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

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 .

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

Return value

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

Exceptions

If an exception is thrown for any reason, this function has no effect ( strong exception safety guarantee ).

Example

#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);
}

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.

See also