Use of initializer list - is it defined?

**Elysia** · 03-26-2010

Does anyone think they can answer if the order of initialization in the initialization list is defined? Ie, is this safe?

Code:

ArrayList(): m_Size(1), m_pArray(new T[m_Size]), m_LastIndex(0)

**laserlight** · 03-26-2010

It depends on the order in which the members were declared in the class definition.

**Elysia** · 03-26-2010

Then, if I throw it around to something like

Code:

	int m_Size;
	int m_LastIndex;
	T* m_pArray;
	
public:
	ArrayList(): m_Size(1), m_pArray(new T[m_Size]), m_LastIndex(0)

Will it be defined behavior?

**laserlight** · 03-26-2010

Yes, but risky due to a possible change under maintenance, so I wouldn't do it.

**Elysia** · 03-26-2010

I see your point. Bad idea.
Thanks.

**anon** · 03-26-2010

It may not be a best idea, but the compiler might at least be able to warn, if the initialization doesn't occur in the good order (e.g that you initialize m_lastIndex after m_pArray here).

**brewbuck** · 03-26-2010

Originally Posted by anon

It may not be a best idea, but the compiler might at least be able to warn, if the initialization doesn't occur in the good order (e.g that you initialize m_lastIndex after m_pArray here).

The initialization ALWAYS occurs in the order the members are declared. If the initializer list is in a different order, the compiler will probably warn, but it doesn't change the initialization order.

If the members were initialized in the order given in the list, then two constructors could exist which initialize the members in two different orders. Upon destruction, the members would then potentially be destructed in the wrong order.

**anon** · 03-26-2010

Yes, I know that. May-be I worded it badly, but in the example, the initializer list is in an incorrect order and you might expect a warning.

**tabstop** · 03-26-2010

Originally Posted by anon

Yes, I know that. May-be I worded it badly, but in the example, the initializer list is in an incorrect order and you might expect a warning.

I.e., gcc gives you such:

Code:

temp.cpp: In constructor `ArrayList::ArrayList()':
temp.cpp:5: warning: `ArrayList::m_Size' will be initialized after
temp.cpp:4: warning:   `int*ArrayList::m_pArray'
temp.cpp:12: warning:   when initialized here
temp.cpp:4: warning: `ArrayList::m_pArray' will be initialized after
temp.cpp:3: warning:   `int ArrayList::m_LastIndex'
temp.cpp:12: warning:   when initialized here

**Elysia** · 03-26-2010

Visual C++ does not, however. And given that it might be a bad idea, since the order of declarations can easily change, I will not pursue it.

**whiteflags** · 03-26-2010

It's your code, you should know whether that will change.

**Elysia** · 03-26-2010

I most certainly would, but I don't want to introduce dependencies on variable declaration order.
What if I add or remove some of them sometime? Shall I put in comments saying: this variable MUST be declared at this very exact position, and ABSOLUTELY before this and after this? Annoying.

**whiteflags** · 03-26-2010

What if I add or remove some of them sometime?

When would you do that and not also need to edit the big three?

**Elysia** · 03-26-2010

I'm just saying I'd rather stay on the safe side. The whole issue is easily avoided in this case by using a constant.

**laserlight** · 03-26-2010

Originally Posted by whiteflags

When would you do that and not also need to edit the big three?

When you re-arrange the declarations for cosmetic purposes, or maybe with the hope of better packing.

Anyway, moving the new[] to the constructor body would be consistent with what one would do if more than one new or new[] was to be used in the constructor.

Use of initializer list - is it defined?

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