为什么std :: vector :: data和std :: string :: data不同？ [英] Why are std::vector::data and std::string::data different?

问题描述

向量的新方法 data（）提供了一个const和非const版本。

不过string的 data / code>方法只提供一个const版本。

Vector's new method data() provides a const and non-const version.
However string's data() method only provides a const version.

我认为他们改变了 std :: string ，以便字符现在需要是连续的（如 std :: vector ）。

I think they changed the wording about std::string so that the chars are now required to be contiguous (like std::vector).

code> std :: string :: data 刚刚错过了？或者是只允许const访问字符串底层字符的好理由。

Was std::string::data just missed? Or is the a good reason to only allow const access to a string's underlying characters?

注意： std :: vector :: data 有另一个不错的功能，它不是未定义的行为调用 data（）在一个空向量。而& vec.front（）是未定义的行为。

note: std::vector::data has another nice feature, it's not undefined behavior to call data() on an empty vector. Whereas &vec.front() is undefined behavior if it's empty.

推荐答案

在C ++ 98/03中，由于字符串常常被实现为COW，因此没有非const data（）的好理由。如果引用计数大于1，则非const data（）将需要一个副本。虽然可能，这在C ++ 98中并不可取/ 03。

In C++98/03 there was good reason to not have a non-const data() due to the fact that string was often implemented as COW. A non-const data() would have required a copy to be made if the refcount was greater than 1. While possible, this was not seen as desirable in C++98/03.

2005年10月，委员会在 LWG 464 添加了const和非const data（）到 vector 添加const和非const 到到映射。当时， string 没有被更改，因此取缔COW。但后来，通过C ++ 11，COW string 不再符合。 string spec在C ++ 11中也被收紧，因此它需要是连续的，并且总是有一个由 ]（size（））。在C ++ 03中，终止null只是由 operator [] 的const重载保证。

In Oct. 2005 the committee voted in LWG 464 which added the const and non-const data() to vector, and added const and non-const at() to map. At that time, string had not been changed so as to outlaw COW. But later, by C++11, a COW string is no longer conforming. The string spec was also tightened up in C++11 such that it is required to be contiguous, and there's always a terminating null exposed by operator[](size()). In C++03, the terminating null was only guaranteed by the const overload of operator[].

简单来说，对于C ++ 11 string ，非const data（）看起来更合理。据我所知，从未提出过。

So in short a non-const data() looks a lot more reasonable for a C++11 string. To the best of my knowledge, it was never proposed.

更新：

charT* data() noexcept;

在 basic_string =http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4582.pdf =nofollow> C ++ 1z工作草案N4582 作者 David Sankel的P0272R1 在2月份的杰克逊维尔会议上。

was added basic_string in the C++1z working draft N4582 by David Sankel's P0272R1 at the Jacksonville meeting in Feb. 2016.

很好的工作David！

Nice job David!

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