为什么vector::clear不在foreach循环内工作?

我的代码:

vector<int> v[10];
const int x = 3;
void clearBySimpleLoop(){
    for (int i = 0; i < x; i++){
        v[i].clear();
    }
}
int main()
{
    for (int i = 0; i < x; i++){
        v[i].push_back(11+i);
        v[i].push_back(11+i+1);
        v[i].push_back(11+i+2);
    }
    for (auto vec : v) vec.clear(); //#01. Contents are not cleared. Size of the contained vectors remains same.
    clearBySimpleLoop(); //#02. Contents are cleared. Size of the contained vectors becomes zero. 
    return 0;
}

问题是为什么代码在 前额 循环(01)无法清除数组中的向量,而简单的 对于 循环(02)成功吗?

演示: https://onlinegdb.com/B1m8-2jG4