c++中-1

今天在写kmp算法的时候遇到一个很神奇的现象
代码如下:

#include <iostream>

using namespace std;

int main(){

    string needle = "ll";
    int needle_length = needle.length();
    cout<<"(-1 < (needle.length()))  "<<(-1 < (needle.length()))<<endl;
    cout<<"(-1 < needle_length)      "<<(-1 < needle_length)<<endl;

    return 0;
}

输出是:

(-1 < (needle.length()))  0
(-1 < needle_length)      1

后来上网搜索得知str.length()返回的是unsigned int类型,与int比较会得不到预期的结果,需要强制转化为int才能得到正确的结果.