std::basic_string::find

size_type find( const basic_string& str, size_type pos = 0 ) const	(1)
size_type find( const CharT* s, size_type pos, size_type count ) const;	(2)
size_type find( const CharT* s, size_type pos = 0 ) const;	(3)
size_type find( CharT ch, size_type pos = 0 ) const;	(4)
template < class T > size_type find( const T& t, size_type pos = 0 ) const;	(5)	(since C++17)

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 first count characters of the character string pointed to by s. s can include 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.

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 only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, 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) (none)	(until C++11)
1,4) `noexcept` specification: noexcept 2,3) (none)	(since C++11) (until C++14)
1) `noexcept` specification: noexcept 2,3,4) (none)	(since C++14)
5) Throws nothing unless the initialization of `sv` throws an exception.	(since C++17)

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 2946	C++17	`string_view` overload causes ambiguity in some cases	avoided by making it a template

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

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Null-terminated strings
Byte strings
Multibyte strings
Wide strings
Classes
basic_string
basic_string_view (C++17)
char_traits