What does string::npos mean in this code?

Question

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

found = str.find(str2);

if (found != std::string::npos)
    std::         


        

Answer 1

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

Answer 2

It means not found.

It is usually defined like so:

static const size_t npos = -1;

It is better to compare to npos instead of -1 because the code is more legible.

Answer 3

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

Answer 4

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
...
}

Answer 5

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

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

Answer 6

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.