liuyanghejerry liuyanghejerry - 1 month ago 11
C++ Question

How to match empty arguments pack in variadic template

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 compile 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've made some experiments with
enable_if
, but got no lucky.
Is there any solution to drop the
void
and make the compiler happy?

Answer

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<>()
{
}