[C++知识库]数字的小数输出以及C语言与C++的内存申请区别

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#include<cstdio>
#include<cstdlib>
#include<iostream>
#include <iomanip>

using namespace std;
int main()
{
	double a = 3.1415926;
	cout << setprecision(2) << a << endl;
	cout << fixed << setprecision(2) << a << endl;//设置精度语句写一次即可
	double s = 121.345;
	cout << s << endl;
	cout << setprecision(3) << s << endl;
	cout.setf(ios::showpoint);//输出小数点
	cout << s << endl;
	cout.setf(ios::showbase);
	cout << s << endl;
	//C语言中的内存申请
	/*int* pc = (int*)malloc(sizeof(int));
	free(pc);
	pc = nullptr;
	//C++中的内存申请 new delete
	int* pcpp = new int; //申请内存
	delete pcpp; //释放内存
	pcpp = nullptr;
	//申请一段内存  
	//C语言
	int* pArrayC = (int*)malloc(sizeof(int) * 4);
	free(pArrayC);
	pArrayC = nullptr;
	//C++
	int* pArrayCPP = new int[4];
	delete[] pArrayCPP;//数组(一段)内存的释放过程
	pArrayCPP = nullptr; 

	char* str = new char[3];
	delete[] str;
	//申请内存并且做初始化 calloc
	//赋值一个数据
	int* pint = new int(100);
	cout << *pint << endl;
	cout << pint[0] << endl;//同上等价
	delete pint;
	pint = nullptr;
	//赋值一堆数据*/
	int* pArray = new int[4]{ 1,2,3,4 };
	for (int i = 0; i < 4; i++)
	{
		cout << pArray[i] << "\t";
	}
	//delete[] pArray;
//	pArray = nullptr;
	return 0;
}

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