基于无辜范围的循环无效

基于无辜范围的循环无效

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问题描述

限时删除!!

以下内容不会进行编译:

  #include< iostream> 

int main()
{
int a {},b {},c {},d {};

for(auto& s:{a,b,c,d}){
s = 1;
}
std :: cout<< << std :: endl;
返回0;
}



编译器错误为:错误:只读引用的分配'



现在在我的实际情况下,列表由类中的成员变量组成。



现在,这不起作用,因为表达式变成了 initializer_list< int> ,实际上复制了a,b,c和d-因此也不允许修改。



我的问题有两个方面:



是否存在不允许以这种方式编写基于范围的for循环的任何动机? 例如。



什么是纠正语法的句法整洁方法是什么?循环的类型?



最好使用以下方式:

  for(auto& s:something(a,b,c,d)){
s = 1;
}

我认为指针间接寻址不是一个好的解决方案(即 {& a,& b,& c,& d} )-在取消迭代器引用时,任何解决方案都应直接提供元素引用

解决方案

根据标准§11.6.4List-initialization / p5 [dcl.init.list] [重点矿井]:

因此,您的编译器在合理地抱怨(即 auto& s 扣除为 int const& s ,您不能

您可以通过引入容器而不是容器来缓解此问题。带有 std :: reference_wrapper的初始值设定项列表(例如 std :: vector):

  #include< iostream> 
#include< vector>
#include< functional>

int main()
{
int a {},b {},c {},d {};

for(auto& s:std :: vector< std :: reference_wrapper< int>> {a,b,c,d}){
s.get()= 1 ;
}
std :: cout<< << std :: endl;
返回0;
}


The following does not compile:

#include <iostream>

int main()
{
    int a{},b{},c{},d{};

    for (auto& s : {a, b, c, d}) {
        s = 1;
    }
    std::cout << a << std::endl;
    return 0;
}

Try it on godbolt

Compiler error is: error: assignment of read-only reference 's'

Now in my actual case the list is made of member variables on a class.

Now, this doesn't work because the expression becomes an initializer_list<int> that actually copies a,b,c, and d - hence also not allowing modification.

My question is two-fold:

Is there any motivation behind not allowing to write a range-based for loop in this way ? eg. perhaps there could be a special case for naked brace expressions.

What is a syntactical neat way of fixing this type of loop ?

Something along this line would be preferred:

for (auto& s : something(a, b, c, d)) {
    s = 1;
}

I do not consider pointer indirection a good solution (that is {&a, &b, &c, &d}) - any solution should give the element reference directly when the iterator is de-referenced.

解决方案

According to the standard §11.6.4 List-initialization/p5 [dcl.init.list] [Emphasis Mine]:

Thus, your compiler is complaining legitimately (i.e., auto &s deducts to int const& s and you cannot assign to s in the ranged for loop).

You could alleviate this problem by introducing a container instead of an initializer list (e.g., `std::vector’) with ‘std::reference_wrapper’:

#include <iostream>
#include <vector>
#include <functional>

int main()
{
    int a{},b{},c{},d{};

    for (auto& s : std::vector<std::reference_wrapper<int>>{a, b, c, d}) {
        s.get()= 1;
    }
    std::cout << a << std::endl;
    return 0;
}

Live Demo

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09-06 09:13