模板构造函数奇怪 [英] Template constructor weirdness

问题描述

可能重复：

后续问题，（我在编辑2中发现了这种情况）

following up on my previous question, (I found this situation in edit 2)

在代码中简单地显示：

#include <iostream>

struct Printer
{
  Printer() { std::cout << "secret code" << std::endl; }
};

template <class A>
struct Class
{
  template <class B, class C>
  Class(B arg)
  {
      C c; /* the 'secret code' should come from here */
      std::cout << arg << std::endl;
  }

  Class(double arg) { std::cout << "double" << std::endl; }
  Class(float arg) { std::cout << "float" << std::endl; }

  /* this forbids the use of printer in the first parameter */
  Class(Printer printer) { throw std::exception(); /* here be dragons */ }
};

int main()
{
  Class<int> c(1.0f);
  Class<int>* ptr = new Class<int>((double)2.0f);
  return 0;
}

// Can anyone print 'secret code' while creating an object of type 'Class' ?

详细：对于模板构造函数，可以指定一个不是构造函数参数的模板参数当对象得到实例化时？

Detailed: For a template constructor, can you specify a template argument which is not part of the constructor's arguments when an object get's instantiated?

我认为这值得自己的问题。

I think this deserves a question of its own.

推荐答案

不，这是不可能的。

没有任何语法可以为构造函数模板提供显式模板参数。您只能为类模板作为整体提供显式模板参数。

There is no syntax with which you can provide explicit template parameters to a constructor template. You can only provide explicit template parameters for the class template as a whole.

[temp.arg.explicit] （2003年的措辞，14.8.1 / 5）涵盖了这种情况。虽然该条款是非规范性的，但它向我们解释，作为语法的固有限制，这是不可能的：

The following text from [temp.arg.explicit] (2003 wording, 14.8.1/5) covers the scenario. Though the clause is non-normative, it serves to explain to us that, as an inherent restriction of the grammar, this is not possible:

注意：因为显式模板
参数列表遵循函数
模板名称和因为转换
成员函数模板和
构造函数成员函数模板
被调用而不使用函数
name ，没有办法为
提供
显式模板参数列表这些函数模板。

Note: because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates.

这部分是因为你从来没有实际调用构造函数。当你写，例如， A（）你不像一个函数调用构造函数，即使它看起来像你是（转换成员函数模板和构造函数成员函数调用模板而不使用函数名）。

This, partially, comes out of the fact that you never actually invoke the constructor explicitly yourself. When you write, say, A() you are not calling the constructor like a function, even though it looks as if you are ("conversion member function templates and constructor member function templates are called without using a function name").

这篇关于模板构造函数奇怪的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！