how to create an array of objects initializing each object?

[vaysagekv]

hi,
how to create an array of objects initializing each object?
thanks in advance

 

[Victor Bazarov]

how to create an array of objects initializing each object?

You can initialize them if you provide a brace-enclosed initialiser list
for the array. You can't do it with a dynamic array, however.

V

 

[Paul N]

See FAQ 10.5.

You can initialize them if you provide a brace-enclosed initialiser list
for the array.  You can't do it with a dynamic array, however.

Well, if you use a dynamic array everything does get initialised with
the default constructor. Which may be what the OP wanted. If you want
the elements of the array to be different, you can do this by mesing
about with a static member or a global.

 

[Victor Bazarov]

See FAQ 10.5.


Well, if you use a dynamic array everything does get initialised with
the default constructor.

Actually, that's not entirely accurate. If there is the initializer in
the form "()", it's default-initialized, if there isn't, an array of
built-in types will go *uninitialized*.

Which may be what the OP wanted. If you want
the elements of the array to be different, you can do this by mesing
about with a static member or a global.

V

 

[Virchanza]

You can initialize them if you provide a brace-enclosed initialiser list
for the array.  You can't do it with a dynamic array, however.

If you want to go to extremes:

char unsigned *const cu_pointer = new char
unsigned[ sizeof(MyClass) * N ];

MyClass *const p = reinterpret_cast<MyClass*>(cu_pointer);

::new( p ) MyClass(1,2,3);
::new( p+1 ) MyClass(4,5,6);
::new( p+2 ) MyClass(7,8,9);

// and then at the end:

p[2].~MyClass();
p[1].~MyClass();
p->~MyClass();

delete [] cu_pointer;

 

[Yakov Gerlovin]

*If you have tr1*

#include <array>
#include <iostream>

using namespace std;

struct MyClass
{
MyClass(int i){
cout << "MyClass(int " << i << ")" << endl;
}
MyClass(const char* s) {
cout << "MyClass(const char* " << s << ")" << endl;
}
MyClass(int i,int j, int k) {
cout << "MyClass(this=" << this << " i=" << i << " j=" << j << "
k="<< k << ")" << endl;
}
};

void main()
{
array<MyClass,4> arr = { 1, 2, "str", MyClass(1,2,3) };
cout << "Address of elem at pos 3 is " << &(arr[3]) << endl;
}

 

[Victor Bazarov]

*If you have tr1*

#include<array>
#include<iostream>

using namespace std;

struct MyClass
{
MyClass(int i){
cout<< "MyClass(int "<< i<< ")"<< endl;
}
MyClass(const char* s) {
cout<< "MyClass(const char* "<< s<< ")"<< endl;
}
MyClass(int i,int j, int k) {
cout<< "MyClass(this="<< this<< " i="<< i<< " j="<< j<< "
k="<< k<< ")"<< endl;
}
};

void main()

int main()

{
array<MyClass,4> arr = { 1, 2, "str", MyClass(1,2,3) };
cout<< "Address of elem at pos 3 is "<< &(arr[3])<< endl;
}

And... isn't this still copy-initialization (and not direct
initialization) of the array elements?

V

 

[Yakov Gerlovin]

And... isn't this still copy-initialization (and not direct

initialization) of the array elements?

I'm not sure what exactly do you mean by "direct initialization".
If it is compile time initialization, then naturally it is not direct
initialization.

If it means, that the object are created in-place, without creation of
temp objects and then copy construction, then it is a direct
initialization. At least with g++ 4.3.2. ( I'm not sure whether
standard requires a temp objects in this case or it allows in-place
object creation)

 

[Yakov Gerlovin]

For PODs it is possible to initialize at compile time:

struct POD
{
int i;
const char* s;
};

array<POD,2> arr2 = { 1, "str1", 2,"str2" };

(like Victor said in his first post)

 

[Victor Bazarov]

I'm not sure what exactly do you mean by "direct initialization".
If it is compile time initialization, then naturally it is not direct
initialization.

If it means, that the object are created in-place, without creation of
temp objects and then copy construction, then it is a direct
initialization. At least with g++ 4.3.2. ( I'm not sure whether
standard requires a temp objects in this case or it allows in-place
object creation)

I mean, if you define a copy-ctor in your class (instead of letting the
compiler define one), you might see whether it's called at all during
initialization of the array.

V

 

[Yakov Gerlovin]

Leigh is right, g++ requires a public copy constructor, but does not
actually call it

I made a copy constructor with some prints and it had not been called.
Making it private gives a compiler error.

The question is how can optimizer elide function with side effects
(prints)?

 

[Victor Bazarov]

Leigh is right, g++ requires a public copy constructor, but does not
actually call it

I made a copy constructor with some prints and it had not been called.
Making it private gives a compiler error.

The question is how can optimizer elide function with side effects
(prints)?

Elision of calling the copy constructor is explicitly allowed by the
Standard regardless of its side effects.

V