opencv C++ 向量访问，交换报错

vector temp;

for (int i = 0;i < contoursSize;i++)
for (int j = i+1;j < contoursSize;j++) {
if (contours[i].size() < contours[j].size()) {//contours是vector>
temp = contours[i];
contours[i] = contours[j];
contours[j] = temp;
}
}

http://blog.csdn.net/qq826309057/article/details/71437926

逻辑错误还是代码错误？