我的理解是， string :: find（char c）返回 -1 当它没有被发现。

这是不准确的。根据文档：

返回值

如果找不到
这样的子字符串，找到的子字符串或npos的第一个字符的位置。

所以准确的说，当找不到 std :: string :: find 时，会返回 std :: string :: npos 。重点是 std :: string :: npos 的类型是 std :: string :: size_type ，是一个无符号整数类型。即使它是从 -1 的值初始化的，它不是 -1 ;它仍然没有签名。所以 s.find（'8'）将始终为 false ，因为不可能是负数。

std :: string :: npos ：

  static const size_type npos = -1; 
 

这是一个特殊值，等于 size_type 。

所以你应该使用 std :: string :: npos 检查结果，避免这种混淆。

$ b如果（s.find（'8'）== std :: string :: npos）
cout<< 未找到<< ENDL;
else
cout<< 找到<< ENDL;

---

if（s。 find（'8'）== - 1）可以正常工作，因为 operator == 这里是没有签名的，右边的是签名的。根据算术运算符的规则，

$ b否则，如果无符号操作数的转换等级大于或等于有符号操作数的转换等级，则将有符号操作数转换为

所以 -1 将被转换为无符号，这是 std :: string :: npos 的值，然后按预期工作。

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

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.