string.find() returns true when ==-1 is used but false when

问题

I am trying to find a character within a string but I am getting unexpected results. My understanding is that string::find(char c) returns -1 when it is not found. However, I am getting some unexpected results.

Even though the string does not include an '8', it is still returning true.

std::string s = "123456799";
if(s.find('8')<0)
    cout << "Not Found" << endl;
else
    cout <<  "Found" << endl;

//Output: Found

However, when using == instead the code works as expected.

std::string s = "123456799";
if(s.find('8')==-1)
    cout << "Not Found" << endl;
else
    cout <<  "Found" << endl;

//Output: Not Found

回答1:

My understanding is that string::find(char c) returns -1 when it is not found.

It's not accurate. According to the documentation:

Return value
Position of the first character of the found substring or npos if no such substring is found.

So to be precise, when not found std::string::find will return std::string::npos. The point is that the type of std::string::npos is std::string::size_type, which is an unsigned integer type. Even it's initialized from value of -1, it's not -1; it's still unsigned. So s.find('8')<0 will always be false because it's not possible to be negative.

Documentation of std::string::npos:

static const size_type npos = -1;
This is a special value equal to the maximum value representable by the type size_type.

So you should use std::string::npos for checking the result, to avoid such kind of confusing.

if (s.find('8') == std::string::npos)
    cout << "Not Found" << endl;
else
    cout <<  "Found" << endl;

if(s.find('8')==-1) works fine, because the left-hand operand of operator== here is unsigned, the right-hand one is signed. According to the rules for arithmetic operators,

Otherwise, if the unsigned operand's conversion rank is greater or equal to the conversion rank of the signed operand, the signed operand is converted to the unsigned operand's type.

So -1 will be converted to unsigned, which is the value of std::string::npos and then all work as expected.