如何正确初始化类函数中的指针？

Question

I am using a array of pointer's declared within a class to point to another array. When a pointer array element is initialised to a value within main that value is also carried to the array element it is pointing to. The following code show this and it works fine. However if I initialise the pointer within the init class function I get a segfault. Also if I try to access an element from the pointer array not initialised in main I also get a segfault.

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

using namespace std;

class testclass {
public:
    double  *Wgt;
    double* *LWgt;

    void init() {
        Wgt = new double[26];
        LWgt = new double*[26];

        //segfault from here
        *LWgt[5] = 22.34543; 
    }
};

int main() {

    testclass *node;
    node = new testclass[10];

    for (int i = 0; i < 10; i++) {
        node[i].init();
    }

    for (int i = 0; i < 5; i++) {               //init_nconn here
        node[i].LWgt[23] = &node[i + 5].Wgt[12];
    }

    node[6].Wgt[12] = 50.6987;
    node[8].Wgt[12] = 0.999923;

    cout << *node[1].LWgt[23] << *node[3].LWgt[23] << "--\n";

    //No segfault
    *node[1].LWgt[23] = 33.234;

    cout << node[6].Wgt[12] << "---\n";
    //No segfault here
    cout << *node[1].LWgt[23] << "---\n";

    //Segfault from here
    cout << *node[3].LWgt[5] << "---\n";
}

Is there a way to initialise the pointer array within a class function without leading to a segfault?

Answer 1

    LWgt = new double*[26];

allocates memory for 26 double* but does not allocate memory for the pointers themselves.

The line

    *LWgt[5] = 22.34543; 

causes undefined behavior since memory hasn't been allocated for any of the elements of LWgt .

In order to be able to use that, you have to allocate memory for LWgt[5] first.

LWgt[5] = new double[<SOME_SIZE>];  // Allocate an array

or

LWgt[5] = new double; // Allocate just one element.

Answer 2

That's because none of the pointers stored inside LWgt actually point to anything valid yet. Remember that a pointer will be initialized by the runtime with some random value that may or may not be a valid address. When you try to stuff a value inside an invalid address you're going to get an error.

You need to initialize the pointers inside LWgt to point to some actual double memory locations before you can assign any values to them:

void init() {
    Wgt = new double[26];
    LWgt = new double*[26];

    for(int i = 0; i < 26; i++)
        LWgt[i] = new double;

    //segfault from here
    *LWgt[5] = 22.34543; 
}

As a side note, this kind of initialization is probably best done inside a constructor for your class rather than a special initialization function. The point of constructors is that they always run automatically when the class is instantiated. You don't have to worry about remembering to call an init function right after you create the object.

Answer 3

Lwgt is an array of pointers to double. You'd have to allocate each element in the array or remove the ** in the declaration.