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std::basic_string::find

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size_type find( const basic_string& str, size_type pos = 0 ) const;
 (1)
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size_type find( const CharT* s, size_type pos, size_type count ) const;
 (2)
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size_type find( const CharT* s, size_type pos = 0 ) const;
 (3)
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size_type find( CharT ch, size_type pos = 0 ) const;
 (4)

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).


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

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)

Example

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#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:

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found: is is a string
found: is a string
found: a string
not found

See also

finds the first occurrence of a substring of characters
(function)
finds the first occurrence of a wide string within another wide string
(function)
finds the first occurrence of a character
(function)
finds the first occurrence of a wide character in a wide string
(function)
find the last occurrence of a substring
(public member function)
find first occurrence of characters
(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)
searches for a range of elements
(function template)
doc_CPP
doc_CPP
2025-01-10 15:47:30
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