C++, using this to call a constructor from itself

Question

I was writing some java last night and I had two constructors that looked basically alike, except my default constructor provided some values to my object, it was something like this:

testObject(){
     width=5;
     height=12;
     depth=7;
     //other stuff is the same as the next one
}

testObject(int x, int y, int z){
    width=x; 
    height = y;
    depth = z;
    //All the other stuff is the same as default
}

So in this case, I was able to convert the code to do this instead:

testObject(){
    this(5,12,7);
}

That sent the values from the default constructor back to the constructor as the 3-int constructor to be built as such. Is there any way to get this type of functionality in C++?

Not yet, but there will be! See this [link](http://publib.boulder.ibm.com/infocenter/lnxpcomp/v111v131/index.jsp?topic=/com.ibm.xlcpp111.linux.doc/language_ref/delegating_ctors.html). — Daniel Walker, Sep 14 '11 at 20:37

C. K. Young · Accepted Answer · 2011-09-14T21:08:03.427

5

In C++0x, you can do this:

TestObject() :TestObject{5, 12, 7} {}

See Delegating Constructors for more details. (Curly braces not look familiar to you? They're for preventing narrowing.)

If you don't have C++0x available yet, then in your case, you can use default arguments as mentioned in other answers here.

edited Sep 14 '11 at 21:08

answered Sep 14 '11 at 20:36

C. K. Young

219,335
46
382
435

score 1 · Answer 2 · edited Sep 14 '11 at 20:41

1

You were close, try calling the 3-parameter constructor in your initializer list for your default constructor.

testObject(int x, int y, int z) :
    width(x), 
    height(y),
    depth(z) {
    //All the other stuff is the same as default
}

testObject() : testObject(5,12,7) {
     //other stuff is the same as the next one
}

edited Sep 14 '11 at 20:41

AJG85

15,849
13
42
50

answered Sep 14 '11 at 20:38

John Humphreys

37,047
37
155
255

score 0 · Answer 3 · edited May 23 '17 at 11:57

0

There is no way to do this, but you can create a private method with initialization code and call it from your different constructors.

EDIT: duplicate of Can I call a constructor from another constructor (do constructor chaining) in C++?

edited May 23 '17 at 11:57

Community

1
1

answered Sep 14 '11 at 20:36

Aurelien Ribon

7,548
3
43
54

1

It is impossible to _initialize_ class members from a private method -- they can only be assigned to. – ildjarn Sep 14 '11 at 20:39
Yeah, and that would prevent the definition of read-only (constant) variables. – Aurelien Ribon Sep 14 '11 at 20:40
There is a way... In C++ there is always a way! – Sep 14 '11 at 20:43
Yeah, and when there is really no way, the C++ standard committee decide to cope with that with the addition of new keywords: constexpr, auto , decltype, override, nullptr... :) – Aurelien Ribon Sep 14 '11 at 20:47

score 0 · Answer 4 · answered Sep 14 '11 at 20:37

0

You can use default values in the constructor (i.e. testObject(int x = 5, int y = 12, int z = 7) ) to do what you want.

answered Sep 14 '11 at 20:37

goffrie

461
2
7

score 0 · Answer 5 · answered Sep 14 '11 at 20:38

0

The most interesting thing is that you don't need to do that. Default arguments are aimed at solving problems like this. Here is an example:

#include <iostream>

struct Foo
{
    int x, y, z;

    Foo (int x = 5, int y = 12, int z = 7)
        : x (x), y (y), z (z)
    {}
};

int
main ()
{
    Foo f1, f2 (1, 2, 3);
    std::cout << f1.x << ", " << f1.y << ", " << f1.z << '\n'
         << f2.x << ", " << f2.y << ", " << f2.z << '\n';
}

Just for example sake, you can call a constructor of a class using placement new, like this:

struct Foo
{
    int x, y, z;

    Foo (int x, int y, int z)
        : x (x), y (y), z (z)
    {}

    Foo ()
    {
        this->~Foo ();
        new (this) Foo (5, 12, 7);
    }
};

This feature also comes out of the box in C++11, you can read more here.

answered Sep 14 '11 at 20:38

It's undefined behavior in general to call the destructor from the constructor, since the object's lifetime hasn't begun yet. In this case, though, the class has a trivial destructor so it might be valid. I'm not sure. As you say, though, it's for example only not real use -- there are enough other problems aside from that one. – Steve Jessop Sep 14 '11 at 20:53
@Steve: Yeah, this one is just an example. It will work though, if you a lot of conditions are met (like in a perfect storm :-)). For instance, this cannot be done if this class is a base class and has a virtual destructor etc. In this case destructor shouldn't be called, and example is for example sake, because people rarely come up with crazy ideas like this. Like they say in some videos - do not try to repeat this at home! :) – Sep 15 '11 at 00:00

score 0 · Answer 6 · answered Sep 14 '11 at 20:38

0

You can add default variable values to your class.

testObject::testObject(int x, int y, int z = 5)

Then you can call it with testObject(1,2) and z will retrieve the default value of 5

answered Sep 14 '11 at 20:38

Rontologist

3,568
1
20
23

C++, using this to call a constructor from itself

6 Answers6