vector :: reserve()函数以及C ++ STL中的示例
C++vector::reserve()函数
vector::reserve()是“vector”头的库函数，用于请求矢量分配中的更改。请参阅示例以详细了解。
注意：要使用向量，请包含<vector>标头。
vector::reserve()函数的语法
vector::reserve(n);
参数：intn–接受n作为参数，其中n是输入容量。
返回值：void–在有效请求的情况下不返回任何内容。但是，如果请求的容量大于向量的最大大小（vector::max_size），则会引发length_error异常。
示例：情况1：（不带reserve()）
vector<int> arr1; //通常的动态分配
size = arr1.capacity();
cout << "arr1 growing with usual dynamic allocation:\n";
for (int i = 0; i < 50; ++i) {
arr1.push_back(i);
if (size != arr1.capacity()) {
size = arr1.capacity();
cout << "capacity changed to : " << size << '\n';
}
}在这种情况下，我们没有使用reserve，因此增长是按动态分配进行的，增长了两倍。像1，2，4，8，8，16，32，64，128…..so直到max_size。
示例：案例2：（带有reserve()）
vector<int> arr2; //使用reserve
size = arr2.capacity();
arr2.reserve(50); //使用reserve函数
cout << "arr2 growing with using reverse:\n";
for (int i = 0; i < 50; ++i) {
arr2.push_back(i);
if (size != arr2.capacity()) {
size = arr2.capacity();
cout << "capacity changed to: " << size << '\n';
}
}在这种情况下，我们没有使用储备金，因此增长是按动态分配进行的，增长了两倍。像1，2，4，8，8，16，32，64，128…..so直到max_size。
C++程序演示vector::reserve()函数的示例
#include <iostream>
#include <vector>
using namespace std;
int main(){
vector<int>::size_type size;
vector<int> arr1; //通常的动态分配
size = arr1.capacity();
cout << "arr1 growing with usual dynamic allocation:\n";
for (int i = 0; i < 50; ++i) {
arr1.push_back(i);
if (size != arr1.capacity()) {
size = arr1.capacity();
cout << "capacity changed to : " << size << '\n';
}
}
vector<int> arr2; //使用reserve
size = arr2.capacity();
arr2.reserve(50); //使用reserve
cout << "arr2 growing with using reverse:\n";
for (int i = 0; i < 50; ++i) {
arr2.push_back(i);
if (size != arr2.capacity()) {
size = arr2.capacity();
cout << "capacity changed to: " << size << '\n';
}
}
return 0;
}输出结果
arr1 growing with usual dynamic allocation: capacity changed to : 1 capacity changed to : 2 capacity changed to : 4 capacity changed to : 8 capacity changed to : 16 capacity changed to : 32 capacity changed to : 64 arr2 growing with using reverse: capacity changed to: 50
参考：C++vector::reserve()