Is there a reason you would initialise variables like this? - C / C++ / MFC Discussion Boards

Re: Linker Error :C1900

Malli_S26-Jul-12 20:15

Malli_S

26-Jul-12 20:15

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Compiler job

_Flaviu25-Jul-12 9:26

_Flaviu

25-Jul-12 9:26

I have a similar question, the one that somebody ask me recently : If I have a simple class, with only one method

C++

class Foo
{
public:
    Foo();
    virtual ~Foo();

    void DoSomething();
};

what generate the compiler ? Code for constructor, destructor and method DoSomething() only ? Don't generate a default copy constructor and assigning operator, even is doesn't explicity implemented ?

modified 26-Jul-12 7:22am.

Re: Compiler job

Maximilien25-Jul-12 9:47

Maximilien

25-Jul-12 9:47

The compiler will try to create the copy constructor and the assignment constructor.
Rules for Declaring Constructors[^]

Watched code never compiles.

Re: Compiler job

_Flaviu26-Jul-12 6:22

_Flaviu

26-Jul-12 6:22

Thank you for your answer.

Re: Compiler job

CPallini25-Jul-12 22:16

CPallini

25-Jul-12 22:16

Why don't you try to actually compile such code?

For instance you may discover the missed the return value type in DoSomething declaration.

Veni, vidi, vici.

Re: Compiler job

Malli_S25-Jul-12 23:10

Malli_S

25-Jul-12 23:10

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-Malli...!

Re: Compiler job

_Flaviu26-Jul-12 1:21

_Flaviu

26-Jul-12 1:21

You are perfectly right ! Laugh | :laugh:

Is there a reason you would initialise variables like this?

MarkB77725-Jul-12 1:33

MarkB777

25-Jul-12 1:33

Hi all,

Is there any particular reason you would initialise private members of a class like this...

<br />
SomeClass::Constructor(void): privateVar1(NULL), privateVar2(true) {<br />
<br />
}<br />

As opposed to this?

<br />
SomeClass::Constructor(void) {<br />
    privateVar1 = NULL;<br />
    privateVar2 = true;<br />
}<br />

From what I understand the first instance would be useful when inheritance is involved.. Perhaps this is just a favored style?

Cheers,
Mark

Mark Brock
"We're definitely not going to make a G or a PG version of this. It's not PillowfightCraft." -- Chris Metzen

Re: Is there a reason you would initialise variables like this?

_AnsHUMAN_ 25-Jul-12 1:47

_AnsHUMAN_

25-Jul-12 1:47

Const and reference members can only be initialized in an intializer list. Moreover if we don't initialize the variables in the initializers list the compiler will have to do it for us and later on in the constructor this will be initialized again. Hence, there would be two calls for variable initialization

You talk about Being HUMAN. I have it in my name
AnsHUMAN

Re: Is there a reason you would initialise variables like this?

jschell25-Jul-12 6:40

jschell

25-Jul-12 6:40

_AnsHUMAN_ wrote:
Moreover if we don't initialize the variables in the initializers list the compiler will have to do it for us and later on in the constructor this will be initialized again.

I doubt that.

The easiest way for a compiler to provide default values is to do the following
- Request memory block for the object
- Overwrite the entire block with zeros.
- Continue the construction process.

Providing code to set the default value for each member is thus not required.
Any other initialization, beyond the default value, would occur regardless.

Re: Is there a reason you would initialise variables like this?

Philippe Mori25-Jul-12 10:37

Philippe Mori

25-Jul-12 10:37

In standard C++, member without default constructor are not initialized to 0 automatically but are random.

Philippe Mori

Re: Is there a reason you would initialise variables like this?

jschell27-Jul-12 11:38

jschell

27-Jul-12 11:38

Philippe Mori wrote:
In standard C++, member without default constructor are not initialized to 0 automatically but are random.

Wrong.

Section 4.9.5 C++ specification clearly states that class member variables have a default value if they do not have an initializer.

class Any
{
  ....
  int var1 = 2; // This is an initializer.
  int var2;  // No initializer so set to default value (zero.)
  }

Local variables (those declared in a method) have not such default value.

Re: Is there a reason you would initialise variables like this?

Philippe Mori27-Jul-12 13:08

Philippe Mori

27-Jul-12 13:08

At this time, I program mainly in C# and in that case meber are initialized to (as member of a ref class would also be initialized in C++/CLI).

As far as I can tell, you have misunderstood the standard. A static member will be initialized to 0 but not a regiular member in the general case.

I have found many link that confirm what I have said.

http://www.cplusplus.com/forum/general/17582/[^]

http://stackoverflow.com/questions/1069621/are-members-of-a-c-struct-initialized-to-0-by-default[^]

http://stackoverflow.com/questions/2614809/what-is-the-default-value-for-c-class-members[^]

http://msdn.microsoft.com/en-us/library/s0wk5dy9.aspx[^]

Philippe Mori

Re: Is there a reason you would initialise variables like this?

jschell28-Jul-12 6:13

jschell

28-Jul-12 6:13

Philippe Mori wrote:
As far as I can tell, you have misunderstood the standard.

Yep, you are correct.

Re: Is there a reason you would initialise variables like this?

w-peuker26-Jul-12 21:21

w-peuker

26-Jul-12 21:21

Some compilers initialize memory to zero. But for compatibility reasons, the C++ standard leaves (built-in type) members untouched (I don't know the latest one). Of course, if your members have default constructors, they will be called (before the constructor body) unless you place them in the initializer list.

Re: Is there a reason you would initialise variables like this?

jschell27-Jul-12 11:40

jschell

27-Jul-12 11:40

w-peuker wrote:
Some compilers initialize memory to zero. But for compatibility reasons, the C++ standard leaves (built-in type) members untouched (I don't know the latest one).

Wrong. The only compilers that leave data members non-initialized would be compilers that are not implementing the ANSI C++ specification.

Section 4.9.5 C++ specification clearly states that class member variables have a default value if they do not have an initializer.

   class Any
   {
     ....
     int var1 = 2; // This is an initializer.
     int var2;  // No initializer so set to default value (zero.)
     }
<pre>

This happens regardless of how construction occurs.

Re: Is there a reason you would initialise variables like this?

Philippe Mori27-Jul-12 13:10

Philippe Mori

27-Jul-12 13:10

See my answer above. You have misunderstood the standard. I have found many links that proove otherwise. By the way, your example is not even valid C++ code.

Philippe Mori

modified 27-Jul-12 21:04pm.

Re: Is there a reason you would initialise variables like this?

jschell28-Jul-12 6:14

jschell

28-Jul-12 6:14

Philippe Mori wrote:
See my answer above. You have misunderstood the standard.

Yes, you are correct.

Re: Is there a reason you would initialise variables like this?

Richard MacCutchan25-Jul-12 1:52

Richard MacCutchan

25-Jul-12 1:52

It's just a matter of style, although I think the first option is preferred. MSDN has this to say[^] on the matter. You may also like to check if Bjarne Stroustrup has more to offer.

One of these days I'm going to think of a really clever signature.

Re: Is there a reason you would initialise variables like this?

George L. Jackson25-Jul-12 7:07

George L. Jackson

25-Jul-12 7:07

C++ FAQ by Marshall Cline says: [10.6] Should my constructors use "initialization lists" or "assignment"?
[^].

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Re: Is there a reason you would initialise variables like this?

MarkB77725-Jul-12 22:45

MarkB777

25-Jul-12 22:45

Thanks George Smile | :)

Mark Brock
"We're definitely not going to make a G or a PG version of this. It's not PillowfightCraft." -- Chris Metzen

Re: Is there a reason you would initialise variables like this?

w-peuker26-Jul-12 21:30

w-peuker

26-Jul-12 21:30

...and don't forget that the compiler processes the initializer list following the declaration order given in the class definition.
So you better stick to that order in the list (to prevent confusion). There are tools availabe that check this (e.g. cppcheck)

Re: Is there a reason you would initialise variables like this?

Eugen Podsypalnikov25-Jul-12 7:42

Eugen Podsypalnikov

25-Jul-12 7:42

// Is there any particular reason
Yes Smile | :)

C++

class A
{
  //..
public:
  A();
  A(int);
  A& operator=(int);
};

class B
{
  A m_a;
public:
  B() { m_a = 3; /*second call, after A::A()*/}
  // B() : m_a(3) /*first and last call of A::A(int)*/ {}
};

They sought it with thimbles, they sought it with care;
They pursued it with forks and hope;
They threatened its life with a railway-share;
They charmed it with smiles and soap. Smile | :)

Re: Is there a reason you would initialise variables like this?

Malli_S25-Jul-12 20:58

Malli_S

25-Jul-12 20:58

The difference is that, in the first way of initialization (known as 'Initialization List'), the constructors of the initialized members are called.

While in the second option, all the objects get constructed before reaching the first executable statement of the constructor. Hence in the second way of initialization, the assignment operators gets called.

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-Malli...!

ShellExecute() launch .CHM and PDF file not work in Window 7

Andraw11124-Jul-12 7:31

Andraw111

24-Jul-12 7:31

Dear all,

HINSTANCE handle = ShellExecute(NULL, NULL, strHlpFile, NULL, NULL, SW_SHOW);

I create a application in MFC, it works fine in Window XP and return 42 value, but if run it in Window 7, it cannot open the .CHM or PDF file, the return code is 8.

What's the problem? how can I solve issue?

I try to use system() to launch .CHM or PDF file, it works fine in both Window XP and Window 7, but it has MS-DOS black screen there, how can I let MS-DOS not shown?

Thank you very much!

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