C++四种类型转化

static_cast

该运算符把expression转化为type_name类型，static_cast在编译时使用类型信息执行转换，在转换执行必要时的检测（如指针越界，类型检查），但没有运行时类型检查来保证转换的安全性

• 用于基本类型检查，如把int转换为char，把int转换为enum

enum WeekType{
   sun=0,Mon=1,Tues=2,Wed=3,Thus=4,Fir=5,Sat=6};

int main()
{
   
	int a = 10;
	char ch = 'x';
	double dx = 12.34;
	a = static_cast<int>(ch);
	a = static_cast<int>(dx);
	WeekType x = Mon;
	a = x;
	x = static_cast<WeekType>(a);
}

• 指针转换的限制
  
• 可以就void*转换为其它类型

int main()
{
   
	int a = 10;
	int* ip = nullptr;
	double* dp = nullptr;
	void* vp = &a;
	ip = static_cast<int*>(vp);
	dp = static_cast<double*>(dp);
	return 0;
}

• 弃值表达式

int main()
{
   
	int a = 10, b = 20;
	static_cast<void>(a + b);
}

• 左值到右值
  1.先补充一个小知识点，移动语义

class PtrInt
{
   
private:
	int* pval; // new .heap ; stack . data;
public:
	PtrInt(int* p = nullptr) :pval(p)
	{
   
		cout << "create Ptrint：" << this << endl;
	}
	~PtrInt()
	{
   
		delete pval;
		pval = nullptr;
		cout << "Destory Ptrin：t" << this << endl;
	}
	PtrInt(const PtrInt& it) :pval(new int(0)) //拷贝构造函数
	{
   
		*pval = *it.pval;
		//*pval = it.*pval;
	}
	PtrInt& operator=(const PtrInt& it) //拷贝赋值函数
	{
   
		if (this != &it)
		{
   
			delete pval;
			pval = new int(*it.pval);
			//pval = it.pval;
		}
		cout << "Copy create Ptrint：" << this << endl;
		return *this;
	}
	PtrInt(PtrInt&& it) :pval(it.pval) //移动构造函数
	{
   
		it.pval = nullptr;
		cout << "move copy create Ptrint：" << this << endl;
	}
	PtrInt& operator=(PtrInt&& it) //移动赋值函数
	{
   
		if (this != &it)
		{
   
			delete[]pval;
			pval = it.pval;
			it.pval = nullptr;
		}
		cout << "Copy operator Ptrint：" << this << endl;
		return *this;
	}
};

PtrInt func(int x)
{
   
	PtrInt tmp(new int(x));
	return tmp;
}

int main()
{
   
	PtrInt a(new int(10));
	a = func(100);
	return 0;
}

运行结果：

2.左值与右值之间的转换

#include<iostream>
using namespace std;
class PtrInt
{
   
private:
 int</