invalid use of non-static member function

[rock33]

Hi Community,
i need sort after inser elements, but after the shell show this error:

error: invalid use of non-static member function ‘int compare(const void*, const void*)’
     qsort(this->nodes, this->vertices, sizeof(int), compare)"here";

declared here
 int "declare here" compare (const void * a, const void * b)

class GenericNode
{
private:
    int id;
    int weight;
    GenericNode* Prev;
    list<pair<GenericNode*, int>> *adj;
}

class Graph
{
private:
    int arches;
    int vertices;
    vector<GenericNode*> *nodes;
public:
    Graph()
    {
        this->arches = 0;
        this->vertices = 0;
        this->nodes = new vector<GenericNode*>;
    }
    int Graph::compare (const void * a, const void * b)
    {
        return ( *(int*)a - *(int*)b );
    }
    void setNode(GenericNode* node)
    {
        this->nodes->push_back(node);
    }
    void insert()
    {
        this->vertices=10;
        for(int i=0; i < this->vertices; i++){
            this->setNode(new GenericNode(i+1));
        }
        qsort(this->nodes, this->vertices, sizeof(int), compare);
    }
}

int main()
{
    Graph Q;
    Q.insert();   
}

[helios]

//MUST be static:
static int Graph::compare (const void * a, const void * b)
{
    return ( *(int*)a - *(int*)b );
}

You can also simply pass a lambda to qsort:

qsort(this->nodes, this->vertices, sizeof(int), &[](auto a, auto b){ return ( *(int*)a - *(int*)b ); });

[rock33]

i try but:

error: taking address of temporary [-fpermissive]
 rtices, sizeof(int), &[](auto a, auto b){ return ( *(int*)a - *(int*)b ); });
                                                                           ^

Graph.cpp:103:53: error: cannot convert ‘Graph::insert()::<lambda(auto:1, auto:2)>*’ to ‘__compar_fn_t’ {aka ‘int (*)(const void*, const void*)’}
     qsort(this->nodes, this->vertices, sizeof(int), &[](auto a, auto b){ return ( *(int*)a - *(int*)b ); });
In file included from /usr/include/c++/8/cstdlib:75,
                 from /usr/include/c++/8/ext/string_conversions.h:41,
                 from /usr/include/c++/8/bits/basic_string.h:6400,
                 from /usr/include/c++/8/string:52,
                 from Graph.hpp:5,
                 from Graph.cpp:4:
/usr/include/stdlib.h:828:20: note:   initializing argument 4 of ‘void qsort(void*, size_t, size_t, __compar_fn_t)’
      __compar_fn_t __compar) __nonnull ((1, 4));

and if try static near int:

error: cannot declare member function ‘static int Graph::compare(const void*, const void*)’ to have static linkage [-fpermissive]
 static int Graph::compare (const void * a, const void * b)

[Ganado]

I thought lambdas w/ nothing in the [] can be converted into function pointers.
What compiler are you using?

[jlb]

Have you considered using std::sort instead of qsort()?

[rock33]

g++ in linux

now try sort()

sort( this->nodes->begin(), this->nodes->begin()+this->vertices );

can you see if it is correct?

[jlb]

I question a few things about your implementation, for example why is nodes a pointer to a std::vector? Wouldn't a "normal" non-pointer vector work?

class Graph
{
private:
    int arches;
    int vertices;
    vector<GenericNode*> nodes;
public:
    Graph() : arches(0), vertices(0);
    {
        // Blank body.
    }

For the std::sort you will probably want to overload the less than comparison operator in your GenericNode class.

The std::sort would then be something like:

std::sort(nodes.begin(), nodes.end());

And note that the this-> is usually not required unless you have a "name clash" with a parameter.

[rock33]

ok i use it, i think it works, but it didn't solve the problem i had with the graph, too bad, but thanks for the help.

[jlb]

but it didn't solve the problem i had with the graph,

What problem is that?

You haven't provided enough code for anyone to really see what you're trying to do so you would need to ask specific questions and provide the actual code, not just random snippets that won't even come close to compiling.

[rock33]

I thought my problem was about ordering the graph, but that's not it.
I have a problem on building MST with Prim sometimes it gives me the exact result and other times wrong, then sometimes if I order the file before inserting it it gives me the exact result, other times it gives me the wrong result anyway, I've been going crazy for days I can't to understand the reason.
but I can't post all the code because it's a lot