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

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
...
}