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Effective C++ Third version, sample code compile list(11) Handle assignment to self in operator=

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Effective C++: 55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers

https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879

https://www.amazon.com/dp/0321334876

Sample code from

Yuzhen11/Effective_CPP

https://github.com/Yuzhen11/Effective_CPP


算譜(source code)


ec11.cpp

///Effective C++:  55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers

///https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879
#include <iostream>
#include <cstdlib>
#include <vector>
using namespace std;

const char * msg ="Item 11: Handle assignment to self in operator=";

/// From here "Effective C++
namespace A {
class Widget {
///...
};

void f() {
Widget w;
///...
w = w;
int i=8;
int j=9;
int k=10;
int a[k];
int * px=&i;
int * py=&j;
a[i] = a[j]; // potential assignment to self
*px = *py; // potential assignment to self
//If you try to manage resources yourself, you can fall into the trap of accidentally releasing a resource before you're done using it.
}
}

namespace B {
class Bitmap {
/// ...
};
class Widget {
/// ...
private:
Bitmap* pb;
};
//Here's an unsafe implementation:

Widget& Widget::operator=(const Widget& rhs) {
delete pb;
pb = new Bitmap(*rhs.ph);
return *this;
}
//Traditional way to prevent this error:

Widget& Widget::operator=(const Widget& rhs) {
if (this == &rhs) return *this;
delete pb;
pb = new Bitmap(*rhs.ph);
return *this;
}
//This version still has exception trouble. What if new yield an exception, the Widget will end up holding a pointer to a deleted Bitmap.

//Happily, making operator= exeption-safe typically redners it self-assignment-safe too.

//A careful ordering of statements can yield exception-safe.

Widget& Widget::operator=(const Widget& rhs) {
Bitmap* pOrig = pb; // remember the original pb
pb = new Bitmap(*rhs.pb);
delete pOrig; // delete the original pb
return *this;
}
}
//Now, if new Bitmap throws an excpetion, pb remains unchanged. This code handle self-assignement too but with less efficient(delete and new).

//copy and swap
namespace D {
class Widget {
void swap(Widget& rhs);
};
Widget& Widget::operator=(const Widget& rhs) {
Widget temp(rhs);
swap(temp);
return *this;
}
// A variation, sacrifices clarity at the altar of cleverness.
Widget& Widget::operator=(Widget rhs) {
swap(rhs);
return *this;
}
}
//Things to Remember

//Make sure operator= is well-behaved when an object is assigned to itself. Technqiues include comparing addresses of source and target objects, careful statement ordering, and cpoy-and-swap.
//Make suret that any function operating on more than one object behaves correctly if two or more of the objects are the same.

/// to here from "Effective C++"

int main() {
A::f();
cout << msg << endl;
return EXIT_SUCCESS;
}```
#編纂・実行結果(compile and go)

```shell-session:c1.sh
$ ../c1.sh ec11
$ clang++ ec11.cpp -std=c++2a -Wall
ec11.cpp:43:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
^
noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
^
ec11.cpp:45:26: error: no member named 'ph' in 'B::Widget'
pb = new Bitmap(*rhs.ph);
~~~ ^
ec11.cpp:50:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
^
noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
^
ec11.cpp:53:26: error: no member named 'ph' in 'B::Widget'
pb = new Bitmap(*rhs.ph);
~~~ ^
ec11.cpp:62:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
^
noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
^
ec11.cpp:76:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
^
noexcept
ec11.cpp:73:7: note: previous declaration is here
class Widget {
^



課題(agenda)

1)役に立つまたは意味のあるコンパイルエラーの取り方

2)役に立つまたは意味のある出力

3)役に立つまたは意味のある関数、クラスの呼び出し方


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文書履歴(document history)

ver. 0.10 初稿 20180622