可变参数模板麻烦匹配const和非const std :: string [英] Variadic template trouble matching const and non-const std::string

问题描述

我在构建可变参数模板时遇到了麻烦，其中扩展功能正确地与非const和const std :: string 匹配.我有一个通用的匹配函数，在某些情况下会被调用.

I'm having trouble constructing a variadic template where the expansion functions properly match both a non-const and const std::string. I have a generic matching function which is called instead in some cases.

我已将代码简化为以下示例.不是字符串的东西应该以泛型函数结束.作为字符串的内容应该在第二个函数处结束，该函数另外输出"STRING".

I've reduced the code to the below example. Things which aren't a string should end up at the generic function. Things which are a string should end up at the second function which additionally outputs "STRING".

#include <iostream>
#include <string>

template<typename X, typename T>
void extend_exception( X & ex, T&& value )
{
    ex << value << std::endl;
}

template<typename X>
void extend_exception( X & ex, std::string && value )
{
    ex << "STRING: " << value << std::endl;
}

template<typename X, typename TF, typename ... TR>
void extend_exception( X & ex, TF&& f, TR&& ... rest )
{
    extend_exception( ex, std::forward<TF>(f) );
    extend_exception( ex, std::forward<TR>(rest)... );
}

int main()
{
    extend_exception( std::cout, std::string( "Happy" ) );
    std::string var( "var" );
    extend_exception( std::cout, var );
    extend_exception( std::cout, var, std::string( "Combo" ),  const_cast<std::string const&>(var));
    return 0;
}

在以上两个 var 用法的版本中，不会传递给字符串函数.我尝试了其他各种方法来指定参数，但似乎没有办法: std :: string&&值， std :: string const&值， std :: string const&&值.我也确实尝试过 std :: string&值，然后匹配非常量字符串，但不匹配常量字符串.通过提供两个不同的功能，这至少为我提供了一种解决方法，但是我怀疑我不需要这样做.

In the version above the two var uses do not get passed to the string function. I have tried various other ways of specifying the parameter but none seem to do the trick: std::string && value, std::string const & value, std::string const && value. I did also try just std::string & value and then the non-const strings match, but not the const ones. This provides me with at least a workaround by having two different functions, but I suspect I shouldn't need to do this.

如何在此处编写一个将为const，非const和临时字符串对象调用的函数?

How do I write a single function here that will be called for const, non-const, and temporary string objects?

我正在使用g ++ 4.6.3

推荐答案

我重新研究了这个问题，并提出了一个涉及使用类标记的解决方案.我写了一篇完整解决方案/讨论的文章.

I revisited the problem and came up with a solution which involves using a class marker. I've written an article with the full solution/discussion.

这篇关于可变参数模板麻烦匹配const和非const std :: string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！