可变参数模板“对重载函数的模糊调用”似乎是一个错误的错误

Question

我尝试编写一个模板函数来初始化给定的系统并运行应用程序，最后在初始化的系统上运行关闭函数。

这段代码在我看来应该可以工作，并且智能感知不会给出任何错误，但编译器：

1>C:\VisualStudio\DirectApp\AppMain.cpp(32,2): error C2668: 'initialize_these': ambiguous call to overloaded function
1>C:\VisualStudio\DirectApp\AppMain.cpp(28,13): message : could be 'void initialize_these<WindowManager,>(void)'
1>C:\VisualStudio\DirectApp\AppMain.cpp(18,13): message : or       'void initialize_these<WindowManager>(void)'
1>C:\VisualStudio\DirectApp\AppMain.cpp(29,1): message : while trying to match the argument list '()'
1>C:\VisualStudio\DirectApp\AppMain.cpp(29): message : see reference to function template instantiation 'void initialize_these<Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(29): message : see reference to function template instantiation 'void initialize_these<Heap,Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(29): message : see reference to function template instantiation 'void initialize_these<SystemInfo,Heap,Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(45): message : see reference to function template instantiation 'void initialize_these<Sdl,SystemInfo,Heap,Log,WindowManager>(void)' being compiled

编码：

#include "pch.h"
#include "AppMain.h"

#include "AppRun.h"
#include "Dummy.h"
#include "Heap.h"
#include "Log.h"
#include "Sdl.h"
#include "SystemInfo.h"
#include "WindowManager.h"

static void initialize_these()
{
    AppMain::run<AppRun>();
}

template<class Type = Dummy>
static void initialize_these()
{
    if (AppMain::initialize<Type>()) { return; }

    initialize_these();

    AppMain::shutdown<Type>();
}

template<class  First, class... Rest>
static void initialize_these()
{
    if (AppMain::initialize<First>()) { return; }

    initialize_these<Rest...>();

    AppMain::shutdown<First>();
}

void AppMain::app_main()
{
    initialize_these<
        Sdl,
        SystemInfo,
        Heap,
        Log,
        WindowManager
    >();
}

希望代码足够清晰。

哦，顺便说一句。 如果初始化失败，那些AppMain::initialize<>()调用将返回 true。 我检查了这段代码的不同部分。 在我看来一切都很好，但static void initialize_these()函数的可变参数模板。

编辑：我删除

template<class Type = Dummy>
static void initialize_these()
{
    if (AppMain::initialize<Type>()) { return; }

    initialize_these();

    AppMain::shutdown<Type>();
}

编译器错误：

1>C:\VisualStudio\DirectApp\AppMain.cpp(22,2): error C2672: 'initialize_these': no matching overloaded function found
1>C:\VisualStudio\DirectApp\AppMain.cpp(19): message : see reference to function template instantiation 'void initialize_these<WindowManager,>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(19): message : see reference to function template instantiation 'void initialize_these<Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(19): message : see reference to function template instantiation 'void initialize_these<Heap,Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(19): message : see reference to function template instantiation 'void initialize_these<SystemInfo,Heap,Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(35): message : see reference to function template instantiation 'void initialize_these<Sdl,SystemInfo,Heap,Log,WindowManager>(void)' being compiled
1>C:\VisualStudio\DirectApp\AppMain.cpp(22,2): error C2783: 'void initialize_these(void)': could not deduce template argument for 'First'
1>C:\VisualStudio\DirectApp\AppMain.cpp(18): message : see declaration of 'initialize_these'

Answer 1

Rest可以为空，因此两者都有效。

---

您可以使可变参数接受 2 个或更多参数

template<class Type>
static void f(){
  // do something with Type
}

template<class First, class Second, class... Rest>
static void f(){
    // do something with First

    f<Second,Rest...>();
}

Answer 2

用c++17 if constexpr你也可以这样写

template<class Type, class... Rest>
static void f(){
    // do something with Type
    
    if constexpr(sizeof...(Rest))
        f<Rest...>();
}