为什么char指针被赋予new

问题描述

我构建了我自己的字符串类，其中我使用char * str

下面是一段代码



从下面代码片段，在param构造函数和复制构造函数中

cstr = str; //这部分语句工作正常。



但是为什么我需要使用new然后strcpy来分配内存？

cstr = new char [strlen（str）+1];

strcpy（cstr，str）;

都可以。但为什么选择动态分配方法呢？



我尝试过：

i constructed my own string class where i use char* str
the below is the piece of code

From below code snippet, In param constructor and copy constructor
cstr = str; //this part of statement works fine.

but even then why i need to allocate memory using new and then strcpy required?
cstr = new char[strlen(str) +1];
strcpy(cstr, str);
both works fine. but why to choose dynamic allocation method?

What I have tried:

class CString
{
private:
	char* cstr;
public:
	CString();
	CString(char* str);
	CString(CString& str);
	~CString();

	operator char*();
	//operator const char*();
	CString operator+(const CString& q)const;
	CString operator=(const CString& q);
};
CString::CString()
{
	cout << "constructor" << endl;
	cstr = 0;
}
CString::CString(char *str)
{
	cout << " param constructor" << endl;
	//cstr = str;// this works fine even then we allocating with new why???
	cstr = new char[strlen(str) +1];
	strcpy(cstr, str);
}
CString::CString(CString& q)
{
	cout << " copy constructor" << endl;
	if (this == &q)
		return;
	cstr = q.cstr;
	//cstr = new char[strlen(q.cstr) + 1];
	//strcpy(cstr, q.cstr);
}
CString::~CString()
{
	//if (cstr)
	//	delete[] cstr;
}
CString::operator char*()
{
	cout << " operatorcahr*" << endl;
	return cstr;
}
//CString::operator const char* ()
//{
//	cout << " const operatorcahr*" << endl;
//	return cstr;
//}
CString CString::operator +(const CString &q) const
{
	cout << "operator +*" << endl;
	CString s;
	s.cstr = new char[strlen(cstr) + strlen(q.cstr) + 1];
	strcpy(s.cstr, cstr);
	strcat(s.cstr, q.cstr);
	return s;
}
CString CString::operator =(const CString &q)
{
	cout << "operator =" << endl;
	if (this != &q)
	{
		//if (cstr)
			//delete[] cstr;
		cstr = new char[strlen(q.cstr) + 1];
		strcpy(cstr, q.cstr);
	}
	return *this;
}

int _tmain(int argc, _TCHAR* argv[])
{
	CString a = CString("Hello") + CString(" World");
	cout << a << endl;
	getchar();
	return 0;
}

推荐答案

CString CString::operator =(const CString &q)
{
    cout << "operator =" << endl;
    if (this != &q)
    {
        // No need to check if cstr is NULL.
        // delete does nothing in this case.
        delete[] cstr;
        // But check here if q.cstr is NULL!
        if (q.cstr)
        {
            cstr = new char[strlen(q.cstr) + 1];
            strcpy(cstr, q.cstr);
        }
        // String is empty now
        else
            cstr = 0;
    }
    return *this;
}

CString::~CString()
{
    delete[] cstr;
}

一旦实现了赋值运算符，它就可以被其他函数使用：

Once you have implemented the assignment operator, it can be used by other functions:

CString::CString(const CString& q)
{
    cout << " copy constructor" << endl;
    *this = q;
}

知道这一点，最好有一个 CString :: CString（const char * s）赋值运算符，因为它可以用于多个复制构造函数。

Knowing this it might be better to have a CString::CString(const char *s) assignment operator instead because that can be used for multiple copy constructors.

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