"Effective C + +" study notes-clause 17

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Third, Resource Management

Rule 17:store newed objects in smart pointers in standalone statements

rules 17:< Span style= "Color:rgb (85,85,85); font-family: ' Comic Sans MS '; font-size:18px; Line-height:26px "To place the Newed object in a standalone statement into a smart pointer

1. Phenomena

We want the program to have precedence, so use a function to assign precedence to the object that is being allocated dynamically.

<span style= "Font-family:comic Sans MS;" >int priority ();</span>

<span style= "Font-family:comic Sans MS;" >void Processwidget (std::tr1::shared_ptr<widget> pw, int priority);</span>

You know, we can't use this:

Processwidget (New Widget, priority ());

Instead, it should be:

Processwidget (std::tr1::shared_ptr<widget> (new Widget), priority ());

2. What will the compiler do?

Each argument must be accounted for before the compiler outputs the processwidget calling code. Processwidget The second argument is simple, just call the function, and the first argument is not the same, it is two actions:

--Call new Widget expression

Call Tr1::shared_ptr Constructor

Therefore, the compiler must do the following three things before calling the Processwidget function:

> Call Priority

> Execute "New Widget"

> Calling the Tr1::shared_ptr constructor

The question is, in what order does the C + + compiler perform these three things, it is difficult to judge. It does not perform actions in a specific order like C # or JAVA . In C + +, you can only be sure that the new Widget action takes precedence over the tr1::shared_ptr action.

3. Do you have any questions?

Of course there is a problem, if the order of the three actions is this:

1. Execute "New Widget"

2. Call Priority

3. Call the Tr1::shared_ptr constructor

What happens if an error occurs in the second step that causes an exception? The pointer returned in the first step will be lost, causing a resource leak . Oh no! This big chapter, we have been managing resources, fighting these problems, here again there is a resource leak!

4. So, to solve this problem

solution is also exceptionally simple, do not bother, write two lines. (that is, to separate a sentence into two sentences to write)

< Span style= "line-height:26px" > because, c++ The compiler cannot rearrange the statements, only rearrange the within the statement, which is the question to write:

<span style= "Font-family:comic Sans MS;" >std::tr1::shared_ptr<Widget> PW (New Widget) Processwidget (pw,priority ());</span>

5. Finally, please remember

★ Store newed objects in (placed) smart pointers in separate statements. If you do not, once an exception is thrown, it is possible to cause an imperceptible resource leak.

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"Effective C + +" study notes-clause 17