std::vector 是动态数组，存储的数据在内存上是连续的。

那size 和 capacity 有什么区别呢？虽然看起来很简单，但是不细想一下还是会混淆的。

看个例子就能理解了：

#include <iostream>
#include <iterator>
#include <vector>

void PrintVector(const std::vector<int> &a) {
  std::cout << "\n==== size:" << a.size() << " capacity:" << a.capacity()
            << " ====" << std::endl;
  for (auto p : a) {
    std::cout << p << "\t";
  }
  std::cout << std::endl;
}
int main() {
  std::vector<int> a(3, 3); // size 3 capacity 3
  a.reserve(4); // 元素没有变化因此 size 3 容量提高了 因此 capacity 4
  PrintVector(a);
  a = {1, 2, 3}; // 元素改变因此size 3 原来的容量4足够存储了因此不变 capacity 4
  PrintVector(a);
  a.resize(10, 20); // 元素改变size10 多出来的元素20填充capacity需要提升为10
  PrintVector(a);
  a.resize(5, 30); // 元素减少 因此size 5 多出来的元素直接删除 capacity不变
  PrintVector(a);
  a.reserve(50); // 元素没有变化 因此size 5 capacity 50
  PrintVector(a);
  a.reserve(5); // 元素没有变化 因此size 5 capacity 已经大于5了  因此不变
  PrintVector(a);
  return 0;
}

// ================================================
==== size:3 capacity:4 ====
3       3       3

==== size:3 capacity:4 ====
1       2       3

==== size:10 capacity:10 ====
1       2       3       20      20      20      20      20      20      20

==== size:5 capacity:10 ====
1       2       3       20      20

==== size:5 capacity:50 ====
1       2       3       20      20

==== size:5 capacity:50 ====
1       2       3       20      20

这里是不是发现为什么没有两倍扩容呢？

下面这个例子就会触发两倍扩容，原因是push_back 和 赋值操作（=）是不一样的，push_back 会在原有的内存上不断增加数据，直到超过容量了，重新分配一块新的内存块，而赋值操作则是每次都需要擦除原有数据，capacity 和 size 会保持一致。

#include <iostream>
#include <vector>

int main() {
  std::vector<int> vec;

  std::cout << "Initial size: " << vec.size()
            << ", capacity: " << vec.capacity() << std::endl;

  // 插入元素并观察容量变化
  for (int i = 0; i < 20; ++i) {
    vec.push_back(i);
    std::cout << "After inserting element " << i << ": size = " << vec.size()
              << ", capacity = " << vec.capacity() << std::endl;
  }

  return 0;
}
// ================================================
Initial size: 0, capacity: 0
After inserting element 0: size = 1, capacity = 1
After inserting element 1: size = 2, capacity = 2
After inserting element 2: size = 3, capacity = 4
After inserting element 3: size = 4, capacity = 4
After inserting element 4: size = 5, capacity = 8
After inserting element 5: size = 6, capacity = 8
After inserting element 6: size = 7, capacity = 8
After inserting element 7: size = 8, capacity = 8
After inserting element 8: size = 9, capacity = 16
After inserting element 9: size = 10, capacity = 16
After inserting element 10: size = 11, capacity = 16
After inserting element 11: size = 12, capacity = 16
After inserting element 12: size = 13, capacity = 16
After inserting element 13: size = 14, capacity = 16
After inserting element 14: size = 15, capacity = 16
After inserting element 15: size = 16, capacity = 16
After inserting element 16: size = 17, capacity = 32
After inserting element 17: size = 18, capacity = 32
After inserting element 18: size = 19, capacity = 32
After inserting element 19: size = 20, capacity = 32

总结：

--1 size 是元素个数 capacity 是容量大小

--2 size 随着元素个数变化实时变化 capacity 只有当容量不足的时候才会重新分配

--3 push_back 会触发两倍扩容 而 = 赋值运算并不会