为什么data（）和c_str（）返回char const *，而operator []返回char&amp ;？ [英] Why do data() and c_str() return char const*, while operator[] returns char&?

问题描述

为什么 std :: string :: data 和 std :: string :: c_str（）为什么返回指针到 const 字符，而 std :: string :: operator [] 返回对可变字符的引用？

Why do std::string::data and std::string::c_str() return pointers to const chars, while std::string::operator[] returns references to mutable chars?

std::string string("eightfold is the greatest");

auto s = string.data();
*s = 'r'; // illegal

auto t = &string[0];
*t = 'r'; // totally fine

auto& c = string[0];
c = 'r'; // totally fine

为什么不 std :: string :: data （）和 std :: string :: c_str（）返回 char * ，或为什么 cstd :: string :: operator [] 不返回 char const& ？

Why don’t std::string::data() and std::string::c_str() return char*, or why doesn’t std::string::operator[] return char const&?

这背后的原理是什么？

推荐答案

operator [] 使您可以直接访问 std :: string 对象的受控序列。 c_str（）最初没有。

operator [] gives you direct access to the controlled sequence of std::string object. c_str() originally did not.

在 std :: string的原始规范中不需要将存储的序列作为零终止的字符串。这意味着通常情况下 c_str（）无法返回指向存储序列的直接指针。它必须返回一个指针，该指针指向受控序列的完全独立的，单独分配的临时副本（带有附加的零终止符）。因此，尝试修改 c_str（）返回的C字符串根本没有任何意义。应用于该单独的C字符串的任何修改都不会传播到实际的受控序列。 （实际上，该规范明确禁止任何修改尝试。例如，对于空的 std :: string ，实现可以简单地返回指向字符串文字<$ c $的指针。 c> ，这当然是不可修改的，可以很容易地在所有 std :: string 对象之间共享。）使 c_str（）返回 const char * 的完美意义。

In the original specification of std::string the stored sequence was not required to be a zero-terminated string. This meant that in general case c_str() could not return a direct pointer to the stored sequence. It had to return a pointer to a completely independent, separately allocated temporary copy of the controlled sequence (with an added zero terminator character). For this reason, trying to modify the C-string returned by c_str() made no sense at all. Any modifications applied to that separate C-string would not be propagated to the actual controlled sequence. (In fact, the specification explicitly prohibited any modification attempts. For example, for an empty std::string an implementation could simply return a pointer to a string literal "", which was of course non-modifiable and could be easily shared between all std::string objects.) So, it made perfect sense to make c_str() to return const char *.

C ++ 11更改了 c_str（）的内部规范，使其返回指向实际受控序列的直接指针。但是 c_str（）的外部规范保持不变，以使其与旧规范保持一致。

C++11 changed the internal specification of c_str() making it to return a direct pointer to the actual controlled sequence. But the external spec of c_str() remained unchanged to keep it aligned with the legacy spec.

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