NAME¶

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

Synopsis¶

size_type find( const basic_string& str, size_type pos = (1) (noexcept since C++11)
0 ) const; (constexpr since C++20)
size_type find( const CharT* s, size_type pos, size_type (2) (constexpr since C++20)
count ) const;
size_type find( const CharT* s, size_type pos = 0 ) (3) (constexpr since C++20)
const;
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, (5) (since C++17)
(constexpr since 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.
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 avoided by making it a
cases template
P1148R0 C++11 noexcept for overloads (4,5) were 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 characters in the view
find (public member function of std::basic_string_view<CharT,Traits>)

search searches for a range of elements
(function template)

Category:¶

* conditionally noexcept