c++ new string数组问题详解程序员

今天考虑一个问题：

      string *p = new string[5];

      p[0] = “aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa”;

      p[2] = “bb”;

new操作时是需要知道类型的大小的。但new string[5]时怎么知道string的大小的呢？ 感觉很奇怪。

      经过思考，认为原因如下：

      1.  new string[5]会调用string的默认构造函数。所以会有5个空字符串被创建。

       2. string 的=操作会重新new char[], 更新原来的string内部的char*。

       另外，string的实现参考：

#include <iostream> 
#include <string.h> 
using namespace std; 
class MyString 
{ 
	char* m_str; 
	int m_nLength; 
	void _InitString(); 
	 
public: 
	//1. 默认构造函数 
	MyString(); 
	// 2. 带参构造函数 
	MyString( unsigned int length, const char& ch ); 
	MyString(const char* str); 
	MyString( const char* str, unsigned int length ); 
	MyString( const MyString& str, unsigned int index, unsigned int length ); 
	// 3. 拷贝构造函数 
	MyString( const MyString& s ); 
	//4.析构函数 
	~MyString(); 
 
	// 5. 追加字符串函数 
	MyString& append( const MyString& _str ); 
	MyString& append( const char* str ); 
	MyString& append( const MyString& str, unsigned int index, unsigned int len ); 
	MyString& append( const char* str, unsigned int num ); 
	MyString& append( unsigned int num, char ch ); 
	/* 
	// 6. 赋值函数 
	MyString& assign( const MyString& str ); 
	MyString& assign( const char* str ); 
	MyString& assign( const char* str, unsigned int num ); 
	MyString& assign( const MyString& str, unsigned int index, unsigned int len ); 
	// 	 
	// 7. 返回字符串指针函数 
	const char* c_str() const; 
	// 8. 字符串比较函数 
	int MyString::compare( const MyString& str ); 
	int compare( const char* str ); 
	int compare( unsigned int index, unsigned int length, const MyString& str ); 
	 
	// 9. 拷贝函数 
	unsigned int copy( char* str, unsigned int num, unsigned int index  ); 
	 
	// 	10. 查找函数 
	unsigned int find( const MyString& str, unsigned int index ); 
	unsigned int find( const char* str, unsigned int index ); 
	unsigned int find( char ch, unsigned int index ); 
	 
	// 		 11. 替换函数 
	void replace(const char* findString, const char* replaceString); 
	 
	// 12. 返回子串 
	MyString substr( unsigned int index, unsigned int num  ); 
	 
	// 	// 13. 交换字符串函数 
	void swap(MyString& from ); 
	 
	// 14. 其他函数 
	void clear(); 
	unsigned int length() const; 
	unsigned int size() const; 
	// 15. 重载运算符 
	// 1). 比较运算 
	friend bool operator==(const MyString& c1, const MyString& c2); 
	friend bool operator!=(const MyString& c1, const MyString& c2); 
	// 	 
	// 2). 追加运算 
	friend MyString operator+(const MyString& s1, const MyString& s2 ); 
	friend MyString operator+(const char* s, const MyString& s2 ); 
	// 3). 输入输出运算 
	friend ostream& operator<<( ostream& os, const MyString& s ); 
	friend istream& operator>>( istream& is, MyString& s ); 
	// 	 
	// 4). 赋值运算 
	MyString& operator=( const MyString& s ); 
	MyString& operator=( const char* s ); 
	char& operator[]( unsigned int index ); 
	*/ 
	//显示结果 
	void Show() 
	{ 
		cout<<m_str<<endl; 
	} 
	 
}; 
 
MyString::MyString() 
{ 
	m_str=new char[1]; 
	m_str[0]=0; 
	int m_nLength=0; 
} 
MyString::~MyString() 
{ 
	delete []m_str; 
	m_str=NULL; 
} 
MyString::MyString( unsigned int length, const char& ch ) 
{ 
	m_nLength=length; 
	m_str=new char[m_nLength+1]; 
	for (int i=0;i<m_nLength;++i) 
	{ 
		m_str[i]=ch; 
	} 
	m_str[m_nLength]=0; 
 
} 
MyString::MyString(const char* str) 
{ 
	if (NULL==str) 
	{ 
		m_str=new char[1]; 
		m_str[0]=0; 
		int m_nLength=0; 
	} 
	else 
	{ 
		m_str=new char[strlen(str)+1]; 
		strcpy(m_str,str); 
	    m_nLength=strlen(str); 
	} 
	 
} 
MyString::MyString( const char* str, unsigned int length ) 
{ 
	if (NULL==str) 
	{ 
		m_str=new char[1]; 
		m_str[0]=0; 
		int m_nLength=0; 
	} 
	else 
	{ 
		m_nLength=length; 
		m_str=new char[m_nLength+1]; 
		strncpy(m_str,str,m_nLength); 
		m_str[m_nLength]=0; 
	} 
} 
MyString::MyString( const MyString& str, unsigned int index, unsigned int length ) 
{ 
	m_nLength=length; 
	m_str=new char[m_nLength+1]; 
	strncpy(m_str,str.m_str+index,m_nLength); 
	m_str[m_nLength]=0; 
} 
MyString::MyString( const MyString& s ) 
{ 
	m_str= new char[strlen(s.m_str)+1]; 
	strcpy(m_str,s.m_str); 
} 
/* 
MyString::MyString& append( const MyString& _str ) 
{ 
 
} 
*/ 
MyString& MyString::append( const char* str ) 
{ 
	if (NULL==str) 
	{ 
		m_str=new char[1]; 
		m_str[0]=0;		 
	} 
	else 
	{ 
		strcat(m_str,str); 
	} 
	return *m_str; 
} 
 
 
MyString& MyString::append( const MyString& _str ) 
{ 
	char*p=new char[strlen(m_str)+strlen(_str.m_str)+1]; 
	strcpy(p,m_str); 
	strcat(p,_str.m_str); 
    p[strlen(m_str)+strlen(_str.m_str)]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
} 
 
MyString& MyString::append( const char* str ) 
{ 
	char*p=new char[strlen(m_str)+strlen(str)+1]; 
	strcpy(p,m_str); 
	strcat(p,str); 
	p[strlen(m_str)+strlen(str)]=0; 
	delete []m_str; 
	m_str=p ; 
	 
	return *this; 
} 
MyString& MyString::append( const MyString& str, unsigned int index, unsigned int len ) 
{ 
	char*p=new char[strlen(m_str)+strlen(str.m_str)-index+1]; 
	strcpy(p,m_str); 
	strncat(p,str.m_str+index,len); 
	p[strlen(m_str)+strlen(str.m_str)-index]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
} 
MyString& MyString::append( const char* str, unsigned int num ) 
{ 
	char*p=new char[num+1]; 
	strncpy(p,str,num); 
   p[num]=0; 
	delete []m_str; 
	m_str=p; 
 
	return*this; 
} 
MyString& MyString::append( unsigned int num, char ch ) 
{ 
	char*p=new char[num+1]; 
	for(int i=0;i<num;++i) 
	{ 
		p[i]=ch; 
	} 
	p[num]=0; 
	delete []m_str; 
	m_str=p; 
 
	return *this; 
} 
MyString& MyString::assign( const MyString& str ) 
{ 
<span style="color:#FF0000;">	char*p=new char[strlen(str.m_str)+1]; 
	strcpy(p,str.m_str); 
    p[strlen(str.m_str)]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
} 
MyString& MyString::assign( const char* str ) 
{ 
	char*p=new char[strlen(str)+1]; 
	strcpy(p,str); 
	p[strlen(str)]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
} 
MyString& MyString::assign( const char* str, unsigned int num ) 
{ 
	char*p=new char[num+1]; 
	strncpy(p,str,num); 
	p[num]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
} 
MyString& MyString::assign( const MyString& str, unsigned int index, unsigned int len ) 
{ 
	char*p=new char[len+1]; 
	strncpy(p,str.m_str+index,len); 
	p[len]=0; 
	delete []m_str; 
	m_str=p; 
	return *this; 
}