这两种输入向量元素的方法之间的区别？ 哪个更有效？

Question

Is the memory allocated to the vector dynamically in both cases or statically in 1st case and dynamically in 2nd?

1st

vector<int>::size_type n;
cin>>n;
vector<int> a(n);

for(vector<int>::size_type i=0;i<n;i++)
    cin>>a[i];

2nd

vector<int>::size_type n;
int temp;
cin>>n;
vector<int> a;

for(vector<int>::size_type i=0;i<n;i++)
{
    cin>>temp;
    a.push_back(temp);
}

Answer 1

Memory will be allocated dynamically (from heap) in both cases, but you will get only one allocation for first case and multiple reallocations in second. Actual number of reallocation in second case is somewhere between 1 and n.

Answer 2

In both cases the memory for elements is allocated dynamically. In the second case the memory reallocated almost each time when a new element is added. You could make these two cases equivalent if in the second case you would write

vector<int> a;
a.reserve( n );

Answer 3

在这两种情况下，都必须动态分配内存，因为在两种情况下都可以将更多元素添加到向量中。