decltype 的错误参数 [英] Incorrect argument to decltype

问题描述

两天前我问过关于创建线程以使用 Win32 API 运行非静态类方法的问题，我几乎得到了解决方案，但有一些让我感到困惑的东西，所以我在发布之前的答案之前问了这个问题问题.

I asked two days ago about creating threads to run non-static class methods with the Win32 API, and I almost got a solution but there is something confusing me, so I'm asking this question before posting the answer in my previous question.

我正在尝试使用此代码来线程化返回类型未知的函数:

I'm trying to use this code to thread a function with an unknown return type:

template <class R, R func() >
unsigned int usualfunc() {
   func();
   return 1;
}

template <class R>
int Start(R(*func)()) {
   typedef decltype(&usualfunc<int, func>) D; // I get the error here , I can't get the address of the template function directly I need this
   D p = &usualfunc<R, func>;
   uintptr_t add = (uintptr_t)p;
   CreateThread(0, 0, (LPTHREAD_START_ROUTINE)add, 0, 0, 0);
   func();
   return 1;
}

int main() {
   Start(ltest);
}

当我尝试编译上面的代码时，我得到:

When I try to compile the above code, I get :

错误 3556 'usualfunc':'decltype' 的参数不正确

error 3556 'usualfunc': incorrect argument to 'decltype'

MSDN 上描述了错误描述:

The error description is described on MSDN:

编译器错误 C3556

然而，我在此之前尝试了另一个代码，它工作得很好，但我对语法不是很好:

However, I tried another code before this, and it works just fine, but I wasn't very good with the syntax:

template <class R, R func() >
unsigned int usualfunc() {
   func();
   return 1;
}

 template <class R,R func()>
int Start() {
   typedef decltype(&usualfunc<int, func>) D; // works well
   D p = &usualfunc<R, func>;
   uintptr_t add = (uintptr_t)p;
   CreateThread(0, 0, (LPTHREAD_START_ROUTINE)add, 0, 0, 0);
   func();
   return 1;
}

int main() {
   Start<int,ltest>(); // works
}

我知道这段代码已经足够了，但我想使用 Start(ltest) 而不是 Start().

I know this code is enough, but I'd like to use Start(ltest) instead of Start<int,ltest>().

注意:没有人说我应该在usualfunction中使用函数作为参数，我使用它作为模板参数，因为CreateThread()不能通过一个函数作为参数.

Note: no one say that I should use the function as parameter in usualfunction, I'm using it as a template parameter because CreateThread() can't pass a function as a parameter.

推荐答案

模板参数必须在编译时已知.但是，您尝试使用普通函数参数 func 作为模板参数.

Template parameters must be known at compile-time. However you attempt to use the normal function parameter func as a template argument.

在第二个代码中，您提供了一个模板参数作为模板参数，这很好.

In the second code you give a template parameter as the template argument, which is fine.

由于与此代码类似的原因，您的第一个代码是错误的:

Your first code is wrong for a similar reason as this code:

template<int X> void f() { }

int main(int argc, char **argv) { f<argc>(); }

虽然错误信息有点模糊.

although the error message is a bit more obscure.

从 C++17 开始，您可以通过对第二个代码进行此修改来获得所需的语法:

Since C++17 you can get the syntax you want by making this modification to your second code:

template <auto func>
int Start() {
    using R = decltype(func());
    // proceed as before...

并将其称为Start();.

在 C++17 之前，您可以在第二个代码中使用宏:

Prior to C++17 you could use a macro with your second code:

#define START(func) Start<decltype(func()), func>

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