:34:9: note: (perhaps ‘typename FuncPtrTypes<T>::result_type’ was intended)
// This compiles but is clumsy:
template <typename T, typename FuncPtrTypes<T>::func_ptr_type me>
struct FuncPtr {
typedef typename FuncPtrTypes<T>::input_type input_type;
typedef typename FuncPtrTypes<T>::result_type result_type;
static result_type apply(input_type i) {
return me(i);
}
};
// A non-template example:
struct foo {
typedef int bar;
};
struct baz : foo {};
typedef baz::bar bazbar;
// ^ This compiles... huh??
int main() {}
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