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#include 
   
    #include 
    
     using namespace std;class numbered{public:    numbered();    numbered(string no) :mysn_(no){};    string mysn_;};numbered::numbered() {    string s;    s += to_string(rand());    mysn_ = s;}voidf(numbered s){    cout << s.mysn_ << endl;}intmain() {    numbered a, b = a , c = b ;    cout << "a.mysn_: " << a.mysn_ << endl;    cout << "b.mysn_: " << b.mysn_ << endl;    cout << "c.mysn_: " << c.mysn_ << endl;    cout << "invoke f() for a" << endl;    f(a);    cout << "invoke f() for b" << endl;    f(b);    cout << "invoke f() for c" << endl;    f(c);    return 0;}
    
   

输出

a.mysn_: 1804289383

a.mysn_: 1804289383 a.mysn_: 1804289383 invoke f() for a 1804289383 invoke f() for b 1804289383 invoke f() for c 1804289383

 

修改代码

#include 
   
    #include 
    
     using namespace std;class numbered{public:    numbered();    numbered(string no) :mysn_(no){};    numbered(numbered ¶);    string mysn_;};numbered::numbered() {    string s;    s += to_string(rand());    mysn_ = s;}numbered::numbered(numbered ¶) {    cout << "copy constructor invoked." << endl;    string s;    s += to_string(rand());    mysn_ = s;}voidf(numbered s){    cout << s.mysn_ << endl;}intmain() {    numbered a, b = a , c = b ;    cout << "a.mysn_: " << a.mysn_ << endl;    cout << "b.mysn_: " << b.mysn_ << endl;    cout << "c.mysn_: " << c.mysn_ << endl;    cout << "invoke f() for a" << endl;    f(a);    cout << "invoke f() for b" << endl;    f(b);    cout << "invoke f() for c" << endl;    f(c);    return 0;}
    
   

即增加

numbered(numbered ¶);numbered::numbered(numbered ¶) {    cout << "copy constructor invoked." << endl;    string s;    s += to_string(rand());    mysn_ = s;}

输出

copy constructor invoked.

copy constructor invoked. a.mysn_: 1804289383 b.mysn_: 846930886 c.mysn_: 1681692777 invoke f() for a copy constructor invoked. 1714636915 invoke f() for b copy constructor invoked. 1957747793 invoke f() for c copy constructor invoked. 424238335  

继续修改代码

将

f(numbered s){    cout << s.mysn_ << endl;}

改为

f(numbered &s){    cout << s.mysn_ << endl;}

输出

copy constructor invoked.

copy constructor invoked. a.mysn_: 1804289383 b.mysn_: 846930886 c.mysn_: 1681692777 invoke f() for a 1804289383 invoke f() for b 846930886 invoke f() for c 1681692777  

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