Direct constructor call in C++

Question

I have a function and a class as below

class Vertex {
public:
    int mId;
public:
    Vertex(int info=-1) : mId(info) {

    }
};

class Edge {
public:
    Vertex mStart, mEnd;
    int mWeight;
public:
    Edge(Vertex start=-1, Vertex end=-1, int wt=-1) :
        mStart(start), mEnd(end), mWeight(wt) {
    }
};

class Graph {
    void addEdge(const Edge& e) {
        //Adds this edge to a vector
    }
};

shared_ptr<Graph> mygraph(new Graph(13 //no of vertices
                             , 17   //no of edges
                             , false));

mygraph->addEdge(Edge( 1, 2, 1));
mygraph->addEdge(Edge( 3, 1, 1));
mygraph->addEdge(Edge( 1, 6, 2));
mygraph->addEdge(Edge( 1, 7, 4));
...

Here I am passing direct Edge values in a constructor and get no crash. But I guess there will be a memory leak here. Whats the correct way to pass an object by reference after doing construction?

PS: Assume that Vertex is an implicit constructor accepting int as id.

The code looks perfectly fine. There will be no memory leak as nothing is allocated from the free store except the `Graph` in the `shared_ptr` (bonus points for that). — molbdnilo, Jan 20 '14 at 10:16
@molbdnilo Thanks for the comment. I wanted to confirm whether this way of construction and passing to a function without using objects is fine? — tariq zafar, Jan 20 '14 at 10:37
@tariqzafar It's not only fine, it's usually recommended, although I don't understand what you mean by "without using objects" - you are using objects. — molbdnilo, Jan 20 '14 at 10:45
@molbdnilo I actually meant that instead of writing Edge e(1, 2, 1) I am writing Edge(1, 2, 1). Thanks for clarifying :) — tariq zafar, Jan 20 '14 at 11:06

mexus · Accepted Answer · 2014-01-20T11:37:01.987

There wouldn't be any memory leaks, destructors will be automatically called, as @Borgleader wrote

You didn't use new for the edges -> no memory leak If you don't name objects it doesn't mean they wouldn't be destroyed. Everything's okay.

Although you can perform a little optimisation if you are using C++11. In your code object Edge is created, then passed by a const reference to a function addEdge(), where it will be copied to a vector with a copy constructor. You can avoid this overhead by using std::vector::emplace_back! And templates. Like so:

class Graph {
    public:
    template<class... Args>
    void addEdge(Args&& ...args) {
        //Assuming vector is std::vector<Edge>
        vector.emplace_back(args...);
    }
};
mygraph->addEdge(1, 2, 1);

Of course all created objects will be automatically destroyed when vector is cleared.

And, BTW, if you ever doubt whethere your program has memory leaks, Valgrind is at your service!

Update:

I've coded a tiny and simple example showing the difference, see it at http://ideone.com/ARzhL2. You can scroll directrly to a stdout section to see the result.

Maybe you will find this useful: http://stackoverflow.com/questions/413477/is-there-a-good-valgrind-substitute-for-windows — mexus, Nov 13 '14 at 06:31

Direct constructor call in C++

1 Answers1

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