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

 

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Feature test macros (C++20)

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Concepts library (C++20)

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Strings library

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basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23) basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)   basic_string::swap

Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11) stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

size_type find( const basic_string& str, size_type pos = 0 ) const;	(1)	(noexcept since C++11) (constexpr since C++20)
size_type find( const CharT* s, size_type pos, size_type count ) const;	(2)	(constexpr since C++20)
size_type find( const CharT* s, size_type pos = 0 ) const;	(3)	(constexpr since C++20)
size_type find( CharT ch, size_type pos = 0 ) const;	(4)	(noexcept since C++11) (constexpr since C++20)
template< class StringViewLike > size_type find( const StringViewLike& t,                 size_type pos = 0 ) const noexcept(/* see below */);	(5)	(since C++17) (constexpr since C++20)

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.

If [s, s + count) is not a valid range, the behavior is undefined.

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).

If [s, s + Traits::length(s)) is not a valid range, the behavior is undefined.

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 are 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>>)

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(int id, std::string::size_type n, std::string const& s)
{
    std::cout << id << ") ";
    if (std::string::npos == n)
        std::cout << "not found! n == npos\n";
    else
        std::cout << "found @ n = " << n << ", substr(" << n << ") = "
                  << std::quoted(s.substr(n)) << '\n';
}
 
int main()
{
    std::string::size_type n;
    std::string const s = "This is a string"; /*
                             ^  ^  ^
                             1  2  3          */
 
    // search from beginning of string
    n = s.find("is");
    print(1, n, s);
 
    // search from position 5
    n = s.find("is", 5);
    print(2, n, s);
 
    // find a single character
    n = s.find('a');
    print(3, n, s);
 
    // find a single character
    n = s.find('q');
    print(4, n, s);
}

Output:

1) found @ n = 2, substr(2) = "is is a string"
2) found @ n = 5, substr(5) = "is a string"
3) found @ n = 8, substr(8) = "a string"
4) not found! n == npos

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

strstr	finds the first occurrence of a substring of characters (function)
wcsstr	finds the first occurrence of a wide string within another wide 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 (public member function of std::basic_string_view<CharT,Traits>)
search	searches for the first occurrence of a range of elements (function template)