const_cast和std :: move从非引用中删除const [英] const_cast and std::move to remove constness from non-reference

问题描述

我有一个外部库，我不能修改。库声明一个模板函数，由于某种原因返回 const 非引用对象：

  template< class C> 
 const C foo（）; 
  

我有另一个外部库，我也不能修改。该库声明一个不可复制的类，并且只有一个非const对象的移动构造函数：

  struct bar { 
 bar（）; 
 bar（const bar&）= delete; 
 bar（bar&&）; 
}; 
  

现在我需要使用 foo< bar> 。一个简单的用法：

  bar buz（）{
 return foo< bar>（）; 
} 
  

失败，

  main.cpp：在函数'bar buz（）'：
 main.cpp：13：21：error：use of deleted function'bar :: bar（const bar&）'
 return foo< bar>（）; 
 ^ 
 main.cpp：8：5：note：declaration here 
 bar（const bar&）= delete; 
 ^ ~~ 
  

但是，如果我添加一些更复杂的解决方法：

  bar buz（）{
 return const_cast< bar&&>（std :: move（foo< bar>（））） 
} 
  

它编译并且整个代码按预期工作，但我的真正的代码太）。

但是，它是安全的，或者我遇到一些未定义的行为？是否有更好的解决方法？

---

我已阅读并且理解关于返回 const from functions（ 1 ， 2 ）和常见的答案似乎是在现代C ++中不鼓励返回 const 对象，但是我的问题不是关于它，而是关于如何解决外部库返回 const 对象。

解决方案

到未定义的行为。由于原始对象 C （一个临时）被声明为 const ，所以常量转换和修改是非法的， 。 （我假设，移动构造函数做一些修改movee）。

这就是说，它可能在您的环境中工作，我没有看到更好的解决方法。 p>

I have an external library which I can not modify. The library declares a template function that for some reason returns const non-reference object:

template<class C>
const C foo();

I have another external library which I can not modify too. The library declares a class that is non-copyable and has a move constructor from a non-const object only:

struct bar {
    bar();
    bar(const bar&)=delete;
    bar(bar&&);
};

Now I need to use foo<bar>. A simple usage:

bar buz() {
    return foo<bar>();
}

fails with

main.cpp: In function 'bar buz()':
main.cpp:13:21: error: use of deleted function 'bar::bar(const bar&)'
     return foo<bar>();
                     ^
main.cpp:8:5: note: declared here
     bar(const bar&)=delete;
     ^~~

which makes sense, and no simple workaround make the code compile.

However, if I add some more complex workaround:

bar buz() {
    return const_cast<bar&&>(std::move(foo<bar>()));
}

it compiles and the whole code work as expected (not only the simplified example above, but my real code too).

However, it is safe, or am I running into some undefined behavior? Is there any better workaround?

---

I have read and I understand questions about returning const from functions (1, 2), and the common answer there seems to be that returning const objects is discouraged in modern C++, but my question is not about it, but about how can I workaround the situation when an external library returns const object.

解决方案

Technically speaking, you are exposing your program to undefined behavior. Since original object C (a temporary) was declared const, const-casting and modifying it is illegal and against the standard. (I assume, move constructor does some modifications to the movee).

That being said, it probably works in your environment and I do not see a better workaround.

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