C++ vector::reserve() 函数

vector::reserve()是"vector"头的库函数,用于请求向量分配的改变。详细参考示例了解。

注意:要使用矢量,请包括<vector>标题。

vector::reserve() 函数的语法

    vector::reserve(n);

参数: int n- 它接受n作为参数,其中n是输入容量。

返回值: void- 如果请求有效,则不返回任何内容。但是如果请求的容量大于向量的最大大小(vector::max_size),则会抛出 length_error 异常。

示例:案例 1:(没有 reserve())

vector<int> arr1; //usual dynamic allocationsize = 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,我们没有使用储备,因此增长是根据动态分配的,增加了两倍。比如,1, 2, 4, 8, 16, 32, 64, 128.....直到max_size。

示例:案例 2:(使用 reserve())

vector<int> arr2; //using reservesize = arr2.capacity();arr2.reserve(50); // use of reserve functioncout << "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,因此增长是根据动态分配的,增加了两倍。比如,1, 2, 4, 8, 16, 32, 64, 128.....直到max_size。

C++程序演示vector::reserve()函数的例子

#include <iostream>#include <vector>using namespace std;int main(){    vector<int>::size_type size;    vector<int> arr1; //usual dynamic allocation    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; //using reserve    size = arr2.capacity();    arr2.reserve(50); // use of reserve function    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

arr1 growing with usual dynamic allocation:capacity changed to:1capacity changed to:2capacity changed to:4capacity changed to:8capacity changed to:16capacity changed to:32capacity changed to:64arr2 growing with using reverse:capacity changed to:50

参考:C++ vector::reserve()