Does C 11\'s `string::c_str()` Still Guarantee a Null-Terminated String?

Is C 11's string::c_str() Function No Longer Null-Terminated?

In C , the basic_string class provides various methods to access its characters. One of these methods, c_str(), has been a source of confusion since C 11. Prior to C 11, c_str() was responsible for returning a null-terminated character array representing the string's contents.

However, in C 11, the definition of basic_string::c_str() has changed. It is now defined to be the same as basic_string::data, which in turn returns a pointer to the beginning of the string's internal character buffer. This raised a question: does this change imply that c_str() no longer guarantees a null-terminated string?

The Answer: Strings Require Null-Terminated Buffers Internally

The answer to this question is no. Despite the changes in C 11, strings are still required to use null-terminated buffers internally. This is evident from the definition of the operator[] method (21.4.5) in the C standard:

Requires: pos <p>c_str() is defined in terms of operator[]:</p><pre class="brush:php;toolbar:false">Returns: A pointer p such that p + i == &operator[](i) for each i in [0,size()].

Since c_str() and data are both required to be O(1) operations, the implementation is effectively forced to use null-terminated buffers. Additionally, the return value requirement for operator[] ensures that c_str() returns a pointer to a null-terminated buffer. Therefore, c_str() continues to provide a null-terminated string, despite the changes in C 11.

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