在重载输入输出流时，为什么输入输出流要放在前面

Question

template<typename T>
ostream& operator << ( ostream& ot,vector<T> const& v)
{
    ot << "{";
    for (size_t i = 0; i < v.size(); ++i)
    {
        ot << v[i];
        if (i != v.size() - 1)
        {
            ot << ",";
        }
    }
    ot << "}";
    return ot;
}
int main()
{
    vector<int> a = { 1,2,3,4,5 };
    cout << a << endl;

    vector<string> b = { "ok","fun","hello"};
    cout << b << endl;

    return 0;
}

在重载输入输出流时，为什么输入输出流要放在前面，如上面ot，如果ot和容器位置调换，则重载不成功会报错

Answer 1

重载操作符就是函数

cout<<a
就是
operator<<(cout,a)