移动std :: runtime_error的构造函数 [英] move constructor for std::runtime_error

问题描述

为什么 std :: runtime_error 不提供接受 std :: string&& 的构造函数？查看 std :: string ，它有一个移动构造函数，但 noexcept 规范仅适用于C ++ 14，而不是C ++ 11。

解决方案

显式的runtime_error（）是一个错误， string&&&&;）;

<$ p
<$> $ b

事实证明，符合C ++ 11的 runtime_error 不会在内部存储 std :: string 。原因是 runtime_error 的副本成员不能抛出异常。否则，当编译器在抛出异常对象的过程中复制异常对象时，错误的异常可能会被抛出。

这意味着 runtime_error 需要存储一个不可变引用计数字符串。但是C ++ 11违反了 std :: string 的COW实现。 std :: string 的实现已移至短字符串优化，如果字符串的长度超出短限制，则必须在复制构造上分配。并且用于构造 runtime_error 的字符串的长度没有限制。

有效的C ++ 11 （和forward）包含两个字符串实现：

1. std :: string




2. std :: runtime_error ：这是（或持有）不可变引用计数字符串。

和

显式runtime_error（string&&&）;

type 1字符串到type 2字符串。

Why does std::runtime_error not provide a constructor accepting an std::string&&? Looking at the constructors for std::string, it has a move constructor, but the noexcept specification is only there for C++14, not C++11. Was this a mistake, a deadline that was missed or am I missing something?

解决方案

explicit runtime_error(string&&);

does not exist simply because it would not provide any optimization.

As it turns out, a C++11-conforming runtime_error does not internally store a std::string. The reason is that the copy members of runtime_error must not throw exceptions. Otherwise the wrong exception could get thrown when the compiler copies the exception object in the process of throwing it.

This implies that runtime_error needs to store a non-mutable reference counted string. However C++11 outlaws the COW-implementation for std::string. Implementations of std::string have moved to a "short-string-optimization" which must allocate on copy construction if the length of the string is beyond the "short limit". And there is no limit on the length of strings used to construct a runtime_error.

So effectively C++11 (and forward) contains two implementations of strings:

1. std::string : This is typically a short-string-optimized type with a copy constructor and copy assignment that is capable of throwing exceptions.

2. std::runtime_error : This is (or holds) an immutable reference-counted string. This will never throw on copy construction or copy assignment.

And

explicit runtime_error(string&&);

can never (efficiently) transfer resources from the "type 1" string to the "type 2" string.

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