vector中resize与reserve的区别

resize()

resize() 函数的作用是改变 vector 元素个数。

resize(n,m) 第二个参数可以省略，n 代表改变元素个数为 n ,m 代表初始化为 m 。

• resize() 可以改变有效空间的大小。
• 如果 n 比 vector 的 size 小，结果是 size 减小到 n ，然后删除 n 之后的数据。

示例：

void test1() {
	vector<int> v;
	v.reserve(20);
	v.resize(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
	v.resize(5);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
}

输出：

vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0
vector.size()     = 5
vector.capacity() = 20
0 0 0 0 0

• 如果 n 比 vector 的 size 大，比 vector 的 capacity 小，结果是增加 size 并初始化，如果指定了第二参数，则初始化为指定值，没指定初始化则为缺省值，capacity 不变。

示例：

void test2() {
	vector<int> v;
	v.reserve(20);
	v.resize(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
	v.resize(15, 1);//v.resize(15)
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
}

输出：

vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0
vector.size()     = 15
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1

• 如果 n 比 vector 中的 capacity 大，结果是先增加容量，然后增加 size 并初始化。capacity 和 size 均改变。

示例：

void test1() {
	vector<int> v;
	v.reserve(20);
	v.resize(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
	v.resize(30, 1);//v.resize(30);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
}

输出：

vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0
vector.size()     = 30
vector.capacity() = 30
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

reserve()

reserve() 函数的作用是改变总空间的大小。

reverse(n)只有一个参数。

• 如果 n 的值比大于 vector 的 size，则 capacity 增容到 n ，size 不变。

示例：

void test1() {
	vector<int> v;
	v.reserve(20);
	v.resize(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
	v.reserve(30);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
}

输出：

vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0
vector.size()     = 10
vector.capacity() = 30
0 0 0 0 0 0 0 0 0 0

• 如果 n 的值比小于或者等于 vector 的 size ，则 capacity 不变，size 也不变。

示例：

void test1() {
	vector<int> v;
	v.reserve(20);
	v.resize(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
	v.reserve(10);
	cout << "vector.size()     = " << v.size() << endl;
	cout << "vector.capacity() = " << v.capacity() << endl;
	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << " ";
	}
	cout << endl;
}

输出：

vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0
vector.size()     = 10
vector.capacity() = 20
0 0 0 0 0 0 0 0 0 0