Is Accessing Contiguous Characters in std::string Using `&s[0]` Safe in C ?

Accessing Contiguous Characters in std::string

One may encounter the following code in C :

std::string s;
s.resize(strLength);
memcpy(&s[0], str, strLength);  // Is this safe?

While using &s[0] is safe in std::vector, its usage in std::string raises concerns. This article examines the safety of the presented code.

In C 98/03, a std::string's allocation was not guaranteed to be contiguous. However, C 11 mandates contiguity. In practice, all known implementations use contiguous storage.

Moreover, the C 11 standard guarantees the safety of &s[0], even for an empty string. This is because the operator[] for std::string is defined 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

Furthermore, data() is defined as:

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

Thus, &s[0] provides a valid pointer to the initial character of the string.

Note: Prior to standardization, accessing &s[0] for an empty string was undefined behavior. However, this has since been resolved in later standard drafts.

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