构造函数的调用
// Listing 11.6Use of copy constructor to return an object from a function.
#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
void modify(const char*); // change the array contents
bool operator == (const String&) const; // compare contents
const char* show() const; // return a pointer to the array
} ;
String::String(int length)
{ len = length;
str = allocate( " "); // copy empty string into heap memory
cout < < " Originate: ' " < < str < < " '\n "; }
String::String(const char* s)
{ len = strlen(s); // measure length of incoming text
str = allocate(s); // allocate space, copy 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 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 pointer p can disappear
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);// call the constructor
}
int main()
{ 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]; } // data[j].operator+=(c[j]);
String u = enterData(); // 这里是我的问题，问题在下面
int i;
for (i=0; i < MAX; i++) // i is defined outside of the loop
{ if (data[i] == u) break; } // break if string found
if (i == MAX) // how did we get here?
cout < < " City " < < u.show() < < " is not found\n ";
else
cout < < " City " < < u.show() < < " is found\n ";
return 0;
}
请问在上面那里共调用了几次构造函数，调没调用拷贝构造函数。为什么？谢谢了。
[解决办法]
return String(data); //1次
....
String u = enterData();//2次. 应该是调用String::String(const String& s)
[解决办法]
拷贝构造可能会被系统优化掉的 ～
[解决办法]
String enterData()
{ cout < < " Enter city to find: "; // prompt the user
char data[200]; // crude solution
cin > > data; // accept user input
return String(data);// call the constructor
}
这里的返回是一个 对象，
所以， 在函数中的 string 对象会先通过拷贝构造至一个临时对象，
然后，再把这个临时对象通过拷贝构造复制给 string u .....
但是这个拷贝构造的过程可能会被优化掉～
[解决办法]
在构造函数里加计数，得到多少次就多少次。因为有优化，次数是没有“通常”这个说法的。
