C++ vector

C++ vector::reserve() 函數

vector::reserve()是"vector"頭的庫函數,用于請求向量分配的改變。詳細參考示例了解。

注意:要使用矢量,請包括<vector>標題。

vector::reserve() 函數的語法

    vector::reserve(n);

參數: int n- 它接受n作為參數,其中n是輸入容量。

返回值: void- 如果請求有效,則不返回任何內容。但是如果請求的容量大于向量的最大大小(vector::max_size),則會拋出 length_error 異常。

示例:案例 1:(沒有 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';    }}

在這種情況下,我們沒有使用儲備,因此增長是根據動態分配的,增加了兩倍。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';    }}

在這種情況下,我們沒有使用儲備,因此增長是根據動態分配的,增加了兩倍。比如,1, 2, 4, 8, 16, 32, 64, 128.....直到max_size。

C++程序演示vector::reserve()函數的例子

#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;}

輸出

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

參考:C++ vector::reserve()