What does string::npos mean in this code?

Solution 1:

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.

Solution 2:

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

Solution 3:

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.

Solution 4:

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

Solution 5:

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.