构造函数问题
#include <iostream>
using namespace std;
class String {
char *str; // dynamically allocated char array
int len;
char* allocate(const char* s) // private function
{ char *p = new char[len+1]; // allocate heap memory for object
if (p==NULL) exit(1); // test for success, quit if no luck
strcpy(p,s); // copy text into heap memory
return p; } // return pointer to heap memory
public:
String (int length=0); // conversion/default constructor
String(const char*); // conversion constructor
String(const String& s); // copy constructor
~String (); // deallocate dynamic memory
void operator += (const String&); // concatenate another object
String operator = (const String&); // assignment operator
void modify(const char*); // change the array contents
bool operator == (const String&) const; // compare contents
const char* show() const; // return a pointer to array
} ;
String::String(int length)
{ len = length;
str = allocate( " "); } // copy empty string into heap memory
String::String(const char* s)
{ len = strlen(s); // measure the length of incoming text
str = allocate(s); // allocate space, copy incoming text
cout < < " Created: ' " < < str < < " '\n "; }
String::String(const String& s) // copy constructor
{ len = s.len; // measure length of the source text
str = allocate(s.str); // allocate space, copy incoming text
cout < < " Copied: ' " < < str < < " '\n "; }
String::~String()
{ delete str; } // return heap memory (not the pointer!)
void String::operator += (const String& s) // reference parameter
{ len = strlen(str) + strlen(s.str); // total length
char* p = new char[len + 1]; // allocate enough heap memory
if (p==NULL) exit(1); // test for success
strcpy(p,str); // copy the first part of result
strcat(p,s.str); // add the second part of result
delete str; // important step
str = p; } // now p can disappear
String String::operator = (const String& s)
{ if (&s == this) return *this; // test for self-assignment
delete str; // you do not do it in copy constructor
len = s.len; // copy non-pointer data
str = allocate(s.str); // allocate space, copy incoming text
cout < < " Assigned: ' " < < str < < " '\n "; // for debugging only
return *this; } // return the target object to client
bool String::operator==(const String& s) const // compare contents
{ return strcmp(str,s.str)==0; } // strcmp returns 0 if the same
const char* String::show() const // protect data from changes
{ return str; }
void String::modify(const char a[]) // no memory management here
{ strncpy(str,a,len-1); // protect from overflow
str[len-1] = 0; } // terminate string properly
String enterData()
{ cout < < " Enter city to find: "; // prompt the user
char data[200]; // crude solution
cin > > data; // accept user input
return String(data); } // conversion constructor
int main()
{ cout < < endl < < endl;
enum { MAX = 4} ;
String data[4]; // database of objects
char *c[4] = { "Atlanta ", "Boston ", "Chicago ", "Denver " };
for (int j=0; j <MAX; j++)
{ data[j] = c[j]; } // 问题所在
String u; int i;
u = enterData(); // 问题所在
for (i=0; i <MAX; i++)
{ if (data[i] == u) break; } // if (data[i].operator==(u))
if (i == MAX)
cout < < " City " < < u.show() < < " is not found\n ";
else
cout < < " City " < < u.show() < < " is found\n ";
return 0;
}
请问在标注“问题所在”的地方都调用了什么构造函数，调用的顺序是什么？为什么是这样？谢谢啦！
[解决办法]
调用了String::String(const char* s)和String::String(const String& s)
先调用String::String(const char* s) 再String::String(const String& s)
[解决办法]
首先在data[j]=c[j]时，首先调用operator = (const String& s)；
也就是将c[j]传递给变量s，这时要调用String::String(const char* s)；
然后当调用operator = (const String& s)完成时return *this,这时要建立一个临时变量给data[j]，当建立一个临时变量时this本来就是string型调用String(const String& s)
第二个问题与第一个一样，就是临时变量的问题
[解决办法]
u = enterData();然后到
String enterData()
{ cout < < " Enter city to find: "; // prompt the user
char data[200]; // crude solution
cin > > data; // accept user input
return String(data); } /////这时会调用String::String(const char* s)
接着会到
String String::operator = (const String& s)
{ if (&s == this) return *this; // test for self-assignment
delete str; // you do not do it in copy constructor
len = s.len; // copy non-pointer data
str = allocate(s.str); // allocate space, copy incoming text
cout < < " Assigned: ' " < < str < < " '\n "; // for debugging only
return *this;///这时会调用String::String(const String& s) }
