Error when initializing heap-allocated object

Question

I am trying to initialize a heap-allocated object as follows:

class Ball {
  int radius;
  string colour;
};

int main(){
    Ball *b = new Ball { radius = 5, colour = "red" };
}

Wondering why this is giving me an error? Thanks

Please pick up a [Good Book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Alok Save, Nov 12 '11 at 07:14
If you have a c++0x compiler, have a look here at initializer lists http://en.wikipedia.org/wiki/C%2B%2B11. If you have a c++98 compiler... Anyway in both ways your code is wrong. — joy, Nov 12 '11 at 08:24

score 3 · Answer 1 · answered Nov 12 '11 at 07:01

3

That's not how you initialize an object in C++.

Here's one way to do it:

class Ball {
    int radius;
    string colour;

public:

    //  Define a Constructor
    Ball(int _radius, const string &_colour)
        : radius(_radius)
        , colour(_colour)
    {
    }
};

int main(){
    Ball *b = new Ball(5, "red");

    delete b;  //  Don't forget to free it.
}

answered Nov 12 '11 at 07:01

Mysticial

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Why the `_`s? No ambiguity without them. – Pubby Nov 12 '11 at 07:14
Probably more like "reader" ambiguity. But you're right, it does actually work without the `_`. I was under the impression that the parameters will shadow the fields, but I guess it doesn't apply to initializer lists. – Mysticial Nov 12 '11 at 07:19

score 0 · Answer 2 · answered Nov 12 '11 at 07:00

Use () instead of {}. You're calling a constructor, which is just a special function that instantiates an object.

So, the syntax looks like this:

Ball* b = new Ball(5, "red");

Also note that the parameter order in the constructor (5 before "red") dictates which variable get assigned to which value. So, you don't put the variable name in the constructor call.

score 0 · Accepted Answer · answered Nov 12 '11 at 07:02

0

Many issues!

Try this:

class Ball {
public:
  Ball(int r, const string &s)
  {
    radius = r;
    colour = s;
  }
  int radius;
  string colour;
};

int main(){
    Ball *b = new Ball(5, "red");
    // ....
    // delete b;  <-- dont forget
}

answered Nov 12 '11 at 07:02

masoud

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@Pubby: The question tagged by _c++_, not _c++11_. – masoud Nov 12 '11 at 07:43
I'm talking about in the constructor. `Ball(int r, const string &s) : radius(r), colour(s) {}` – Pubby Nov 12 '11 at 19:23
Ummm, yes, we can use this style too. – masoud Nov 12 '11 at 20:04
@MasoudM. It isn't just a style, you know. – nullpotent Nov 13 '11 at 13:29

score 0 · Answer 4 · answered Nov 12 '11 at 07:11

When creating an object use ( ... ) in place of { ... } and no need to write variable name. just pass the value to be assigned.

Ball *b = new Ball { radius = 5, colour = "red" };` // Wrong

change it to

Ball *b = new Ball ( 5, "red" );

and, don't forget to declare a constructor in your Ball class, in public section.

Ball(int, std::string);

score 0 · Answer 5 · answered Nov 12 '11 at 07:12

0

This works in C++11:

struct Ball {
  int radius;
  std::string colour;
};

int main() {
  Ball* b = new Ball({ 5, "red" });
}

answered Nov 12 '11 at 07:12

Pubby

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Error when initializing heap-allocated object

5 Answers5