How to match empty arguments pack in variadic template

Question

I have code:

template<typename T>
void loadBrush_sub_impl()
{
    // do some work here
}

template<typename T, typename... Targs>
void loadBrush_sub()
{
    loadBrush_sub_impl<T>();
    loadBrush_sub<Targs...>();
}

template<>
void loadBrush_sub<void>()
{
}

// BasicBrush, BinaryBrush, SketchBrush, BasicEraser and MaskBased are class
loadBrush_sub<BasicBrush, BinaryBrush, SketchBrush, BasicEraser, MaskBased, void>();

This is correct when compiling it. However, I really want to drop the void in the call loadBrush_sub<BasicBrush, BinaryBrush, SketchBrush, BasicEraser, MaskBased, void>();.

However, this leads to:

..\CanvasEngine\canvasengine.cpp: In instantiation of 'void loadBrush_sub() [with T = MaskBased; Targs = {}]':
..\CanvasEngine\canvasengine.cpp:36:5:   recursively required from 'void loadBrush_sub() [with T = BinaryBrush; Targs = {SketchBrush, BasicEraser, MaskBased}]'
..\CanvasEngine\canvasengine.cpp:36:5:   required from 'void loadBrush_sub() [with T = BasicBrush; Targs = {BinaryBrush, SketchBrush, BasicEraser, MaskBased}]'
..\CanvasEngine\canvasengine.cpp:114:81:   required from here
..\CanvasEngine\canvasengine.cpp:36:5: error: no matching function for call to 'loadBrush_sub()'
..\CanvasEngine\canvasengine.cpp:36:5: note: candidate is:
..\CanvasEngine\canvasengine.cpp:33:6: note: template<class T, class ... Targs> void loadBrush_sub()
..\CanvasEngine\canvasengine.cpp:33:6: note:   template argument deduction/substitution failed:
..\CanvasEngine\canvasengine.cpp:36:5: note:   couldn't deduce template parameter 'T'
mingw32-make[1]: *** [release/canvasengine.o] Error 1

I ran some experiments with enable_if, but no luck.

Is there any solution to drop the void and make the compiler happy?

Answer 1

The easiest solution is adding another indirection:

template<typename T>
void loadBrush_sub_impl()
{
    // do some work here
}

template<typename... Targs>
void loadBrush_sub();

template<typename T, typename... V>
void loadBrush_sub_helper()
{
    loadBrush_sub_impl<T>();
    loadBrush_sub<V...>();
}

template<typename... Targs>
void loadBrush_sub()
{
    loadBrush_sub_helper<Targs...>();
}

template<>
void loadBrush_sub<>()
{
}

Answer 2

Without helper (notice "= void"):

template<typename T>
void loadBrush_sub_impl()
{
    // do some work here
}

template<typename T = void, typename... Targs>
void loadBrush_sub();

template<typename T, typename... Targs>
void loadBrush_sub()
{
    loadBrush_sub_impl<T>();
    loadBrush_sub<Targs...>();
}

template<>
void loadBrush_sub<>()
{
}