Copy-and-swap

An assignment operator that overloads an exception safe (exception safe) in its own definition class. Created a idiom of this kind. Also called: Create-temporary-and-swap.

To write robust C + + code. Exception security is important.
To be able to give abnormal safety level three:
1> Basic security: Simple to implement, low cost. Should be used as a regular means.
2> is very safe: it is not possible to achieve this step on any occasion. The assignment operator overload for this example is considered to be one.

3> does not throw an exception: can see non-throwing swap.

The so-called "Create-temporary-and-swap". is to apply for new resources first. And then released after use.
1> Use the RAII principle when applying for new resources.
2> after the successful application. Exchange resources using the non-throwing swap principle.

[CPP]View Plaincopy

2. class String {
4. Char * STR;
6. Public:
8. string& operator = (String const &s)
10. {
12. String temp (s); //Copy-constructor--RAII
14. Temp.swap (*this); //non-throwing swap
16. return *this;
18. }
20. void Swap (String &s) throw ()
22. {
24. Std::swap (this->str, s.str);
26. }
28. };

Sometimes it increases the inference of the number of references:

[CPP]View Plaincopy

2. class String {
4. Char * STR;
6. Public:
8. string& operator = (String const &s)
10. {
12. if ( This! = &s)
14. String (s). Swap (*this); //Copy-constructor and non-throwing swap
16. return *this;
18. }
20. void Swap (String &s) throw ()
22. {
24. Std::swap (this->str, s.str);
26. }
28. };

A temporary variable inside a function. can actually be omitted. Just use the pass-through method:

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2. string& operator = (String s) //Pass-by-value
4. {
6. S.swap (*this); //non-throwing swap
8. return *this;
10. }

Such a practice can be optimized in some cases.
Suppose S binds to lvalue, no optimizations. Temporary objects are created inside the stack, and are assumed to be bound to rvalue. The call to copy ctor is usually omitted.

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1. String createstring (); //A function that returns a String object.
2. String s;
3. s = createstring (); //This is an example of rvalue. An assignment operator that uses a pass-through value. can be optimized

In the c++0x standard, this assignment operator is called the "uniform assignment operator" because it combines "copy assignment" and "Move Assignment". The c++0x compiler will always optimize temporary rvalue once it discovers that a class has a move ctor present.
The old compiler certainly does not have move ctor, but is able to rvalue the assignment operation to optimize, also is a somewhat.

Copy-and-swap