[string.access]

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constexpr const_reference operator[](size_type pos) const;
constexpr reference       operator[](size_type pos);

Returns: *(begin() + pos) if pos < size().

Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object to any value other than charT() leads to undefined behavior.

Complexity: Constant time.

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constexpr const_reference at(size_type pos) const;
constexpr reference       at(size_type pos);

Throws: out_of_range if pos >= size().

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constexpr const_reference front() const;
constexpr reference front();

Effects: Equivalent to: return operator[](0);

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constexpr const_reference back() const;
constexpr reference back();

Effects: Equivalent to: return operator[](size() - 1);

27 Strings library [strings]

27.4 String classes [string.classes]

27.4.3 Class template basic_string [basic.string]

27.4.3.6 Element access [string.access]