Variadic templated constructor does not take x arguments

Question

For an inner templated struct of a templated class, I want to have a variadic templated constructor. Unfortunately the constructor (see first constructor below) does not suffice: if I only use that constructor, I obtain C2260 compiler errors stating that the constructor function does not take 3, 4 or 5 arguments. On the other hand, making everything explicit by adding another three constructors (see remaining constructors below) works as intended.

template< typename KeyT, typename ResourceT >
class ResourcePool {

    ...

    template< typename DerivedResourceT >
    struct ResourcePoolEntry final : public DerivedResourceT {

        template< typename... ConstructorArgsT >
        ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ConstructorArgsT... args)
            : DerivedResourceT(args...), m_resource_pool(resource_pool), m_resource_key(resource_key) {}

        ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ID3D11Device2 &x)
            : DerivedResourceT(x), m_resource_pool(resource_pool), m_resource_key(resource_key) {}
        
        ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ID3D11Device2 &x, const wstring &y)
            : DerivedResourceT(x,y), m_resource_pool(resource_pool), m_resource_key(resource_key) {}
        
        template < typename VertexT >
        ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ID3D11Device2 &x, const wstring &y, const MeshDescriptor< VertexT > &z)
            : DerivedResourceT(x, y, z), m_resource_pool(resource_pool), m_resource_key(resource_key) {}

           ...
    }
}

The constructor is called like this:

template< typename KeyT, typename ResourceT >
template< typename... ConstructorArgsT >
std::shared_ptr< ResourceT > ResourcePool< KeyT, ResourceT >::GetResource(KeyT key, ConstructorArgsT... args) {
    return GetDerivedResource< ResourceT, ConstructorArgsT... >(key, args...);
}

template< typename KeyT, typename ResourceT >
template< typename DerivedResourceT, typename... ConstructorArgsT >
std::shared_ptr< ResourceT > ResourcePool< KeyT, ResourceT >::GetDerivedResource(KeyT key, ConstructorArgsT... args) {
    ...
    auto new_resource = std::shared_ptr< ResourcePoolEntry< DerivedResourceT > >(
        new ResourcePoolEntry< DerivedResourceT >(*this, key, args...));
    ...
}

For a primitive like a bool as variadic argument, everything works fine.

Severity    Code    Description Project File    Line    Suppression State
Error   C2660       'mage::ResourcePool<std::wstring,mage::VertexShader>::ResourcePoolEntry<DerivedResourceT>::ResourcePoolEntry': function does not take 3 arguments   MAGE    c:\users\matthias\documents\visual studio 2015\projects\mage\mage\mage\src\resource\resource_pool.tpp   37

Where line 37 corresponds to the constructor call (new ResourcePoolEntry< DerivedResourceT >(*this, key, args...)); in the example above)

What am I doing wrong? (Compiler MSVC++ 14.0)

Minimal Example:

#include <memory>
#include <map>

template < typename T >
using SharedPtr = std::shared_ptr< T >;

template < typename T >
using WeakPtr = std::weak_ptr< T >;

template< typename KeyT, typename ResourceT >
using ResourceMap = std::map< KeyT, WeakPtr< ResourceT > >;

template< typename KeyT, typename ResourceT >
class ResourcePool {

public:

    template< typename... ConstructorArgsT >
    SharedPtr< ResourceT > GetResource(KeyT key, ConstructorArgsT... args);
    template< typename DerivedResourceT, typename... ConstructorArgsT >
    SharedPtr< ResourceT > GetDerivedResource(KeyT key, ConstructorArgsT... args);
        
private:

    ResourceMap< KeyT, ResourceT > m_resource_map;

    template< typename DerivedResourceT >
    struct ResourcePoolEntry final : public DerivedResourceT {

    public:

        template< typename... ConstructorArgsT >
        ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ConstructorArgsT... args)
            : DerivedResourceT(args...), m_resource_pool(resource_pool), m_resource_key(resource_key) {}
    private:

        ResourcePool< KeyT, ResourceT > &m_resource_pool;
        KeyT m_resource_key;
    };
};

template< typename KeyT, typename ResourceT >
template< typename... ConstructorArgsT >
SharedPtr< ResourceT > ResourcePool< KeyT, ResourceT >::GetResource(KeyT key, ConstructorArgsT... args) {
    return GetDerivedResource< ResourceT, ConstructorArgsT... >(key, args...);
}

template< typename KeyT, typename ResourceT >
template< typename DerivedResourceT, typename... ConstructorArgsT >
SharedPtr< ResourceT > ResourcePool< KeyT, ResourceT >::GetDerivedResource(KeyT key, ConstructorArgsT... args) {
    auto it = m_resource_map.find(key);
    if (it != m_resource_map.end()) {
        auto resource = it->second.lock();
        if (resource) {
            return resource;
        }
        else {
            m_resource_map.erase(it);
        }
    }

    auto new_resource = SharedPtr< ResourcePoolEntry< DerivedResourceT > >(
        new ResourcePoolEntry< DerivedResourceT >(*this, key, args...));
    m_resource_map[key] = new_resource;
    return new_resource;
}

#include <d3d11_2.h>

struct A {
};
struct B : public A {
    B(ID3D11Device &device) : A() {}
};


const D3D_FEATURE_LEVEL g_feature_levels[] = {
    D3D_FEATURE_LEVEL_11_1,
    D3D_FEATURE_LEVEL_11_0
};

int main() {

    ID3D11Device *device;
    ID3D11DeviceContext *device_context;
    D3D_FEATURE_LEVEL feature_level;
    D3D11CreateDevice(nullptr, D3D_DRIVER_TYPE_HARDWARE, nullptr, 0, 
        g_feature_levels, _countof(g_feature_levels), D3D11_SDK_VERSION,
        &device, &feature_level, &device_context
    );

    ResourcePool< char, A > *pool = new ResourcePool< char, A >();
    //pool->template GetResource< int & >('a');
    pool->template GetDerivedResource< B, ID3D11Device & >('b', *device);
}

Error:

Severity    Code    Description Line    Suppression State
Error   C2661   'ResourcePool<char,A>::ResourcePoolEntry<DerivedResourceT>::ResourcePoolEntry': no overloaded function takes 3 arguments    66

Answer 1

One thing to note (which may or may not be the ultimate cause of your problem) is that you're not doing quite the right thing with your template parameter forwarding. For example, in the case where you're passing an ID3D11Device2 your DerivedResourceT constructor probably (judging by the signature of your non-variadic constructors) expects a reference - however because of the way template deduction works it will actually get a copy instead (if indeed this is even permissable - if not it would not compile).

To correct this, you need to use the standard forwarding recipe, which allows the correct l- or r-valueness of a passed parameter to be passed on, which includes correctly forwarding references:

template< typename... ConstructorArgsT >
ResourcePoolEntry(ResourcePool< KeyT, ResourceT > &resource_pool,
            KeyT resource_key, ConstructorArgsT&&... args)
            : DerivedResourceT(std::forward<ConstructorArgsT>(args)...), m_resource_pool(resource_pool), m_resource_key(resource_key) {}

In the above, note the && in the parameter type of args... and the std::forward call.