std::basic_string::find(3)

NAME
std::basic_string::find - std::basic_string::find

Synopsis
   size_type find( const basic_string& (until
   str, size_type pos = 0 ) const; C++11)
   size_type find( const basic_string& (since
   str, size_type pos = 0 ) const C++11)
   noexcept; (until
                        C++20)
   constexpr size_type find( const (since
   basic_string& str, C++20)
   size_type pos = 0 ) const noexcept;
   size_type find( const CharT* s, (until
   size_type pos, size_type count ) C++20)
   const;
   constexpr size_type find( const
   CharT* s, (since
   size_type pos, size_type count ) C++20)
   const;
   size_type find( const CharT* s, (until
   size_type pos = 0 ) const; C++20)
   constexpr size_type find( const (since
   CharT* s, size_type pos = 0 ) const; (1) C++20)
   size_type find( CharT ch, size_type (until
   pos = 0 ) const; C++11)
                        (since
   size_type find( CharT ch, size_type (2) C++11)
   pos = 0 ) const noexcept; (until
                        C++20)
   constexpr size_type find( CharT ch, (since
   size_type pos = 0 ) const noexcept; (3) C++20)
   template < class StringViewLike > (since
   size_type find( const (4) C++17)
   StringViewLike& t, size_type pos = 0 (until
   ) const noexcept(/* see below */); C++20)
   template < class StringViewLike >
                        (5)
   constexpr size_type find( const (since
   StringViewLike& t, C++20)
   size_type pos = 0 ) const
   noexcept(/* see below */);
   Finds the first substring equal to the given character sequence. Search begins at
   pos, i.e. the found substring must not begin in a position preceding pos.
   1) Finds the first substring equal to str.
   2) Finds the first substring equal to the range [s, s+count). This range may contain
   null characters.
   3) Finds the first 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).
   4) Finds the first character ch (treated as a single-character substring by the
   formal rules below).
   5) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT,
   Traits> sv = t;, then finds the first substring equal to 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.
   Formally, a substring str is said to be found at position xpos if all of the
   following is true:
     * xpos >= pos
     * xpos + str.size() <= size()
     * for all positions n in str, Traits::eq(at(xpos+n), str.at(n))
   In particular, this implies that
     * a substring can be found only if pos <= size() - str.size()
     * an empty substring is found at pos if and only if pos <= size()
     * for a non-empty substring, if pos >= size(), the function always returns npos.

Parameters
   str - string to search for
   pos - position at which to start the search
   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.

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

Example
// Run this code
 #include <string>
 #include <iostream>
 void print(std::string::size_type n, std::string const &s)
 {
     if (n == std::string::npos) {
         std::cout << "not found\n";
     } else {
         std::cout << "found: " << s.substr(n) << '\n';
     }
 }
 int main()
 {
     std::string::size_type n;
     std::string const s = "This is a string";
     // search from beginning of string
     n = s.find("is");
     print(n, s);
     // search from position 5
     n = s.find("is", 5);
     print(n, s);
     // find a single character
     n = s.find('a');
     print(n, s);
     // find a single character
     n = s.find('q');
     print(n, s);
 }

Output:
 found: is is a string
 found: is a string
 found: a string
 not found
  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 2064 C++11 overload (3) and (4) were noexcept removed
   LWG 2946 C++17 string_view overload causes ambiguity in avoided by making it a
                       some cases template
   P1148R0 C++11 noexcept for overload (4)/(5) was restored
            C++17 accidently dropped by LWG2064/LWG2946

See also
   strstr finds the first occurrence of a substring of characters
                     (function)
                     finds the first occurrence of a wide string within another wide
   wcsstr string
                     (function)
   strchr finds the first occurrence of a character
                     (function)
   wcschr finds the first occurrence of a wide character in a wide string
                     (function)
   rfind find the last occurrence of a 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)
   find find characters in the view
   (C++17) (public member function of std::basic_string_view<CharT,Traits>)
   search searches for a range of elements
                     (function template)