vector::capacity

vector::capacity（容量）

Return size of allocated storage capacity
Returns the size of the storage space currently allocated for the vector, expressed in terms of elements.

This capacity is not necessarily equal to the vector size. It can be equal or greater, with the extra space allowing to accommodate
for growth without the need to reallocate on each insertion.

Notice that this capacity does not suppose a limit on the size of the vector. When this capacity is exhausted and more is needed,
it is automatically expanded by the container (reallocating it storage space). The theoretical limit on the size of a vector is
given by member max_size.

The capacity of a vector can be explicitly altered by calling member vector::reserve.

Parameters

none

Return Value

The size of the currently allocated storage capacity in the vector, measured in terms of the number elements it can hold.

Member type size_type is an unsigned integral type.

capacity的意思是容量，此方法返回的是该vector对象最多能容纳多少个元素。
size的意思是大小，此方法是返回该vector对象当前有多少个元素。

// comparing size, capacity and max_size
#include <iostream>
#include <vector>
using namespace std;
int main ()
{
std::vector<int> myvector;

// set some content in the vector:
for (int i=0; i<100; i++) myvector.push_back(i);

cout << "size: " << (int) myvector.size() << '\n';
cout << "capacity: " << (int) myvector.capacity() << '\n';
cout << "max_size: " << (int) myvector.max_size() << '\n';
return 0;
}