最近在看《Effective STL》，【条款17：使用“交换技巧”修整过剩容量】中提到容器的成函数void swap(container& from)，即实现容器对象与from对象的交换。
另外，对于连续内存容器vector和string，还有shrink to fit（收缩到合适，根据容器的实际size设置capacity，减少实际内存分配）的功能。
以vector为例，首先说下，size和capacity、resize和reserve的含义及区别。
1. size和capacity、resize和reserve
size：对象的元素个数，除reserve外的几乎所有vector操作都可以影响size；
capacity：容器预留容量，即占用真实内存量，capacity >= size。push_back、insert、resize、reserve操作可以影响capacity，其特点为只增不减（即使erase和clear操作）；
resize：可以改变size，如果resize的元素个数大于capacity，则同时改变capacity，增加对象内存空间；
reserve：不改变size，如果reserve的元素个数大于capacity，则改变capacity，增加对象内存空间，反之，则不影响capacity；
代码示例如下：
#include <iostream>
#include <vector> using namespace std; int main()
{
vector<int> v;
cout << "initial: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.insert(v.end(), , );
cout << "after insert: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.erase(v.end()-, v.end());
cout << "after erase: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.reserve();
cout << "after reserve: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.resize();
cout << "after resize: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.reserve();
cout << "after reserve: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.clear();
cout << "after clear: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; return ;
}
输出结果为：
initial:
size of v is
capacity of v is after insert:
size of v is
capacity of v is after erase:
size of v is
capacity of v is after reserve:
size of v is
capacity of v is after resize:
size of v is
capacity of v is after reserve:
size of v is
capacity of v is after clear:
size of v is
capacity of v is
根据上面几个概念及Demo，可以看出来，vector对象的实际内存（即capacity）在多次insert和erase操作后大于实际元素个数（size），即使在clear操作后，capacity依然不会减小为零。
所以，有必要对这种基于内存池的容器做内存收缩。
2. swap：shrink to fit
修改vector对象多余内存空间的方法：
vector<int>(v).swap(v); // v为一个vector对象
vector<int>(v)基于v拷贝构造了一个临时vector，其分配了v.size()个元素的内存空间，即capacity为v.size()，然后vector<int>(v)与v交换数据，交换后，v的capacity也减少为v.size()，而临时vector对象vector<int>(v)被销毁。
另外，vector<int>().swap(v); // 可以清除v的内存空间
代码示例如下：
#include <iostream>
#include <vector> using namespace std; int main()
{
vector<int> v(, );
cout << "initial: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; v.erase(v.end()-, v.end());
cout << "after erase: " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; vector<int>(v).swap(v);
cout << "after swap (shrink to fit): " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; vector<int>().swap(v);
cout << "after swap (clear): " << endl;
cout << "size of v is " << v.size() << endl;
cout << "capacity of v is " << v.capacity() << endl << endl; return ;
}
输出结果为：
initial:
size of v is
capacity of v is after erase:
size of v is
capacity of v is after swap (shrink to fit):
size of v is
capacity of v is after swap (clear):
size of v is
capacity of v is
如果使用C++11及更新标准的话，可以直接使用shrink_to_fit函数。