Passing a pointer of constructor

[seank76]

While going through my company's existing codebase, I saw a bunch of
weird lines.

Take a look at this one:

class A
{
public:
A(Foo *f) : _f(f) {}

private:
Foo *_f;
};

class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}
};

How can you pass "Foo()" as the argument to the constructor of A ?
Wouldn't the memory that is allocated to Foo() get destroyed as soon
as we are out of the scope of B()?

What does this mean?

[pmouse]

Interesting...A re you sure this is compilable code?
g++ returns the following error :
error: invalid cast of an rvalue expression of type 'Foo' to type
'Foo&'
On May 3, 1:14 pm, seank76 <sean...@gmail. comwrote:

While going through my company's existing codebase, I saw a bunch of
weird lines.

Take a look at this one:

class A
{
public:
A(Foo *f) : _f(f) {}

private:
Foo *_f;

};

class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}

};

How can you pass "Foo()" as the argument to the constructor of A ?
Wouldn't the memory that is allocated to Foo() get destroyed as soon
as we are out of the scope of B()?

What does this mean?

[seank76]

On May 3, 2:16 pm, pmouse <pmo...@cogeco. cawrote:

Interesting...A re you sure this is compilable code?
g++ returns the following error :
error: invalid cast of an rvalue expression of type 'Foo' to type
'Foo&'

On May 3, 1:14 pm, seank76 <sean...@gmail. comwrote:

While going through my company's existing codebase, I saw a bunch of
weird lines.

Take a look at this one:

class A
{
public:
A(Foo *f) : _f(f) {}

private:
Foo *_f;

};

class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}

};

How can you pass "Foo()" as the argument to the constructor of A ?
Wouldn't the memory that is allocated to Foo() get destroyed as soon
as we are out of the scope of B()?

What does this mean?

I can definitely compile this using Sun forte 6.2 compiler.
Did you make sure to define class Foo correctly?

[Gianni Mariani]

seank76 wrote:

While going through my company's existing codebase, I saw a bunch of
weird lines.

Take a look at this one:

class A
{
public:
A(Foo *f) : _f(f) {}

private:
Foo *_f;
};

class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}
};

How can you pass "Foo()" as the argument to the constructor of A ?
Wouldn't the memory that is allocated to Foo() get destroyed as soon
as we are out of the scope of B()?

What does this mean?

It means you have problems.

The first thing that comes to mind is that &(Foo &)Foo()) might return
the address of a temporary. This means that A(Foo *f) : _f(f) {} is
being initialized to somthing that will be destroyed shortly after the
constructor for A returns.

All around, a very bad thing to be doing.

[seank76]

On May 3, 2:51 pm, Gianni Mariani <gi3nos...@mari ani.wswrote:

seank76 wrote:

While going through my company's existing codebase, I saw a bunch of
weird lines.

Take a look at this one:

class A
{
public:
A(Foo *f) : _f(f) {}

private:
Foo *_f;
};

class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}
};

How can you pass "Foo()" as the argument to the constructor of A ?
Wouldn't the memory that is allocated to Foo() get destroyed as soon
as we are out of the scope of B()?

What does this mean?

It means you have problems.

The first thing that comes to mind is that &(Foo &)Foo()) might return
the address of a temporary. This means that A(Foo *f) : _f(f) {} is
being initialized to somthing that will be destroyed shortly after the
constructor for A returns.

All around, a very bad thing to be doing.

That was my first response when I first saw this code.
However, a colleague of mine has claimed although it is "temporary" ,
the scope of it belongs to the class instance, NOT the constructor,
hence it is ok.
I don't know if this is right. (The code has been in production for
years so he must be right though...)
I will try to create a test program to validate.

In any case, I just wanted to know if this is a common programming
tactic by anybody.

[Gianni Mariani]

seank76 wrote:
....

>
That was my first response when I first saw this code.
However, a colleague of mine has claimed although it is "temporary" ,
the scope of it belongs to the class instance, NOT the constructor,
hence it is ok.

That would be one weird compiler.

In standard C++ it is not, it's probably not even standard C++.

I don't know if this is right. (The code has been in production for
years so he must be right though...)

or lucky.

I will try to create a test program to validate.

In any case, I just wanted to know if this is a common programming
tactic by anybody.

I have never used it.

[Alf P. Steinbach]

* seank76:

On May 3, 2:51 pm, Gianni Mariani <gi3nos...@mari ani.wswrote:

>seank76 wrote:

>>While going through my company's existing codebase, I saw a bunch of
weird lines.
Take a look at this one:
class A
{
public:
A(Foo *f) : _f(f) {}
private:
Foo *_f;
};
class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}
};

That was my first response when I first saw this code.
However, a colleague of mine has claimed although it is "temporary" ,
the scope of it belongs to the class instance, NOT the constructor,
hence it is ok.

Bullshit.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[seank76]

On May 3, 4:09 pm, "Alf P. Steinbach" <[e-mail] rote:

* seank76:

On May 3, 2:51 pm, Gianni Mariani <gi3nos...@mari ani.wswrote:

seank76 wrote:
While going through my company's existing codebase, I saw a bunch of
weird lines.
Take a look at this one:
class A
{
public:
A(Foo *f) : _f(f) {}
private:
Foo *_f;
};
class B : public A
{
public:
B() : A(&(Foo &)Foo())) {}
};

That was my first response when I first saw this code.
However, a colleague of mine has claimed although it is "temporary" ,
the scope of it belongs to the class instance, NOT the constructor,
hence it is ok.

Bullshit.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

Just compiled and ran a test program without any errors using Sun
forte and gcc1.2

the test program is the following:

#include <iostream>
#include <fstream>

using namespace std;

class A
{
public:
A() {}

void doThat() { cout << "HELLOSE SEAN!" << endl; }
};

class B
{
protected:
A* _a;

public:
B(A *a) : _a(a) {}

};

class C : B
{
public:
C() : B((A *)&(const A &)A()) {}

void dodo() { _a->doThat(); }
};

int main()
{
cout << "Starting test." << endl;

C c;

cout << "About the run C.dodo() " << endl;

c.dodo();
return 0;
}

[Victor Bazarov]

seank76 wrote:

[..]
Just compiled and ran a test program without any errors using Sun
forte and gcc1.2

the test program is the following:

[..code involving undefined behaviour redacted..]

Go buy a lottery ticket, today is your lucky day.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[seank76]

On May 3, 4:28 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

seank76 wrote:

[..]
Just compiled and ran a test program without any errors using Sun
forte and gcc1.2

the test program is the following:

[..code involving undefined behaviour redacted..]

Go buy a lottery ticket, today is your lucky day.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

Wow.

I didn't know there were so many highschool kids in this forum.
Real professionals know they see all kinds of weird **it in the
previous programmers' codes all the time.

But then... I guess this is not really the place to share weird
behaviors of a c++ compiler.