Default constructed std::string c_str() value

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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.

  1. Does "CAN have 0 added to it" == "MUST and ALWAYS have 0 added to it"?

  2. 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?

stdstring c++
1 Answers

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 the std::string object, which is an empty string for both s1.

The second assertion is not guaranteed to succeed. There is nothing requiring the c_str() returned from a std::string to 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-string configure option).

Note that prior to C++11, data() did not have the same effect as c_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 replacing c_str() with data() in your examples would, prior to C++11, result in undefined behavior on the call to strlen.

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 that data() + i == &operator[](i) for all i in the range [0,size()] and operator[] is specified in [strings.access]/2 to return a reference to a CharT() (aka a null character) for operator[](size()) without any conditions.

The strange wording has also been replaced via editorial change in 2018, see https://github.com/cplusplus/draft/pull/1879.

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