stl reserve_vector :: reserve（）函数以及C ++ STL中的示例

stl reserve

C ++ vector :: reserve()函数 (C++ vector::reserve() function)

vector::reserve() is a library function of "vector" header, which is used to request change in vector allocation. Refer to example to understand in details.

vector :: reserve()是“ vector”头的库函数，用于请求矢量分配的更改。请参阅示例以详细了解。

Note: To use vector, include <vector> header.

注意：要使用向量，请包含<vector>标头。

Syntax of vector::reserve() function

vector :: reserve()函数的语法

    vector::reserve(n);

Parameter(s): int n – It accepts n as a parameter where n be the input capacity.

参数： int n –接受n作为参数，其中n是输入容量。

Return value: void – It returns nothing in case of valid request. But if the capacity requested is greater than the maximum size of the vector (vector::max_size), a length_error exception is thrown.

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。

Example: Case 1: (without reserve())

示例：情况1 ：(没有reserve())

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';
}
}

In this case, we have not used reserve thus the growth is as per dynamic allocation, which increases in a factor of two. Like, 1, 2, 4, 8, 16, 32, 64, 128…..so on till max_size.

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1，2，4，8，16，16，32，64，128…..so直到max_size 。

Example: Case 2: (with reserve())

示例：情况2 ：(带有reserve())

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';
}
}

In this case, we have not used reserve thus the growth is as per dynamic allocation, which increases in a factor of two. Like, 1, 2, 4, 8, 16, 32, 64, 128…..so on till max_size.

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1，2，4，8，16，16，32，64，128…..so直到max_size 。

C ++程序演示vector :: reserve()函数的示例 (C++ program to demonstrate example of vector::reserve() function)

#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;
}

Output

输出量

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

Reference: C++ vector::reserve()

参考： C ++ vector :: reserve()