[c++] What does string::npos mean in this code?

What does the phrase std::string::npos mean in the following snippet of code?

found = str.find(str2);

if (found != std::string::npos)
    std::cout << "first 'needle' found at: " << int(found) << std::endl;

Value of string::npos is 18446744073709551615. Its a value returned if there is no string found.

we have to use string::size_type for the return type of the find function otherwise the comparison with string::npos might not work. size_type, which is defined by the allocator of the string, must be an unsigned integral type. The default allocator, allocator, uses type size_t as size_type. Because -1 is converted into an unsigned integral type, npos is the maximum unsigned value of its type. However, the exact value depends on the exact definition of type size_type. Unfortunately, these maximum values differ. In fact, (unsigned long)-1 differs from (unsigned short)-1 if the size of the types differs. Thus, the comparison

idx == std::string::npos

might yield false if idx has the value -1 and idx and string::npos have different types:

std::string s;
...
int idx = s.find("not found"); // assume it returns npos
if (idx == std::string::npos) { // ERROR: comparison might not work
...
}

One way to avoid this error is to check whether the search fails directly:

if (s.find("hi") == std::string::npos) {
...
}

However, often you need the index of the matching character position. Thus, another simple solution is to define your own signed value for npos:

const int NPOS = -1;

Now the comparison looks a bit different and even more convenient:

if (idx == NPOS) { // works almost always
...
}

An answer for these days of C++17, when we have std::optional:

If you squint a bit and pretend std::string::find() returns an std::optional<std::string::size_type> (which it sort of should...) - then the condition becomes:

auto position = str.find(str2);

if ( position.has_value() ) {
    std::cout << "first 'needle' found at: " << found.value() << std::endl;
}

std::string::npos is implementation defined index that is always out of bounds of any std::string instance. Various std::string functions return it or accept it to signal beyond the end of the string situation. It is usually of some unsigned integer type and its value is usually std::numeric_limits<std::string::size_type>::max () which is (thanks to the standard integer promotions) usually comparable to -1.

found will be npos in case of failure to find the substring in the search string.

string::npos is a constant (probably -1) representing a non-position. It's returned by method find when the pattern was not found.

The document for string::npos says:

npos is a static member constant value with the greatest possible value for an element of type size_t.

As a return value it is usually used to indicate failure.

This constant is actually defined with a value of -1 (for any trait), which because size_t is an unsigned integral type, becomes the largest possible representable value for this type.

size_t is an unsigned variable, thus 'unsigned value = - 1' automatically makes it the largest possible value for size_t: 18446744073709551615

npos is just a token value that tells you that find() did not find anything (probably -1 or something like that). find() checks for the first occurence of the parameter, and returns the index at which the parameter begins. For Example,

  string name = "asad.txt";
  int i = name.find(".txt");
  //i holds the value 4 now, that's the index at which ".txt" starts
  if (i==string::npos) //if ".txt" was NOT found - in this case it was, so  this condition is false
    name.append(".txt");

$21.4 - "static const size_type npos = -1;"

It is returned by string functions indicating error/not found etc.

static const size_t npos = -1;

Maximum value for size_t

npos is a static member constant value with the greatest possible value for an element of type size_t.

This value, when used as the value for a len (or sublen) parameter in string's member functions, means "until the end of the string".

As a return value, it is usually used to indicate no matches.

This constant is defined with a value of -1, which because size_t is an unsigned integral type, it is the largest possible representable value for this type.