Here's an issue I ran across while playing with variadic templates. I have some code that uses specialization to count "interesting" types in a parameter pack like so:
template<typename... _Pp>
struct count;
template<>
struct count<>
{
static const int value = 0;
};
// ignore uninteresting types
template<typename _First, typename... _Rest>
struct count<_First, _Rest...>
{
static const int value = count<_Rest...>::value;
};
// add 1 for a pointer
template<typename _First, typename... _Rest>
struct count<_First*, _Rest...>
{
static const int value = 1 + count<_Rest...>::value;
};
// add 1 for a reference
template<typename _First, typename... _Rest>
struct count<_First&, _Rest...>
{
static const int value = 1 + count<_Rest...>::value;
};
// add 1 for an int
template<typename... _Rest>
struct count<int, _Rest...>
{
static const int value = 1 + count<_Rest...>::value;
};
This code works fine, but I run into problems if I want to use the same approach to count class templates:
// add 1 for a vector
template<typename... _Rest>
struct count<vector, _Rest...>
{
static const int value = 1 + count<_Rest...>::value;
};
The above code fails to compile, error is "expected a type, got 'vector'" on the line beginning with "struct count". I'm also unable to something simpler, all class templates accepting a single argument:
// add 1 for a class template with 1 type parameter
template<template<typename> class _First, typename... _Rest>
struct count<_First, _Rest...>
{
static const int value = 1 + count<_Rest...>::value;
}
This code also fails to compile, complaining of "expected a type, got '_First'" once again on the line beginning with "struct count". Is someone familiar with a way to accomplish this goal using this approach (i.e. some modification that I can make to one or both of the specializations that will get them to compile and perform the desired calculation at compile time)?
EDIT: I want the parameter pack for vector to be unbound, similar to the following code for a simple container wrapper with variadic template-template parameters that also specializes on std::vector:
// pass a container as a parameter using variadic template-template
parameter
template<typename _Tp, template<typename...> class _C>
struct success
{
// not specialized for any container
static const bool is_specialized = false;
// data member of container type
_C<_Tp> c_;
};
// partial specialization of above for std::vector
template<typename _Tp>
struct success<_Tp, std::vector>
{
// specialized for vector
static const bool is_specialized = true;
// again, data member of container type
std::vector<_Tp> c_;
};
EDIT Seems like the final answer is that what I want to do can't be accomplished, but I have found a way to reframe the problem so that I cans solve it. Many thanks to those who helped.
If I understand correctly what you want... yes, you can create a templated struct that can count "class templates", so you can write something like
count<std::vector, std::map, std::set, std::pair>::value
but you can't mix class templates and simple typenames, so you can't write something like
count<std::vector, int &, float, std::set>::value
The problem is that if you define
template <typename... _Pp>
struct count;
you can pass std::vector<int> to it, because std::vector<int> is a typename, but you can't pass std::vector to it because std::vector isn't a typename; it's a template<typename...> class (or template template) that it's a total different things.
You can write something like the following struct countC
template <template<typename...> class ...>
struct countC;
template <>
struct countC<>
{ static const int value = 0; };
// ignore uninteresting templates
template<template<typename...> class F, template<typename...> class ... R>
struct countC<F, R...>
{ static const int value = countC<R...>::value; };
template <template<typename...> class ... R>
struct countC<std::vector, R...>
{ static const int value = 1 + countC<R...>::value; };
The following is a working complete example where I've rewritten your struct count as struct countT for count selected types, I've added a struct countC to count selected "class templates" and I've added a struct countV to count selected values of a fixed typename.
#include <map>
#include <set>
#include <vector>
#include <utility>
#include <iostream>
// countC for templates
template <template<typename...> class ...>
struct countC;
template <>
struct countC<>
{ static const int value = 0; };
// ignore uninteresting templates
template<template<typename...> class F, template<typename...> class ... R>
struct countC<F, R...>
{ static const int value = countC<R...>::value; };
template <template<typename...> class ... R>
struct countC<std::vector, R...>
{ static const int value = 1 + countC<R...>::value; };
template <template<typename...> class ... R>
struct countC<std::map, R...>
{ static const int value = 1 + countC<R...>::value; };
template <template<typename...> class ... R>
struct countC<std::pair, R...>
{ static const int value = 1 + countC<R...>::value; };
// countV for for values of a fixed type
template <typename T, T ... v>
struct countV;
template <typename T>
struct countV<T>
{ static const int value = 0; };
// ignore uninteresting values
template <typename T, T f, T ... r>
struct countV<T, f, r...>
{ static const int value = countV<T, r...>::value; };
// count only int odd values
template <int f, int ... r>
struct countV<int, f, r...>
{ static const int value = (f % 2) + countV<int, r...>::value; };
// countT for typenames
template <typename...>
struct countT;
template <>
struct countT<>
{ static const int value = 0; };
// ignore uninteresting types
template <typename F, typename ... R>
struct countT<F, R...>
{ static const int value = countT<R...>::value; };
template <typename F, typename ... R>
struct countT<F*, R...>
{ static const int value = 1 + countT<R...>::value; };
template<typename F, typename ... R>
struct countT<F&, R...>
{ static const int value = 1 + countT<R...>::value; };
template<typename ... R>
struct countT<int, R...>
{ static const int value = 1 + countT<R...>::value; };
int main()
{
std::cout << "countC vector + map + set + pair = "
<< countC<std::vector, std::map, std::set, std::pair>::value
<< std::endl;
std::cout << "countT int + float + bool* + double& + bool + int& = "
<< countT<int, float, bool*, double&, bool, int&>::value
<< std::endl;
std::cout << "countV int, 1 + 4 + 4 + 5 + 7 + 10 + 11 + 16 + 15 = "
<< countV<int, 1, 4, 4, 5, 7, 10, 11, 16, 15>::value
<< std::endl;
std::cout << "countV long, 1 + 4 + 4 + 5 + 7 + 10 + 11 + 16 + 15 = "
<< countV<long, 1, 4, 4, 5, 7, 10, 11, 16, 15>::value
<< std::endl;
return 0;
}
p.s.: sorry for my bad English.