c – 在向量中找到中间项,为什么要使用“mid = beg(end-beg)/ 2

mid = beg + (end - beg) / 2;

为什么要这样写,为什么不能写成：

mid = (beg + end) /2

是中间=(乞求(结束-1))/ 2一个可行的替代方案？

我很难理解背后的原因.

vector<string> text = {"apple","beer","cat","dog"};
    string sought = "beer";

    auto beg = text.begin(),end = text.end();
    auto mid = text.begin() + (end - beg) / 2;
    while (mid != end && *mid != sought){
        if(sought < *mid){
            end = mid;
        } else {
            beg = mid + 1;
        }
        mid = beg + (end - beg) / 2;
    }