Accessing Strings

In C++, you can access individual characters in a std::string using array subscript notation [] or the at() method. The subscript operator is fast but does not check bounds, while at() checks the index and throws an exception if it is out of range.

Both operator[] and at() return references for non-const strings, allowing modification; for const strings they return const references.

You can modify characters in a string using either array notation or the at() method. Since C++ strings are mutable, the changes are applied directly to the string (provided you have a non-const string).

The at() method checks whether the index is within the valid range and throws std::out_of_range if it is not. By contrast, using array notation with an invalid index results in undefined behavior.

Time complexity for both [] and at() is constant; the difference is the bounds check and potential exception.

C++11 added the front() and back() methods to access the first and last characters of a string. These methods make the intent clearer and avoid manual index calculations.

operator[], at(), front(), and back() return char& on non-const strings and const char& on const strings. To modify characters, you must have a non-const string (or a non-const reference to it).

Taking references or iterators to string elements is fine, but remember they can be invalidated by operations that reallocate the string (e.g., growth).

c_str() returns a pointer to a null-terminated read-only character array. Do not write through it.

Since C++17, data() returns a writable pointer for non-const strings. You may write through it within the current size of the string, but any operation that causes reallocation invalidates the pointer. Never write past size().

std::string stores bytes, not Unicode code points. With UTF-8 text, a user-visible character may span multiple bytes. Indexing by [] or at() steps bytes, so it can land in the middle of a multi-byte sequence and corrupt your string.

If you need Unicode-aware character access, use a proper Unicode library or encode as std::u16string / std::u32string (and handle grapheme clusters with a library).