std::basic_string::rfind(3)

✨ NAME ✨

okay so bestie, this is std::basic_string::rfind from the C++ Standard Libary 💅. She’s the backwards-search icon of std::basic_string, and we’re gonna get you comfy with her fr fr.

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🫶 Synopsis 🫶

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  (5) (since C++17)
(constexpr since C++20)
noexcept(/* see below */);

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. no cap, that’s UB territory.

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. 💖

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🌸 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

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🎀 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.

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💀 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). ✨

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🦋 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

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🧚 Defect reports 🧚

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

• LWG 847 — applied to: C++98; behavior as published: “there was no exception safety guarantee”; correct behavior: “added strong exception safety guarantee”.
• LWG 2064 — applied to: C++11; behavior as published: “overloads (3,4) were noexcept”; correct behavior: “removed”.
• LWG 2946 — applied to: C++17; behavior as published: “overload (5) caused ambiguity in some cases”; correct behavior: “avoided by making it a template”.
• P1148R0 — applied to: C++11 / C++17; behavior as published: “noexcept for overloads (4,5) were accidently dropped by LWG2064/LWG2946”; correct behavior: “restored”.

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🌈 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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👑 Category 👑

• conditionally noexcept

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that’s the whole vibe, bestie 💖 — source: cppreference.com, dated 2024.06.10, page: std::basic_string::rfind(3). mwah, go slay your backwards searches ✨💅