Array not work (stupid question)

[rock33]

hi community,
i have a problem with array, i don't understand because it go in Segmentation error.

class Graph
{
private:
    int nodes;
    int edges;
    pair <long long, pair<int, int> > p[MAX];  
public:
void init()
{
     DisjointSet *Q;
cout << "before init"<<endl;
     Q->init();
}

const int MAX = 9000;  

class DisjointSet
{
private:
    int id[MAX];
public:
 void init()  
 {  
    for(int i = 0;i < MAX;++i)  
        id[i] = i; 
    cout << "after init" << endl; 
 }
}

 Graph g;
    string filename = "name.txt";

cout << "sono main" << endl;

    if(!g.file(filename))
    {
        cerr << "errore file" << endl;
    }
    return 0;

error: Segmentation error

shell show:
before init
Segmentation error

[Ganado]

     DisjointSet *Q;
cout << "before init"<<endl;
     Q->init();

Your pointer is uninitialized.
Nowhere do you create a DisjointSet object.

This would create an actual object:
DisjointSet Q;

Of course, that object only exists for the duration of the function... so in your example it's quite useless.

BTW, unless there's a good reason not to, any "initialization" work should be done in the constructor, that's what a constructor is for.

[rock33]

if try Q only, this is the error:

in function `Graph::file(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
main4.cpp:(.text._ZN5Graph4fileENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN5Graph4fileENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x25): undefined reference to `DisjointSet::DisjointSet()'
collect2: error: ld returned 1 exit status

for initialization in the costructor, call init in constructor?

[Ganado]

Sure, you can call init in the constructor. Or just move the code that's in init into the constructor. Whatever you feel is the best organization.

Can you show the current code you have that produces that error?
Show where you declare and implement the DisjointSet constructor. What you are presenting to us does not match what you have in your code, therefore you are not accurately showing your classes (for example, you are attempting to use a function called "file" but no such function exists).

[rock33]

ok i have moved the init code in constructor, now work, thanks.

I have another question related to similar questions,
if I wanted to put a dynamic value in place of MAX,
how should I proceed to then initialize the array?
the dynamic value is in another class

[Ganado]

If the size of the array needs to be dynamic, I would switch to using a std::vector.

A vector can be used in the same way as an array, except you would declare it like:
vector<int> id; // empty vector

vector<int> id(42); // vector with 42 ints

id[3]; // index 3 (4th element) of vector

See example at bottom of https://en.cppreference.com/w/cpp/container/vector

[rock33]

thanks for all :D