Dynamic array of objects - initialization

[Spoon]

Hello everyone,

I want to create an array of objects at run-time.

AFAIU, operator new[] will call the default constructor for each object
in the array. In other words, the following program will print INSIDE
DEFAULT CTOR five times.

#include <vector>
#include <cstdio>

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;
};

int main()
{
Foo *ww = new Foo[5];
return 0;
}

$ g++ -Wall -g3 vectest.cxx
vectest.cxx: In function `int main()':
vectest.cxx:20: warning: unused variable 'ww'
$ ./a.out
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR

What if I want to use a different constructor?

For example, how can I get the program to create an array of 5 objects
that hold a 123-byte buffer?

Foo *ww = new Foo(123)[5];

is a syntax error. Am I missing something obvious?

I suppose I could add a static variable to class Foo and have the
default constructor use the value of that variable...

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;
static int defaultsize;
};

int Foo::defaultsize = 0;

int main()
{
Foo::defaultsize = 123;
Foo *ww = new Foo[5];
return 0;
}

But that feels like a kludge. Is there a better solution?

On a related note, would a vector help in this situation?

I could write something along the lines of

std::vector < Foo > v;
v.reserve(N);
for (int i=0; i < N; ++i)
{
Foo *curr = new Foo(size)
v.push_back(*curr);
}

But that feels somewhat like a kludge too.

Regards.

 

[Victor Bazarov]

I want to create an array of objects at run-time.

AFAIU, operator new[] will call the default constructor for each
object in the array. In other words, the following program will print
INSIDE DEFAULT CTOR five times.

#include <vector>
#include <cstdio>

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;
};

int main()
{
Foo *ww = new Foo[5];
return 0;
}

$ g++ -Wall -g3 vectest.cxx
vectest.cxx: In function `int main()':
vectest.cxx:20: warning: unused variable 'ww'
$ ./a.out
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR

What if I want to use a different constructor?

You're SOL.

For example, how can I get the program to create an array of 5 objects
that hold a 123-byte buffer?

Foo *ww = new Foo(123)[5];

is a syntax error. Am I missing something obvious?

Not really. There is no way to do what you want in a single statement.

I suppose I could add a static variable to class Foo and have the
default constructor use the value of that variable...

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;
static int defaultsize;
};

int Foo::defaultsize = 0;

int main()
{
Foo::defaultsize = 123;
Foo *ww = new Foo[5];
return 0;
}

But that feels like a kludge. Is there a better solution?

Use std::vector.

On a related note, would a vector help in this situation?
Yep.

I could write something along the lines of

std::vector < Foo > v;
v.reserve(N);
for (int i=0; i < N; ++i)
{
Foo *curr = new Foo(size)
v.push_back(*curr);
}

But that feels somewhat like a kludge too.

Whatever you choose to call it. I call it "a work-around".

V

 

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

Hello everyone,

I want to create an array of objects at run-time.

AFAIU, operator new[] will call the default constructor for each object
in the array. In other words, the following program will print INSIDE
DEFAULT CTOR five times.

#include <vector>
#include <cstdio>

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;

};

int main()
{
Foo *ww = new Foo[5];
return 0;

}

$ g++ -Wall -g3 vectest.cxx
vectest.cxx: In function `int main()':
vectest.cxx:20: warning: unused variable 'ww'
$ ./a.out
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR
INSIDE DEFAULT CTOR

What if I want to use a different constructor?

For example, how can I get the program to create an array of 5 objects
that hold a 123-byte buffer?

Foo *ww = new Foo(123)[5];

is a syntax error. Am I missing something obvious?

I suppose I could add a static variable to class Foo and have the
default constructor use the value of that variable...

struct Foo
{
Foo(int u) {
puts("INSIDE CTOR"); p = new char;
}
Foo() {
puts("INSIDE DEFAULT CTOR"); p = new char[666];
}
~Foo() {
puts("INSIDE DTOR"); delete[] p;
}
char *p;
static int defaultsize;

};

int Foo::defaultsize = 0;

int main()
{
Foo::defaultsize = 123;
Foo *ww = new Foo[5];
return 0;

}

But that feels like a kludge. Is there a better solution?

On a related note, would a vector help in this situation?

I could write something along the lines of

std::vector < Foo > v;
v.reserve(N);
for (int i=0; i < N; ++i)
{
Foo *curr = new Foo(size)
v.push_back(*curr);

}

std::vector<Foo> v(5, Foo(4));

 

[Victor Bazarov]

[..]

std::vector<Foo> v(5, Foo(4));

... along with a proper implementation of the copy c-tor.

V