Does "&s[0]" point to contiguous characters in a std::string?

Question

I'm doing some maintenance work and ran across something like the following:

std::string s; s.resize( strLength );   // strLength is a size_t with the length of a C string in it.   memcpy( &s[0], str, strLength ); 

I know using &s[0] would be safe if it was a std::vector, but is this a safe use of std::string?

Answer 1

A std::string's allocation is not guaranteed to be contiguous under the C++98/03 standard, but C++11 forces it to be. In practice, neither I nor Herb Sutter know of an implementation that does not use contiguous storage.

Notice that the &s[0] thing is always guaranteed to work by the C++11 standard, even in the 0-length string case. It would not be guaranteed if you did str.begin() or &*str.begin(), but for &s[0] the standard defines operator[] as:

Returns: *(begin() + pos) if pos < size(), otherwise a reference to an object of type T with value charT(); the referenced value shall not be modified

Continuing on, data() is defined as:

Returns: A pointer p such that p + i == &operator[](i) for each i in [0,size()].

(notice the square brackets at both ends of the range)

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Notice: pre-standardization C++0x did not guarantee &s[0] to work with zero-length strings (actually, it was explicitly undefined behavior), and an older revision of this answer explained this; this has been fixed in later standard drafts, so the answer has been updated accordingly.

Answer 2

Technically, no, since std::string is not required to store its contents contiguously in memory.

However, in almost all implementations (every implementation of which I am aware), the contents are stored contiguously and this would "work."