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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../string/basic_string/operator_at.html below:

std::basic_string<CharT,Traits,Allocator>::operator[] - cppreference.com

CharT& operator[]( size_type pos );

(1) (constexpr since C++20)

const CharT& operator[]( size_type pos ) const;

(2) (constexpr since C++20)

Returns a reference to the character at specified location pos if pos < size(), or if pos == size():

1)

The behavior is undefined.

(until C++11)

Returns a reference to CharT(), if the object referred by the returned reference is modified to any value other than CharT(), the behavior is undefined.

(since C++11)

2) Returns a reference to CharT().

If pos > size() is true, the behavior is undefined.

(until C++26)

If pos > size() is true:

If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under âobserveâ evaluation semantic, the behavior is undefined.
If the implementation is not hardened, the behavior is undefined.

(since C++26) [edit] Parameters pos - position of the character to return [edit] Return value

1) *(begin() + pos) if pos < size(), or a reference to CharT() if pos == size()(since C++11).

2) *(begin() + pos) if pos < size(), or a reference to CharT() if pos == size().

[edit] Complexity

Constant.

[edit] Example

#include <iostream>
#include <string>
 
int main()
{
    const std::string e("Exemplar");
    for (unsigned i = e.length() - 1; i != 0; i /= 2)
        std::cout << e[i];
    std::cout << '\n';
 
    const char* c = &e[0];
    std::cout << c << '\n'; // print as a C string
 
    // Change the last character of s into a 'y'
    std::string s("Exemplar ");
    s[s.size() - 1] = 'y'; // equivalent to s.back() = 'y';
    std::cout << s << '\n';
}

Output:

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 259 C++98 overload (1) could return const lvalue
data()[pos], which was ill-formed changed to return
*(begin() + pos) LWG 2475 C++11 if pos == size(), the behavior of modifying the
object referred by the returned reference was undefined well-defined if
modified to CharT() [edit] See also accesses the specified character with bounds checking
(public member function) [edit] accesses the first character
(public member function) [edit] accesses the last character
(public member function) [edit] accesses the specified character
(public member function of std::basic_string_view<CharT,Traits>) [edit]

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