std::string s1;
std::string s2;
assert(strlen(s1.c_str()) == 0);
assert(s1.c_str() == s2.c_str());
这两个断言总是正确的吗？
我使用C ++ 11，并且已经检查了标准，§21.4.2中的表63说：
data（）一个可复制的非挂钩指针，可以将0添加到它
size（）0
容量（）未指定的值
我认为 c_str（）与 data（）相同。但是我对这种定义有一些疑问。
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“ 可以添加0个“ ==” 必须并且始终添加了0？
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所有默认构造的std :: String是否共享相同的底层缓冲区？
我在GCC上测试，这两个断言是正确的。我想知道所有编译器总是正确的吗？
std::string s1;
std::string s2;
assert(strlen(s1.c_str()) == 0);
assert(s1.c_str() == s2.c_str());
Does these two assert always true?
I use C++11, and I have checked the standard, the Table 63 in §21.4.2 says:
data() a non-null pointer that is copyable and can have 0 added to it
size() 0
capacity() an unspecified value
I think c_str() is the same as data(). But I have some question about this defination.
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Does "CAN have 0 added to it" == "MUST and ALWAYS have 0 added to it"?
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Does all default constructed std::string shared a same underlay buffer?
I test on gcc, these two assert is true. I wonder does these always true for all compiler?
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评论（1）
第一断言可以成功。
c_str（）始终将指针返回到null端接的字符串，该字符串与std :: String :: String对象相同的字符串内容，这是两个<<代码> S1。不能保证第二个断言能成功。如果内容相同，则没有什么需要
c_str（）从std :: string返回的。默认结构的字符串不需要共享相同的基础缓冲区。那将是特定标准库实施的实施细节。 （我认为libstdc ++会根据配置做出一些反兼容（？）原因（？）的原因，如果我记得正确，请参见- 启用 - dynamic-stringconfigureoption）。请注意，在C ++ 11之前，
data（）did 不是的效果与c_str（）。data（）不能保证给指向零端的字符串的指针。如果字符串是空的，则不允许将其返回的指针被删除。因此，用c_str（）用data（）在您的示例中，在C ++ 11之前，会导致对strlen 。措辞“ and可以将0添加到它”有些怪异，我不确定它应该传达什么，但是对于C ++ 11（草稿N3337）
data（）的返回值在这样，data（） + i ==＆amp; operator []（i）在范围内 in范围[0， size（）]和operator []在 [strings.access]/2 返回Chart（）（aka a aka a null cartem）的参考）没有任何条件。奇怪的措辞也已通过2018年的社论更改替换，请参见 https://github.com/ cplusplus/draft/pult/1879 。
The first assertion is guaranteed to succeed.
c_str()always returns a pointer to a null-terminated string with the same string contents as held by thestd::stringobject, which is an empty string for boths1.The second assertion is not guaranteed to succeed. There is nothing requiring the
c_str()returned from astd::stringto be the same if the content is the same. Default-constructed strings do not need to share the same underlying buffer. That would be an implementation detail of a particular standard library implementation. (I think libstdc++ does something like this depending on configuration for some backwards-compatibility(?) reasons if I remember correctly, see the--enable-fully-dynamic-stringconfigureoption).Note that prior to C++11,
data()did not have the same effect asc_str().data()was not guaranteed to give a pointer to a null-terminated string. If the string was empty, then the pointer returned by it was not allowed to be dereferenced. So replacingc_str()withdata()in your examples would, prior to C++11, result in undefined behavior on the call tostrlen.The wording "and can have 0 added to it" is somewhat weird and I am not completely sure what it is supposed to convey, but for C++11 (draft N3337)
data()'s return value is further specified in [string.accessors]/1 so thatdata() + i == &operator[](i)for alliin the range[0,size()]andoperator[]is specified in [strings.access]/2 to return a reference to aCharT()(aka a null character) foroperator[](size())without any conditions.The strange wording has also been replaced via editorial change in 2018, see https://github.com/cplusplus/draft/pull/1879.