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]

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constexpr const_reference operator[](size_type pos) const; constexpr reference operator[](size_type pos);
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Preconditions: pos <= size().
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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.
3
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Throws: Nothing.
4
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Complexity: Constant time.
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constexpr const_reference at(size_type pos) const; constexpr reference at(size_type pos);
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Returns: operator[](pos).
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Throws: out_of_range if pos >= size().
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constexpr const charT& front() const; constexpr charT& front();
7
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Preconditions: !empty().
8
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Effects: Equivalent to: return operator[](0);
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constexpr const charT& back() const; constexpr charT& back();
9
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Preconditions: !empty().
10
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Effects: Equivalent to: return operator[](size() - 1);