std::basic_string<CharT,Traits,Allocator>::find_first_of

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basic_string::find_first_of
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size_type find_first_of( const basic_string& str, size_type pos = 0 ) const;
 (1) (noexcept since C++11)
(constexpr since C++20)
size_type find_first_of( const CharT* s,
                         size_type pos, size_type count ) const;
 (2) (constexpr since C++20)
size_type find_first_of( const CharT* s, size_type pos = 0 ) const;
 (3) (constexpr since C++20)
size_type find_first_of( CharT ch, size_type pos = 0 ) const;
 (4) (noexcept since C++11)
(constexpr since C++20)
template< class StringViewLike >

size_type
    find_first_of( const StringViewLike& t,

                   size_type pos = 0 ) const noexcept(/* see below */);
 (5) (since C++17)
(constexpr since C++20)

Finds the first character equal to one of the characters in the given character sequence. The search considers only the range [pos, size()). If none of the characters in the given character sequence is present in the range, npos will be returned.

1) Finds the first character equal to one of the characters in str.
2) Finds the first character equal to one of the characters in 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 first character equal to one of the characters in 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 first 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 first character equal to one of the characters in 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.

Parameters

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

Return value

Position of the found character or npos if no such character is found.

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).

Notes

Traits::eq() is used to perform the comparison.

Example

Run this code
#include <cassert>
#include <iostream>
#include <string>
#include <string_view>
 
int main()
{
    using namespace std::literals;
    std::string::size_type sz;
 
    // (1)
    sz = "alignas"s.find_first_of("klmn"s);
    //     └────────────────────────┘
    assert(sz == 1);
 
    sz = "alignof"s.find_first_of("wxyz"s);
    // no match
    assert(sz == std::string::npos);
 
    // (2)
    sz = "consteval"s.find_first_of("xyzabc", 0, 3);
    // no match (× are not targets)     ×××
    assert(sz == std::string::npos);
 
    sz = "consteval"s.find_first_of("xyzabc", 0, 6);
    //    └───────────────────────────────┘
    assert(sz == 0);
 
    // (3)
    sz = "decltype"s.find_first_of("xyzabc");
    //      └────────────────────────────┘
    assert(sz == 2);
 
    // (4)
    sz = "co_await"s.find_first_of('a');
    //       └──────────────────────┘
    assert(sz == 3);
 
    // (5)
    sz = "constinit"s.find_first_of("int"sv);
    //      └─────────────────────────┘
    assert(sz == 2);
 
    std::cout << "All tests passed.\n";
}

Output: 

All tests passed.

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
accidentally dropped by LWG2064/LWG2946 		restored

See also

 finds the first occurrence of the given substring
(public member function)
find the last occurrence of a substring
(public member function)
find first absence of characters
(public member function)
find last occurrence of characters
(public member function)
find last absence of characters
(public member function)
find first occurrence of characters
(public member function of std::basic_string_view<CharT,Traits>)
returns the length of the maximum initial segment that consists
of only the characters found in another byte string
(function)
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