Using 'new' keyword with struct in c++

Question

#include "PQueue.h"

struct arcT;

struct coordT {
    double x, y;
};

struct nodeT {
    string name;
    coordT* coordinates;
    PQueue<arcT *> outgoing_arcs;
};

struct arcT {
    nodeT* start, end;
    int weight;
};

int main(){
    nodeT* node = new nodeT; //gives error, there is no constructor
}

My purpose is to create a new nodeT in heap. Error is:

error C2512: 'nodeT' : no appropriate default constructor available

There's nothing wrong with that code - `coordT` has an implicitly-generated constructor, which does nothing. Do you really get an error from that code? What exactly does it say? — Mike Seymour, Oct 14 '11 at 12:07
error C2512: 'nodeT' : no appropriate default constructor available — SegFault, Oct 14 '11 at 12:08
You have a different problem. The code you are providing compiles fine (Have you declared any constructor?) You can try pasting it to ideone or any other similar page to verify this. — David Rodríguez - dribeas, Oct 14 '11 at 12:08
Are you sure you don't define a parameterized constructor in coordT? — Luc Touraille, Oct 14 '11 at 12:09
@SegFault: there is no `nodeT` in the code you posted, so whatever you're really compiling, it has some error that this code doesn't. — Steve Jessop, Oct 14 '11 at 12:09
Are you sure that's your complete code? If you've given your class a constructor with arguments, then that will suppress the implicit default constructor; you will get that error then. And what's a `nodeT`? — Mike Seymour, Oct 14 '11 at 12:10
@Mike Seymour, I though I could simplify the question, but now I guess problem is not what I though, because coordT* coord = new coordT works fine but nodeT does not if used in same way — SegFault, Oct 14 '11 at 12:19

score 5 · Accepted Answer · answered Oct 14 '11 at 12:20

PQueue<arcT *> does not have an appropriate default constructor, so a default constructor for nodeT cannot be generated by the compiler. Either make an appropriate default constructor for PQueue<arcT *> or add a user-defined default constructor for nodeT which constructs outgoing_arcs appropriately.

score 4 · Answer 2 · answered Oct 14 '11 at 12:21

If the currently posted code in the question is an exact copy, then the only possible cause for this error is that PQueue<…> doesn’t define a default constructor, and defines another constructor instead.

Otherwise this code would compile.

More precisely, since you didn’t define a constructor for your structures, C++ tries to auto-generate them. It can only do this, though, as long as all its member variables are appropriately default constructible or initialisable. std::string has a default constructor, and coordT*, being a pointer, can be initialised. So only PQueue<…> remains as the culprit.

score 3 · Answer 3 · answered Oct 14 '11 at 12:36

3

THis may not be your problem but you've only declared one pointer on this line in arcT :-

nodeT* start, end;

You've declared start as a pointer and end as an actual nodeT object. Is this what you wanted to do?

answered Oct 14 '11 at 12:36

jcoder

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No, this is not what I wanted to do, I wanted both of them to be pointer, you saved me a lot of time – SegFault Oct 14 '11 at 12:50

Using 'new' keyword with struct in c++

3 Answers3