[C++知识库]c语言的内存动态申请：malloc、realloc、calloc区别

//c语言申请内存 malloc 和realloch calloc
#include<stdio.h>
#include<string>
#include<iostream>
using namespace std;

void printff(int array[],int arrayNum)
{
	for (int i = 0; i < arrayNum; i++)
	{
		cout << array[i] <<"\t"<< "";
	}
	cout << endl;
}
void testCalloc()
{

	int *pMNum = (int *)malloc(sizeof(int*) * 3);
	if (pMNum == nullptr)//申请后记得安全处理
		return;
	memset(pMNum, 0, sizeof(int) * 3); //memset(pMNum, 2, sizeof(int) * 3);//calloc等于malloc加memset函数初始化，且初始化的值都是0字节，也就没有数据，值自然是为0
	printff(pMNum, 3);//malloc不做初始化，输出数组内的值为系统随机值,
	delete(pMNum);
	pMNum = nullptr;

	int *pCNum = (int *)calloc(3, sizeof(int));
	printff(pCNum, 3);//calloc初始化数组的值为0
	delete(pCNum);
	pCNum = nullptr;

	//realloc是再次申请，当之前申请不够时，扩大使用。
	int *pint = (int *)malloc(sizeof(int));
	if (pint == nullptr)
		return;
	*pint = 1999;
	cout << *pint << endl;
	pint = (int*)realloc(pint, sizeof(int)*3);//再次申请三个int大小的内存，接在之前的内存后方
	pint[1] = 13;
	pint[2] = 45;
	cout << pint[0] << "\t" << pint[1] << "\t" << pint[4] << "\t" << endl;//可以看出pint[4]没有做初始化为0字节的操作
	delete(pint);

}

void testAuto()
{
	auto p = new int[3]{1, 2, 3};//auto xxx这里变量会隐式转换为*XXX的指针变量
	for (int i = 0; i < 3; i++)
	{
		cout << p[i] <<"\t"<< "";
	}
	cout << endl;
}

int main()
{
	testCalloc();
	testAuto();
}