Basically I am trying to create a templated class that can take either be instantiated in a user friendly way or in a more complex way offering more configurability. I want to offer these two instantiating methods WITHOUT HAVING TO DUPLICATE THE API.
The class would look something along the lines of:
// templates that offer a lot of configurability
template<typename T, typename some_probably_awful_to_type_nested_class>
class has_my_api {
};
// templates that are easy to use
template<typename T, int i>
class has_my_api {
};
The idea is that I can build a generic version of some_probably_awful_to_type_nested_class
with i
but the two classes would call the exact same api.
I know I could do this by having an inner type with the actual API and having the API of each of these two classes just make calls to that inner class but I am looking for a way to do this without duplicating code.
Here is a more concrete example of what I'm looking for:
#define DEFAULT_CONFIG 0
//////////////////////////////////////////////////////////////////////
// Nested classes themselves
template<typename T>
class inner_nested_class {
// some fairly complex api
};
template<typename T,
typename inner_nested_class_t,
int configuration_flags = DEFAULT_CONFIG>
class outer_nested_class {
// some data structure of inner_nested_class
};
//////////////////////////////////////////////////////////////////////
// Helpers for creating nested_class from int seed
template<typename T, int i>
struct nested_class_creator;
template<typename T>
struct nested_class_creator<T, 0> {
typedef inner_nested_class<T> type;
};
template<typename T, int i>
struct nested_class_creator {
typedef outer_nested_class<T, typename nested_class_creator<T, i - 1>::type> type;
};
template<typename T, typename outer_nested_class_t>
class manager_class {
manager_class() = default;
int foo();
int bar();
int baz();
// some fairly involved additional API...
};
/* What i do NOT want to do
template<typename T, typename outer_nested_class_t>
class manager_class_api {
manager_class<T, outer_nested_class_t> inner_m;
int foo() { return inner_m.foo() };
int bar() { return inner_m.bar() };
int baz() { return inner_m.baz() };
};
template<typename T, int i>
class manager_class_api {
manager_class<T, typename nested_class_creator<T, i>::type> inner_m;
int foo() { return inner_m.foo() };
int bar() { return inner_m.bar() };
int baz() { return inner_m.baz() };
};
*/
/*
have tried... where the unified api could be called through _manager_class
template<typename T, int i>
using _manager_class = manager_class <T, typename nested_class_creator<T, i>::type>;
template<typename T, typename outer_nested_class_t>
using _manager_class = manager_class <T, outer_nested_class_t>;
But obviously it does not work...
*/
int
main() {
manager_class <int, outer_nested_class<int, outer_nested_class<int, inner_nested_class<int>, 0x4>, 0x3>> manager_with_user_specified_configs;
// how do I do this?
//manager_class <int, 3> manager_with_simply_api;
}
Is this possible to do? If so how can I do it?
I am happy using any version of C++ >= 11
Note: I am aware that I could use a preprocessor macro for this as a worse case scenario. If possible I would rather find a solution so the user could simply specify an int or type.
I don't think we can have template which accept type or value at the same type.
As workaround, you might wrap your value in a type (std::integral_constant
) (and possibly provide UDL to allow 3_c
instead of std::integral_constant<int, 3>
).
template <char... cs>
auto operator ""_c ()
{
constexpr int n = [](){
auto res = 0;
for (auto c : {cs...}) {
res *= 10;
res += c - '0';
}
return res;
}();
return std::integral_constant<int, n>{};
}
And so you would have:
template<typename T, typename outer_nested_class_t>
class manager_class {
manager_class() = default;
int foo();
int bar();
int baz();
// some fairly involved additional API...
};
template<typename T, int i>
class manager_class<T, std::integral_constant<i>> :
manager_class<T, typename nested_class_creator<T, i>::type>
{
};
You might avoid inheritance with alias wrapper:
template<typename T, typename outer_nested_class_t>
class manager_class_api {
using type = manager_class<T, outer_nested_class_t>;
};
template<typename T, int i>
class manager_class_api<T, std::integral_constant<i>>
{
using type = manager_class<T, typename nested_class_creator<T, i>::type>;
};
template<typename T, typename outer_nested_class_t>
using manager_class_api_t = typename manager_class_api<T, outer_nested_class_t>::type;
with usage:
manager_class_api_t<int, outer_nested_class<int, outer_nested_class<int, inner_nested_class<int>, 0x4>, 0x3>> manager_with_user_specified_configs;
manager_class_api_t<int, decltype(3_c)> manager_with_simply_api;
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