Re: C++ Compiler behavior regarding struct constructors

From: Carl Daniel [VC++ MVP] (cpdaniel_remove_this_and_nospam_at_mvps.org.nospam)
Date: 12/19/04 

Date: Sun, 19 Dec 2004 08:29:00 -0800

Karl M wrote:
> Hi everyone,
> I just notice some strange behaviors on the MS C++ compiler regarding
> struct default constructor

You're just not used to the initialization rules for C++ (which are arcane
and obscure).

When you write...

int a[5];

...the contents of a[] are undefined.

Likewise...

struct Z
{
   int a[5];
};

Z z;

...the contents of z are undefined.

8.5/9 of the C++ standard says

<quote>
If no initializer is specified for an object, and the object is of (possibly
cv-qualified) nonPOD class type (or array thereof), the object shall be
default-initialized; if the object is of const-qualified type, the
underlying class type shall have a user-declared default constructor.
Otherwise, if no initializer is specified for an object, the object and its
sub-objects, if any, have an indeterminate initial value; if the object or
any of its sub-objects are of const-qualified type, the program is
ill-formed.
</quote>

You're falling afoul of the "Otheriwse..." part of that quotation.

> struct MyStruct
> {
> int a[5];
> };
> class MyClass
> {
> public:
> MyClass();
> ~MyClass();
> protected:
> MyStruct m_MyStruct;
> };
> //#1:
> MyClass::MyClass() //The default ctor for the class does not init the
> struct with NULL(maybe there is no default ctor for struct yet?)
> {

Here m_MyStruct has undefined content because MyStruct is a POD (which
implies no user defined constructors) so it's not initialized.

> }
> //#2 if I define the class ctor like this:
> MyClass::MyClass() : m_MyStruct() //The default ctor for the class
> does init the struct with NULL(call m_MyStruct() default ctor, now
> there is one!) {

Correct. Since you've forced the creation of a default contructor for
MyStruct, that default constructor zero-fills the memory.

> }
> //#3 or like this:
> MyClass::MyClass()
> {
> m_MyStruct = MyStruct(); // dose init the struct with NULL using a
> local stack var init with NULL

Likewise, you've forced the creation (and calling) of a default constructor
for MyStruct() which zero-fills the memory.

> }
> //Now if I define the struct like this:
> struct MyStruct
> {
> MyStruct()
> {
> int i;
> for(i=0;i<5;i++)
> a[i] = NULL;
> }
> int a[5];
> };

This is equivalent to the default constructor that the compiler will
generate for you, so it shouldn't be surprising that the results are nearly
identical.

> //#1 will init the struct with NULL because uses the defined struct
> ctor but generate almost the same bin (asm) code as case #2 and #3
> (the init stack var) without the struct ctor defined!
> I guess I don't understand when the compiler knows about the struct
> default constructor and when dose not (and expect for one defined).

It always knows about it, your example #1 simply doesn't force the compiler
to explicitly create a default constructor. Getting back to the quote from
8.5 of the standard - once you force a struct to have a constructor, it's no
longer a POD, so the code falls under the first sentence of that passage -
the object is default-initialized.

The best advice is: if you need initialization, write a constructor.

HTH

-cd

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