Template specialization on template member of template class - c++

Question

This is probably only a syntax problem.
So i have this template class :
template <typename String, template<class> class Allocator>
class basic_data_object
{
template<typename T>
using array_container = std::vector<T, Allocator<T>>;
};
And another one :
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value
{
};
Now i want to specialize the second one's T parameter with the first one's inner typedef array_container for any given type.
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value
<String, Allocator,
typename basic_data_object<String, Allocator>::template array_container<T>>
{
};
But this specialization doesn't seem to be matched when i pass an std::vector as the last parameter.
If i create a temporary hard coded typedef:
typedef basic_data_object<std::string, std::allocator<std::string>> data_object;
And use it for the specialization, everything works :
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value
<String, Allocator,
data_object::template array_container<T>>
{
};
What did i miss ? :)
Alternatively what is the best (smallest / cleanest) way to make this work ?

Answer 1

The C++ standard says, in [temp.class.spec.match] paragraph 2:
A partial specialization matches a given actual template
argument list if the template arguments of the partial
specialization can be deduced from the actual template
argument list (14.8.2).
14.8.2 is [temp.arg.deduct] i.e. the clause describing template argument deduction for function templates.
If you modify your code to use a similar function template and attempt to call it, you will see that the arguments cannot be deduced:
template <typename String, typename T>
void deduction_test(String,
typename basic_data_object<String, std::allocator>::template array_container<T>)
{ }
int main()
{
deduction_test(std::string{}, std::vector<int, std::allocator<int>>{});
}
(I removed the Allocator parameter, since there's no way to pass a template template parameter as a function argument and in the basic_data_object type it's a non-deduced context, I don't believe it affects the result.)
Both clang and GCC say they cannot deduce T here. Therefore the partial specialization will not match the same types used as template arguments.
So I haven't really answered the question yet, only clarified that the reason is in the rules of template argument deduction, and shown an equivalence with deduction in function templates.
In 14.8.2.5 [temp.deduct.type] we get a list of non-deduced contexts that prevent deduction, and the following rule in paragraph 6:
When a type name is specified in a way that includes a non-deduced context, all of the types that comprise that type name are also non-deduced.
Since basic_data_object<String, Allocator> is in a non-deduced context (it is a nested-name-specifier, i.e. appears before ::) that means the type T is also non-deduced, which is exactly what Clang and GCC tell us.
With your temporary hardcoded typedef there is no non-deduced context, and so deduction for T succeeds using the deduction_test function template:
template <typename String, typename T>
void deduction_test(String,
typename data_object::template array_container<T>)
{ }
int main()
{
deduction_test(std::string{}, std::vector<int, std::allocator<int>>{}); // OK
}
And so, correspondingly, your class template partial specialization can be matched when it uses that type.
I don't see a way to make it work without changing the definition of get_data_object_value, but if that's an option you can remove the need to deduce the array_container type and instead use a trait to detect whether a type is the type you want, and specialize on the result of the trait:
#include <string>
#include <vector>
#include <iostream>
template <typename String, template<class> class Allocator>
class basic_data_object
{
public:
template<typename T>
using array_container = std::vector<T, Allocator<T>>;
template<typename T>
struct is_ac : std::false_type { };
template<typename T>
struct is_ac<array_container<T>> : std::true_type { };
};
template <typename String, template<class> class Allocator, typename T, bool = basic_data_object<String, Allocator>::template is_ac<T>::value>
struct get_data_object_value
{
};
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value<String, Allocator, T, true>
{
void f() { }
};
int main()
{
get_data_object_value<std::string,std::allocator,std::vector<short>> obj;
obj.f();
}
This doesn't really scale if you wanted several class template partial specializations, as you would need to add several bool template parameters with default arguments.

Answer 2

For some reason, the problem seems to stem from the double level of templates. I'll leave you check the 3 test cases below, they are simple:
Remove the template arguments of First: works as expected
Make First a template, but the inner type a plain one: works as expected
Make both First and the inner type templates: compiles but the output is unexpected
Note: the template template parameter Allocator is useless to reproduce the issue, so I left it out.
Note: both GCC (ideone's version, 4.8.1 I believe) and Clang (Coliru version, 3.4) compile the code, and yet produce the same baffling result
From the 3 above examples, I deduce:
that this is NOT a non-deducible context issue; otherwise why would (2) work ?
that this is NOT an alias issue; otherwise why would (1) work ?
And therefore that either the problem is much hairier than the current hints would make us believe OR that both gcc and Clang have a bug.
EDIT: Thanks to Jonathan Wakely who patiently educated me enough that I could finally understand both the Standard wording related to this case and how it applied. I will now attempt to explain this (again) in my own words. Please refer to Jonathan's answer for the exact Standard quotes (it all sits in [temp.deduct.type])
When deducing template parameters (Pi), whether for functions or classes, the deduction is done independently for each and every argument.
Each argument need provide zero or one candidate Ci for each parameter; if an argument would provide more than one candidate, it provides none instead.
Thus, each argument produces a dictionary Dn: Pi -> Ci which maps a subset (possibly empty) of the template parameters to be deduced to their candidate.
The dictionaries Dn are merged together, parameter by parameter:
if only one dictionary has a candidate for a given parameter, then this parameter is accepted, with this candidate
if several dictionaries have the same candidate for a given parameter, then this parameter is accepted, with this candidate
if several dictionaries have different incompatible (*) candidates for a given parameter, then this parameter is rejected
If the final dictionary is complete (maps each and every parameter to an accepted candidate) then deduction succeeds, otherwise it fails
(*) there seems to be a possibility for finding a "common type" from the available candidates... it is of no consequence here though.
Now we can apply this to the previous examples:
1) A single template parameter T exists:
pattern matching std::vector<int> against typename First::template ArrayType<T> (which is std::vector<T>), we get D0: { T -> int }
merging the only dictionary yields { T -> int }, thus T is deduced to be int
2) A single template parameter String exists
pattern matching std::vector<int> against String, we get D0: { String -> std::vector<int> }
pattern matching std::vector<int> against typename First<String>::ArrayType we hit a non-deducible context (many values of String could fit), we get D1: {}
merging the two dictionaries yields { String -> std::vector<int> }, thus String is deduced to be std::vector<int>
3) Two template parameters String and T exist
pattern matching std::vector<char> against String, we get D0: { String -> std::vector<char> }
pattern matching std::vector<int> against typename First<String>::template ArrayType<T> we hit a non-deducible context, we get D1: {}
merging the two dictionaries yields { String -> std::vector<char> }, which is an incomplete dictionary (T is absent) deduction fails
I must admit I had not considered yet that the arguments were resolved independently from one another, and therefore than in this last case, when computing D1 the compiler could not take advantage of the fact that D0 had already deduced a value for String. Why it is done in this fashion, however, is probably a full question of its own.
Without the outer template, it works, as in it prints "Specialized":
#include <iostream>
#include <vector>
struct First {
template <typename T>
using ArrayType = std::vector<T>;
};
template <typename T>
struct Second {
void go() { std::cout << "General\n"; }
};
template <typename T>
struct Second < typename First::template ArrayType<T> > {
void go() { std::cout << "Specialized\n"; }
};
int main() {
Second < std::vector<int> > second;
second.go();
return 0;
}
Without the inner template, it works, as in it prints "Specialized":
#include <iostream>
#include <vector>
template <typename String>
struct First {
using ArrayType = std::vector<int>;
};
template <typename String, typename T>
struct Second {
void go() { std::cout << "General\n"; }
};
template <typename String>
struct Second < String, typename First<String>::ArrayType > {
void go() { std::cout << "Specialized\n"; }
};
int main() {
Second < std::vector<int>, std::vector<int> > second;
second.go();
return 0;
}
With both, it fails, as in it prints "General":
#include <iostream>
#include <vector>
template <typename String>
struct First {
template <typename T>
using ArrayType = std::vector<T>;
};
template <typename String, typename T>
struct Second {
void go() { std::cout << "General\n"; }
};
template <typename String, typename T>
struct Second < String, typename First<String>::template ArrayType<T> > {
void go() { std::cout << "Specialized\n"; }
};
int main() {
Second < std::vector<char>, std::vector<int> > second;
second.go();
return 0;
}

Answer 3

The answer of Jonathan Wakely gives the reason why your code does not work.
My answer shows you how to solve the problem.
In your example, the container type over which you want to specialize is defined outside of basic_data_object thus you can of course use it directly in your specialization:
template <typename S, template<class> class A, typename T>
struct get_data_object_value<S,A,std::vector<T,A>>
{ };
This definitely conforms with the standard and works with all compilers.
In the case where the type is defined in basic_data_object, you can move it out of the class.
Example: Instead of
template<typename S, template<class> class A>
struct a_data_object
{
template<typename T>
struct a_container
{ };
};
write this:
template<typename S, template<class> class A, typename T>
// you can perhaps drop S and A if not needed...
struct a_container
{ };
template<typename S, template<class> class A, typename T>
struct a_data_object
{
// use a_container<S,A,T>
};
Now you can specialize with:
template <typename S, template<class> class A, typename T>
struct get_data_object_value<S,A,a_container<S,A,T>>
{ };
Note: The next "solution" is apparently a bug with GCC 4.8.1.
If the container is only defined in an enclosing template and can not be moved out you can do this:
Get the container type out of basic_data_object:
template<typename S, template<class> class A, typename T>
using bdo_container = basic_data_object<S,A>::array_container<T>;
Write a specialization for this type:
template <typename S, template<class> class A, typename T>
struct get_data_object_value<S,A,bdo_container<S,A,T>>
{ };

Answer 4

Alternatively what is the best (smallest / cleanest) way to make this work?
Arguably, it is:
Write a SFINAE trait template Tr<String,Allocator,T> that determines whether T is the
same as basic_data_object<String,Allocator>::array_container<T::E>
for some type E - if such there be - that is T::value_type.
Provide template get_data_object_value with a 4th parameter
defaulting to Tr<String,Allocator,T>::value
Write partial specializations of get_data_object_value instantiating that
4th parameter as true, false respectively.
Here is a demo program:
#include <type_traits>
#include <vector>
#include <iostream>
template <typename String, template<class> class Allocator>
struct basic_data_object
{
template<typename T>
using array_container = std::vector<T, Allocator<T>>;
};
template<typename T, typename String, template<class> class Allocator>
struct is_basic_data_object_array_container
/*
A trait template that has a `static const bool` member `value` equal to
`true` if and only if parameter type `T` is a container type
with `value_type E` s.t.
`T` = `basic_data_object<String,Allocator>::array_container<T::E>`
*/
{
template<typename A>
static constexpr bool
test(std::is_same<
A,
typename basic_data_object<String,Allocator>::template
array_container<typename A::value_type>
> *) {
return std::is_same<
A,
typename basic_data_object<String,Allocator>::template
array_container<typename A::value_type>
>::value;
}
template<typename A>
static constexpr bool test(...) {
return false;
}
static const bool value = test<T>(nullptr);
};
template <
typename String,
template<class> class Allocator,
typename T,
bool Select =
is_basic_data_object_array_container<T,String,Allocator>::value
>
struct get_data_object_value;
template <
typename String,
template<class> class Allocator,
typename T
>
struct get_data_object_value<
String,
Allocator,
T,
false
>
{
static void demo() {
std::cout << "Is NOT a basic_data_object array_container" << std::endl;
}
};
template <
typename String,
template<class> class Allocator,
typename T>
struct get_data_object_value<
String,
Allocator,
T,
true
>
{
static void demo() {
std::cout << "Is a basic_data_object array_container" << std::endl;
}
};
#include <list>
#include <memory>
using namespace std;
int main(int argc, char **argv)
{
get_data_object_value<string,allocator,std::vector<short>>::demo();
get_data_object_value<string,allocator,std::list<short>>::demo();
get_data_object_value<string,allocator,short>::demo();
return 0;
}
Built with gcc 4.8.2, clang 3.4. Output:
Is a basic_data_object array_container
Is NOT a basic_data_object array_container
Is NOT a basic_data_object array_container
VC++ 2013 will not compile this for lack of constexpr support. To accommodate that
compiler the following less natural implementation of the trait may be used:
template<typename T, typename String, template<class> class Allocator>
struct is_basic_data_object_array_container
{
template<typename A>
static
auto test(
std::is_same<
A,
typename basic_data_object<String, Allocator>::template
array_container<typename A::value_type>
> *
) ->
std::integral_constant<
bool,
std::is_same<
A,
typename basic_data_object<String, Allocator>::template
array_container<typename A::value_type>
>::value
>{}
template<typename A>
static std::false_type test(...);
using type = decltype(test<T>(nullptr));
static const bool value = type::value;
};

Answer 5

Edit: This answer only works because of a bug in GCC 4.8.1
Your code works as expected if you drop the keyword template in your specialization:
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value
{
void foo() { std::cout << "general" << std::endl; }
};
template <typename String, template<class> class Allocator, typename T>
struct get_data_object_value
<String, Allocator,
typename basic_data_object<String, Allocator>::array_container<T>>
// ^^^^^^ no template!
{
void foo() { std::cout << "special" << std::endl; }
};
Example tested with GCC 4.8.1:
int main() {
get_data_object_value<std::string,std::allocator,std::vector<int>> obj;
obj.foo(); // prints "special"
}