What does the "n" argument of std::string::find_first_of(cstr, pos, n) mean?

[ElusiveTau]

As I understand it, string::find_first_of looks though a string for any character found in cstr in the interval [pos, pos+n).

So in the code below, the substring being searched should be "con", which contains the character "c", found at position 0. So shouldn't the function return 0? Why is string::npos (not found) returned?

// C++11
#include <iostream>
#include <string>
#include <cstddef>   // std::size_t

using std::string;
using std::cout;

int main ()
{
  // What is the substring being searched? Isn't it "con"? 
  size_t idx = string("consteval").find_first_of("xyzabc", 0, 3);
  cout << "Found: " << idx << "\n";

  return 0;
}

Output

Found: 4294967295

Example is taken from here [1].

[1] https://en.cppreference.com/w/cpp/string/basic_string/find_first_of

Edited

Looking at this example:

string("czonsteval").find_first_of("xyzabc", 0, 3); // Returns 1 

It seems n specifies the number of characters of "xyzabc" (cstr) to use for searching. For n=3, we're searching for any characters in "xyz" that are found in "czonsteval". The "z" is found at position 1.

If we do:

string("czonsteval").find_first_of("xyzabc", 0, 2); // Returns npos 

We searching for anyone of characters "xy" in "czonsteval" which does not exist. So npos is returned.

I think the documentation and example code on cppreference and cplusplus is pretty ambiguous when discussing this parameter :(

[mbozzi]

To answer the title question, it is the length of the array of characters whose first element is pointed to by cstr.

[Peter87]

mbozzi is right.

This expression

string("consteval").find_first_of("xyzabc", 0, 3)

means the same as

string("consteval").find_first_of(string("xyzabc", 3), 0)

Note that it is only when you pass a const char* that you can provide a third argument. This could be useful for example if the string is not null terminated.

[ElusiveTau]

mbozzi wrote:

To answer the title question, it is the length of the array of characters whose first element is pointed to by cstr.

Peter87 wrote:

Note that it is only when you pass a const char* that you can provide a third argument. This could be useful for example if the string is not null terminated.

Well put, thanks.

[keskiverto]

ElusiveTau wrote:

I think the documentation and example code on cppreference and cplusplus is pretty ambiguous when discussing this parameter :(

cplusplus wrote:

size_t find_first_of (const char* s, size_t pos, size_t n) const;

s
Pointer to an array of characters.
If argument n is specified (3), the first n characters in the array are searched for.

cppreference wrote:

constexpr size_type find_first_of( const CharT* s,
                                   size_type pos, size_type count ) const;

Finds the first character equal to one of the characters in the range [s, s+count). This range can include null characters.

count - length of character string identifying characters to search for
s - pointer to a character string identifying characters to search for

I might look those in biased way, but they don't seem terribly ambiguous.

When one would use a range that contains nulls?