Item 17: Put the new object into a smart pointer in a separate statement effective C + + notes

Item 17:store newed objects in smart pointers in standalone statements.

Puts the new object into a smart pointer in a separate statement, which is a resource leak caused by other expressions throwing exceptions. Because C + + differs from other languages, the order of calculation of function parameters is largely determined by the compiler.

If you're doing Windows programming, or DLL development, May often encounter similar __cdecl , __stdcall and other keywords. They are to specify the parameters in the stack order. A discussion of functions and parameters can be consulted: C + + Manuscripts: Functions and Parameters

or to give an example of clarity:

processwidget   ( shared_ptr  <  widget  >   (  new  widget   priority   ());  

In the preceding code, the processWidget arguments are evaluated first before the function is called. The process can be considered to consist of three parts:

2. Implementation new Widget ;
4. Structure shared_ptr<Widget> ;
6. Called priority() .

In most cases the compiler has the right to determine the order of the three parts of the process, and if it is unfortunate for some efficiency reason, the compiler considers the order 1, 3, 2 to be, namely:

2. Implementation new Widget ;
4. Called priority() .
6. Structure shared_ptr<Widget> ;

So if priority throws an exception, the new widget will never find it back." Although we use smart pointers everywhere, the resources are leaking!

So in a more robust implementation, you should separate the statements that create the resource and initialize the smart pointer:

shared_ptr  <  widget  >  PW  =  shared_ptr  <  widget  >   ( new   Widget   processwidget   ( PW  ,  priority   ());  

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Item 17: Put the new object into a smart pointer in a separate statement effective C + + notes