开辟栈内存速度 v.s. 开辟堆内存速度

在函数内直接定义多维数组，即可在栈上开辟内存。

用栈上指针 new 出堆内存，定义多维数组，即在堆上开辟内存。

可以比较这两种的速度，做个小实验，心里有个数。

代码如下：

#include<iostream>
using namespace std;

#include<cmath>
#include<time.h>

#define n 1000

void open_stack_memory(){
    int a[n][n];
}

void open_heap_memory(){
    int **a = new int *[n];
    for(int i=0;i<n;i++)
        a[i] = new int [n];
    for(int i=0;i<n;i++)
        delete [] a[i];
    delete [] a;
}

int main(){

    int repeat1 = 1E7;

    clock_t t_start = clock();
    for(int i=0;i<repeat1;i++)
        open_stack_memory();
    clock_t t_end = clock();
    cout<<" It took me "<<(double)(t_end - t_start)/CLOCKS_PER_SEC<<" s to open stack memory "<<repeat1<<" times repeatedly"<<endl;

    int repeat2 = 1E1;
    t_start = clock();
    for(int i=0;i<repeat2;i++)
        open_heap_memory();
    t_end = clock();
    cout<<" It took me "<<(double)(t_end - t_start)/CLOCKS_PER_SEC<<" s to open  and delete heap memory "<<repeat2<<" times repeatedly"<<endl;

    return 0;
}

运行结果为：

 It took me 0.03125 s to open stack memory 10000000 times repeatedly
 It took me 0.0625 s to open  and delete heap memory 10 times repeatedly

所以，直接征用栈内存，是 new 出堆内存的 1E6 倍速度。