C++通過實現String類描述運算符重載和臨時字符串對象調用c_str方法帶來的隱藏問題

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;

class String
{
public:
	String()
	{
		//cout << "無參構造函數" << endl;
		m_psz = NULL;
		m_length = 0;
	}

	String(const char* psz)
	{
		cout << "有參構造函數" << endl;
		if (psz)
		{
			m_length = strlen(psz);
			m_psz = new char[m_length + 1];
			strcpy(m_psz, psz);
		}
	}

	String(String& str)
	{
		cout << "拷貝構造函數" << endl;
		m_length = str.m_length;
		if (str.m_psz)
		{
			m_psz = new char[m_length + 1];
			strcpy(m_psz, str.m_psz);
		}
		else
			m_psz = NULL;
	}

	~String()
	{
		printf("析構函數被調用，m_psz的地址爲%p\n", m_psz);
		if (m_psz)
			delete[]m_psz;
	}

	String operator+(String& str)
	{
		//cout << "+重載函數,String&" << endl;
		if (str.m_psz)
		{
			String tmp;
			tmp.m_length = str.m_length;
			tmp.m_psz = new char[tmp.m_length + 1];
			strcpy(tmp.m_psz, str.m_psz);
			return tmp;
		}
		else
			return *this;
	}

	//必須返回一個新的對象
	String operator+(char* psz)
	{
		//cout << "+重載函數,char*" << endl;
		if (psz)
		{
			int len = m_length + strlen(psz);
			char* tmp = new char[len + 1];
			tmp[len] = 0;
			strcpy(tmp, m_psz);
			strcat(tmp, psz);
			String str = tmp;//調用有參構造函數
			delete []tmp;
			return str;
		}
		else
			return *this;
	}

	friend ostream& operator<<(ostream& obj, String& str);

	String& operator=(String& str)
	{
		//cout << "=重載函數,String&" << endl;
		if (m_psz)
		{
			delete []m_psz;
			m_length = 0;
		}
		if (str.m_psz)
		{
			m_length = str.m_length;
			m_psz = new char[m_length + 1];
			strcpy(m_psz, str.m_psz);
		}
		return *this;
	}

	String& operator=(char* psz)
	{
		//cout << "=重載函數,char*" << endl;
		if (m_psz)
		{
			delete []m_psz;
			m_length = strlen(psz);
			m_psz = new char[m_length + 1];
			strcpy(m_psz, psz);
		}
		return *this;
	}

	char* c_str()
	{
		return m_psz;
	}

	int length()
	{
		return m_length;
	}


private:
	char* m_psz;
	int m_length;
};

ostream& operator<<(ostream& obj, String& str)
{
	printf("%s",str.m_psz);
	return obj;
}

class TestClass
{
public:
	String f1()
	{
		String s = "hello";
		return s;
	}
};

int main()
{
	TestClass tc;
	char* psz = tc.f1().c_str();
	printf("psz地址爲%p\n", psz);
	getchar();
	return 0;
}

VC++運行結果