为什么C ++模板接受数组不比一个接受指针(bis)更专业? [英] Why C++ template accepting array is not more specialized than one accepting pointer (bis)?

问题描述

_关于

_{In reference to this question, which has indeed the same title but for which I found an answer in the standard. I have continued to dig the subject and finaly find out an example code for which this answer does not apply.}

让我们考虑这段代码:

template<class T> void func(T* buf);           //template I
template<size_t N> void func(char (&buf) [N]); //template II

void g(char (&buf)[3])
   {
   func(buf) //Error: ambiguous function call (Clang, GCC, ICC, MSVC)
   }

根据 [temp.func.order]中的部分排序规则和 [temp.deduct.partial] ，如果人们通过执行这段代码来解释此规则，那么 template II 应该比 template I 更专业:

According to the partial ordering rules in [temp.func.order] and [temp.deduct.partial], template II shall be more specialized than template I if one interpreted this rules through the execution of this piece of code:

template <class T> void func1(T* buf) {}
template <std::size_t N> void func2(char (&buf)[N]) {}

struct invented_T{};
constexpr std::size_t invented_N=42;

void is_template_I_more_specialized(invented_T* buf)
  {
  func2(buf);
  //DO NOT COMPILE
  // => template I is not more specialized than func2
  }

void is_template_II_more_specialized(char (&buf)[invented_N])
  {
  func1(buf);
  //DO COMPILE
  // => template II is more specialized than func1
  }

因此，根据这种解释，模板II 应该是更专业的.为什么不是这样?

So according to this interpretation, template II should be more specialized. Why would it not be the case?

推荐答案

注释中指出，原因是无法从类型char (&buf)[invented_N]推导出类型T*.

As n.m. pointed out in the comment, the reason is that type T* cannot be deduced from type char (&buf)[invented_N].

在is_template_II_more_specialized中，根据

如果P不是引用类型:

If P is not a reference type:

• 如果A是数组类型，则使用数组到指针标准转换生成的指针类型代替A进行类型推导；否则，

• If A is an array type, the pointer type produced by the array-to-pointer standard conversion is used in place of A for type deduction; otherwise,

...

此规则仅适用于从函数调用中推断模板参数 .为了推断部分订购期间期间的模板参数，不应用这种转换.

This rule only applies for deducing template arguments from a function call. For deducing template arguments during partial ordering, there is no such conversion applying.

_{在[temp.deduct.partial]/} 6 ， 7 .

_{Conversions that can be applied during partial ordering are described in [temp.deduct.partial]/5,6,7.}

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