Vector's new method data() provides a const and non-const version.
However string's data() method only provides a const version.
Vector的新方法data()提供了一个const和非const版本。但是string的data()方法只提供const版本。
I think they changed the wording about std::string so that the chars are now required to be contiguous (like std::vector).
我认为他们改变了关于std::string的措辞，这样chars现在就需要是连续的(比如std:::vector)。
Was std::string::data just missed? Or is the a good reason to only allow const access to a string's underlying characters?
std::string:数据只是错过了?还是仅仅允许const访问字符串的底层字符的好理由?
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.
注意:std:::vector::data还有一个很好的特性，在一个空的向量上调用data()不是没有定义的行为。然而，front()是没有定义的行为，如果它是空的。
4 个解决方案
#1
29
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.
在c++ 98/03中，有充分的理由不使用非常量数据()，因为字符串通常被实现为COW。如果refcount大于1，则需要复制一个非const数据()。虽然可能，但在c++ 98/03中并没有被认为是可取的。
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[].
2005年10月，委员会对LWG 464进行了投票，将const和non-const数据()添加到vector中，并将const和non-const at()添加到map中。当时还没有改弦，以取缔牛。但后来，在c++ 11中，牛字符串不再符合标准。在c++ 11中，字符串规范也得到了加强，因此要求它是连续的，而且操作符[](size())总是暴露一个终止null。在c++ 03中，只有操作符的const过载才能保证终止null[]。
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.
因此，对于c++ 11字符串来说，非const数据()看起来要合理得多。据我所知，它从未被提出过。
Update
更新
charT* data() noexcept;
was added basic_string in the C++1z working draft N4582 by David Sankel's P0272R1 at the Jacksonville meeting in Feb. 2016.
在2016年2月杰克逊维尔会议上，David Sankel的P0272R1在c++ 1z工作草案N4582中添加了basic_string。
Nice job David!
大卫好工作!
#2
2
Historically, the string data has not been const because it would prevent several common optimizations, like copy-on-write (COW). This is now, IIANM, far less common, because it behaves badly with multithreaded programs.
从历史上看，字符串数据并不是const，因为它会阻止一些常见的优化，比如copy-on-write (COW)。这是现在，IIANM，不太常见，因为它在多线程程序中表现很差。
BTW, yes they are now required to be contiguous:
顺便说一下，它们现在被要求是连续的:
[string.require].5: The char-like objects in a basic_string object shall be stored contiguously. That is, for any basic_string object s, the identity &*(s.begin() + n) == &*s.begin() + n shall hold for all values of n such that 0 <= n < s.size().
[string.require]。5: basic_string对象中的charlike对象应该连续存储。也就是说，对于任何basic_string对象s, identity &*(s.begin() + n) = &*s.begin() + n应该包含所有n的值，以便0 <= n < s.size()。
Another reason might be to avoid code such as:
另一个原因可能是避免使用以下代码:
std::string ret;
strcpy(ret.data(), "whatthe...");
Or any other function that returns a preallocated char array.
或返回预分配字符数组的任何其他函数。
#3
1
Although I'm not that well-versed in the standard, it might be due to the fact that std::string doesn't need to contain null-terminated data, but it can and it doesn't need to contain an explicit length field, but it can. So changing the undelying data and e.g. adding a '\0' in the middle might get the strings length field out of sync with the actual char data and thus leave the object in an invalid state.
虽然我对标准不是很熟悉，但这可能是因为std::string不需要包含以null结尾的数据，但它可以，也不需要包含显式长度字段，但它可以。因此，更改undelying数据并在中间添加“\0”可能会使字符串长度字段与实际的char数据不同步，从而使对象处于无效状态。
#4
0
@Christian Rau
@Christian劳
From the time the original Plauger (around 1995 I think) string class was STL-ized by the committee (turned into a Sequence, templatified), std::string has always been std::vector plus string-related stuff (conversion from/to 0-terminated, concatenation, ...), plus some oddities, like COW that's actually "Copy on Write and on non-const begin()/end()/operator[]".
从最初的Plauger(我认为是在1995年左右)string类被委员会stl化(转换为序列，模板化)开始，std::string就一直被std::vector + string相关的东西(从/到0终止，连接，……)，加上一些奇怪的东西，比如COW，实际上是“写上复制，非结束/开始操作”()
But ultimately a std::string is really a std::vector under another name, with a slightly different focus and intent. So:
但是最终的std::string实际上是另一个名称下的std::vector，其焦点和意图略有不同。所以:
- just like
std::vector,std::stringhas either a size data member or both start and end data members; - 就像std::vector, std::string有大小数据成员，也有开始和结束数据成员;
- just like
std::vector,std::stringdoes not care about the value of its elements, embedded NUL or others. - 就像std::vector、std::string并不关心它的元素、嵌入的NUL或其他元素的值。
std::string is not a C string with syntax sugar, utility functions and some encapsulation, just like std::vector<T> is not T[] with syntax sugar, utility functions and some encapsulation.
string不是带有语法糖、实用函数和一些封装的C字符串，就像std::vector
#1
29
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.
在c++ 98/03中，有充分的理由不使用非常量数据()，因为字符串通常被实现为COW。如果refcount大于1，则需要复制一个非const数据()。虽然可能，但在c++ 98/03中并没有被认为是可取的。
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[].
2005年10月，委员会对LWG 464进行了投票，将const和non-const数据()添加到vector中，并将const和non-const at()添加到map中。当时还没有改弦，以取缔牛。但后来，在c++ 11中，牛字符串不再符合标准。在c++ 11中，字符串规范也得到了加强，因此要求它是连续的，而且操作符[](size())总是暴露一个终止null。在c++ 03中，只有操作符的const过载才能保证终止null[]。
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.
因此，对于c++ 11字符串来说，非const数据()看起来要合理得多。据我所知，它从未被提出过。
Update
更新
charT* data() noexcept;
was added basic_string in the C++1z working draft N4582 by David Sankel's P0272R1 at the Jacksonville meeting in Feb. 2016.
在2016年2月杰克逊维尔会议上，David Sankel的P0272R1在c++ 1z工作草案N4582中添加了basic_string。
Nice job David!
大卫好工作!
#2
2
Historically, the string data has not been const because it would prevent several common optimizations, like copy-on-write (COW). This is now, IIANM, far less common, because it behaves badly with multithreaded programs.
从历史上看，字符串数据并不是const，因为它会阻止一些常见的优化，比如copy-on-write (COW)。这是现在，IIANM，不太常见，因为它在多线程程序中表现很差。
BTW, yes they are now required to be contiguous:
顺便说一下，它们现在被要求是连续的:
[string.require].5: The char-like objects in a basic_string object shall be stored contiguously. That is, for any basic_string object s, the identity &*(s.begin() + n) == &*s.begin() + n shall hold for all values of n such that 0 <= n < s.size().
[string.require]。5: basic_string对象中的charlike对象应该连续存储。也就是说，对于任何basic_string对象s, identity &*(s.begin() + n) = &*s.begin() + n应该包含所有n的值，以便0 <= n < s.size()。
Another reason might be to avoid code such as:
另一个原因可能是避免使用以下代码:
std::string ret;
strcpy(ret.data(), "whatthe...");
Or any other function that returns a preallocated char array.
或返回预分配字符数组的任何其他函数。
#3
1
Although I'm not that well-versed in the standard, it might be due to the fact that std::string doesn't need to contain null-terminated data, but it can and it doesn't need to contain an explicit length field, but it can. So changing the undelying data and e.g. adding a '\0' in the middle might get the strings length field out of sync with the actual char data and thus leave the object in an invalid state.
虽然我对标准不是很熟悉，但这可能是因为std::string不需要包含以null结尾的数据，但它可以，也不需要包含显式长度字段，但它可以。因此，更改undelying数据并在中间添加“\0”可能会使字符串长度字段与实际的char数据不同步，从而使对象处于无效状态。
#4
0
@Christian Rau
@Christian劳
From the time the original Plauger (around 1995 I think) string class was STL-ized by the committee (turned into a Sequence, templatified), std::string has always been std::vector plus string-related stuff (conversion from/to 0-terminated, concatenation, ...), plus some oddities, like COW that's actually "Copy on Write and on non-const begin()/end()/operator[]".
从最初的Plauger(我认为是在1995年左右)string类被委员会stl化(转换为序列，模板化)开始，std::string就一直被std::vector + string相关的东西(从/到0终止，连接，……)，加上一些奇怪的东西，比如COW，实际上是“写上复制，非结束/开始操作”()
But ultimately a std::string is really a std::vector under another name, with a slightly different focus and intent. So:
但是最终的std::string实际上是另一个名称下的std::vector，其焦点和意图略有不同。所以:
- just like
std::vector,std::stringhas either a size data member or both start and end data members; - 就像std::vector, std::string有大小数据成员，也有开始和结束数据成员;
- just like
std::vector,std::stringdoes not care about the value of its elements, embedded NUL or others. - 就像std::vector、std::string并不关心它的元素、嵌入的NUL或其他元素的值。
std::string is not a C string with syntax sugar, utility functions and some encapsulation, just like std::vector<T> is not T[] with syntax sugar, utility functions and some encapsulation.
string不是带有语法糖、实用函数和一些封装的C字符串，就像std::vector