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在学习vector时看到capacity()和size()都是求容器在内存中分配的大小，为什么两者得到的结果却不一样呢？下面把代码和结果贴出来和大家分享下！
注：以下结果是在vs2005下编译的。
void capacity()
{
vector<int>v;
cout << "初始值" << endl;
cout << "v.size() = " << v.size() << "\tv.capacity() = " << v.capacity() << endl;
cout << "变化后的值" << endl;
for(int i = 0; i < 15; i++)
{
v.push_back(i);
cout << "v.size() = " << v.size() << "\tv.capacity() = " << v.capacity() << endl;
}
}得到的结果为：
初始值
v.size() = 0 v.capacity() = 0
变化后的值
v.size() = 1 v.capacity() = 1
v.size() = 2 v.capacity() = 2
v.size() = 3 v.capacity() = 3
v.size() = 4 v.capacity() = 4
v.size() = 5 v.capacity() = 6
v.size() = 6 v.capacity() = 6
v.size() = 7 v.capacity() = 9
v.size() = 8 v.capacity() = 9
v.size() = 9 v.capacity() = 9
v.size() = 10 v.capacity() = 13
v.size() = 11 v.capacity() = 13
v.size() = 12 v.capacity() = 13
v.size() = 13 v.capacity() = 13
v.size() = 14 v.capacity() = 19
v.size() = 15 v.capacity() = 19
为什么两者的结果不一样？
我再试了下下面的情况：
void capacity()
{
vector<int>v;
cout << "初始值" << endl;
cout << "v.size() = " << v.size() << "\tv.capacity() = " << v.capacity() << endl;
cout << "变化后的值" << endl;
for(int i = 0; i < 15; i++)
{
v.push_back(i);
v.resize(12);
cout << "v.size() = " << v.size() << "\tv.capacity() = " << v.capacity() << endl;
}
}得到的结果是：
初始值
v.size() = 0 v.capacity() = 0
变化后的值
v.size() = 12 v.capacity() = 12
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
v.size() = 12 v.capacity() = 18
为什么size()的大小就不会变化了呢？为什么capacity()的大小直接从12就跳到了18？
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