基于非类型模板参数的重载 [英] Overloading based on non-type template parameter

问题描述

我们熟悉基于函数参数的重载.但是为什么我们不能基于非类型模板参数进行重载呢?使用这种重载，您不必仅出于重载目的而添加额外的函数参数，否则可能会对运行时性能产生负面影响. las，以下代码无法编译:

We are familiar with overloading based on function parameters. But why can't we have overloading based on non-type template parameters? With such overloading, you don't have to add extra function parameters just for overloading purposes, which may have a negative impact on runtime performance. Alas, the following code does not compile:

template <bool>
void func() {}

template <int>
void func() {}

int main() {
  func<0>();
}

产生的错误消息是

error: call of overloaded 'func()' is ambiguous
       func<0>();
               ^
note: candidate: void func() [with bool <anonymous> = false]
     void func() {}
          ^
note: candidate: void func() [with int <anonymous> = 0]
     void func() {}
          ^

请注意，这可能比

void func(bool) {}

void func(int) {}

允许这种用法有问题吗?

Is there any problem in allowing this usage?

推荐答案

如果您愿意接受一些新增的语法，则可以使用:

If you are open to a bit of added syntax, you can use:

// No default implementation.
template <typename T, T value> struct Impl;

// Implement the bool/true version
template <> struct Impl<bool, true>
{
   void operator()() {}
};

// Implement the bool/false version
template <> struct Impl<bool, false>
{
   void operator()() {}
};

// Implement the int version
template <int N> struct Impl<int, N>
{
   void operator()() {}
};

template <typename T, T value>
void func()
{
   Impl<T, value>()();
};

int main()
{
   func<bool, true>();
   func<int, 10>();
}

免责声明

我不知道这样做是否会比调用func(true)更好.

I have no idea whether this will perform better than calling func(true).

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