Error on return using a struct method - C++

Question

I separated from a single file into a header and body files, but now have a problem.

I guess I am declaring something wrong but can't figure out what.

Body:

# include "GraphAL.h"


// A utility function to create a new adjacency list node
struct AdjListNode* GraphAL::newAdjListNode(int dest, int weight)
{
    struct AdjListNode* newNode =
            (struct AdjListNode*) malloc(sizeof(struct AdjListNode));
    newNode->dest = dest;
    newNode->weight = weight;
    newNode->next = NULL;
    return newNode;
}

Header:

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>


class GraphAL{
public:

    struct AdjListNode* newAdjListNode(int dest, int weight);


    // A structure to represent a node in adjacency list
    struct AdjListNode
    {
        int dest;
        int weight;
        struct AdjListNode* next;
    };

    // A structure to represent an adjacency liat
    struct AdjList
    {
        struct AdjListNode *head; // pointer to head node of list
    };

    // A structure to represent a graph. A graph is an array of adjacency lists.
    // Size of array will be V (number of vertices in graph)
    struct Graph
    {
        int V;
        struct AdjList* array;
    };
};

The error is: " cannot convert ‘GraphAL::AdjListNode*’ to ‘AdjListNode*’ in return "

struct AdjListNode* newAdjListNode(int dest, int weight); must come after the definition of struct AdjListNode. Also the use of malloc causes undefined behaviour, you could use new instead — M.M
– M.M, Commented Aug 29, 2019 at 23:30
lol indeed, thanks! I changed it to the end and at the body file I changed to "struct GraphAL::AdjListNode* GraphAL::newAdjListNode(int dest, int weight)". Also, thanks for the advice on malloc, you are right. — João P.
– João P., Commented Aug 29, 2019 at 23:38
You don't need to put struct in cases where you refer to a variable in C++ like: struct AdjList* array, you can just put AdjList* array — Mirko
– Mirko, Commented Aug 30, 2019 at 3:39

Red.Wave · Accepted Answer · 2019-08-30 00:20:04Z

You need to fully qualify return type in the implementation site:

GraphAL::AdjListNode* GraphAL::newAdjListNode(int dest, int weight)

In c++ use of malloc/free is discouraged for a couple of reasons; we tend to use new/delete instead:

AdjListNode* newNode =new AdjListNode;

There are a bunch of other things that can overally modernize and improve this snippet, but that I guess might cause confusion - if mentioned right now.

You may go ahead an do this:

auto GraphAL::newAdjListNode(int dest, int weight){
    //...
}

And:

auto newNode =new AdjListNode {dest, weight,nullptr};

Where nullptr is the proper keyword for null pointer value and auto is used for automatic type deduction.

I can go furthur and rewrite your code in a few short lines of declaration in header:

#include <forward_list>
#include <utility>

class GraphAL{
public: 
    typedef std::forward_list<std::pair<int,int>> Graph;
};

Then:

#include <GraphAL.h>
GraphAL::Graph gr1;
gr1.emplace_front(1,1);

Choice of std container(vector, list, ...) depends on detailed implementation.

Have fun.

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