# std::basic_string<CharT,Traits,Allocator>::rfind

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< cpp‎ | string‎ | basic string

size_type rfind( const basic_string& str, size_type pos = npos ) const; | (1) | (noexcept since C++11) (constexpr since C++20)
size_type rfind( const CharT* s, size_type pos, size_type count ) const; | (2) | (constexpr since C++20)
size_type rfind( const CharT* s, size_type pos = npos ) const; | (3) | (constexpr since C++20)
size_type rfind( CharT ch, size_type pos = npos ) const; | (4) | (noexcept since C++11) (constexpr since C++20)
template< class StringViewLike > size_type rfind( const StringViewLike& t, size_type pos = npos ) const noexcept(/* see below */); | (5) | (since C++17) (constexpr since C++20)

Finds the last substring that is equal to the given character sequence. The search begins at pos and proceeds from right to left (thus, the found substring, if any, cannot begin in a position following pos). If npos or any value not smaller than size() - 1 is passed as pos, the whole string will be searched.

1) Finds the last substring equal to str.
2) Finds the last substring equal to the range `[`s`,` s + count`)`. This range can include null characters.
If `[`s`,` s + count`)` is not a valid range, the behavior is undefined.
3) Finds the last substring equal to the character string pointed to by s. The length of the string is determined by the first null character using Traits::length(s).
If `[`s`,` s + Traits::length(s)`)` is not a valid range, the behavior is undefined.
4) Finds the last character equal to ch.
5) Implicitly converts t to a string view sv as if by std::basic_string_view <CharT, Traits> sv = t;, then finds the last substring equal to the contents of sv.
This overload participates in overload resolution only if std::is_convertible_v <const StringViewLike&,
std::basic_string_view <CharT, Traits>> is true and std::is_convertible_v <const StringViewLike&, const CharT*> is false.

In all cases, equality is checked by calling `Traits::eq`.

### Parameters

str | - | string to search for
pos | - | position at which to begin searching
count | - | length of substring to search for
s | - | pointer to a character string to search for
ch | - | character to search for
t | - | object (convertible to std::basic_string_view) to search for

### Return value

Position of the first character of the found substring or npos if no such substring is found. Note that this is an offset from the start of the string, not the end.

If searching for an empty string (i.e., str.size(), count, or Traits::length(s) is zero), the empty string is found immediately and `rfind` returns:

- pos, if pos < size();
- size() otherwise, including the case where pos == npos.

Otherwise, if size() is zero, npos is always returned.

### Exceptions

1,4) Throws nothing.
5)
`noexcept` specification:
noexcept( std::is_nothrow_convertible_v <const T&, std::basic_string_view <CharT, Traits>>)

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <string>

    void print(std::string::size_type n,
               std::string::size_type len,
               std::string const &s)
    {
        if (n == std::string::npos)
            std::cout << "not found\n";
        else
            std::cout << "found: " << std::quoted(s.substr(n, len)) << " at " << n << '\n';
    }

    int main()
    {
        std::string::size_type n;
        std::string const s = "This is a string";

        // search backwards from end of string
        n = s.rfind("is");
        print(n, 2, s);

        // search backwards from position 4
        n = s.rfind("is", 4);
        print(n, 2, s);

        // find a single character
        n = s.rfind('s');
        print(n, 1, s);

        // find a single character
        n = s.rfind('q');
        print(n, 1, s);

        // find the prefix (see also s.starts_with("This"))
        n = s.rfind("This", 0);
        print(n, 4, s);
    }

Output:

    found: "is" at 5
    found: "is" at 2
    found: "s" at 10
    not found
    found: "This" at 0

### 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 847 | C++98 | there was no exception safety guarantee | added strong exception safety guarantee
LWG 2064 | C++11 | overloads (3,4) were noexcept | removed
LWG 2946 | C++17 | overload (5) caused ambiguity in some cases | avoided by making it a template
P1148R0 | C++11 C++17 | noexcept for overloads (4,5) were accidently dropped by LWG2064/LWG2946 | restored

### See also

find | finds the first occurrence of the given substring (public member function)
find_first_of | find first occurrence of characters (public member function)
find_first_not_of | find first absence of characters (public member function)
find_last_of | find last occurrence of characters (public member function)
find_last_not_of | find last absence of characters (public member function)
rfind | find the last occurrence of a substring (public member function of `std::basic_string_view<CharT,Traits>`)

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