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How do pointers inside a struct work?

rgork (14)
So, I have the following:

struct Example {string info, Data* data};

A class named Data incidentally.

vector<Data> this;

Now, the problem. The this-vector has a few objects in it. What I want to do is make an Example variable and make the Data* point to the first member of the this-vector. I thought about making something like this:

Example doesnotwork = {"", 0};
doesnotwork.data = this.at(0);

But it doesn't work. Exactly how do I do it?
Last edited on
Catfish3 (666)
this is a C++ keyword. Use another name instead "this".
rgork (14)
Oops, didn't think of that, but that's not the issue. I simply made up all the variable names for the sake of the example as I don't have the code at the moment.
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ne555 (10692)
> But it doesn't work
It doesn't work is not an error message.

> How do pointers inside a struct work?
Same as outside.

> make the Data* point to the first member of the this-vector
Consider that if the vector makes a reallocation, you'll have an invalid pointer.
Smac89 (1727)
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#include <iostream>
#include <vector>
DELETED


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#include <iostream>
#include <vector>

using namespace std;

typedef union Data{
    int m;
    double u;
    long f;
    
    Data(int kj, double lh, long fg)
    {
        m = kj;
        u = lh;
        f = fg;
    }
    
    Data& operator= (const Data& mydat)
    {
        m = mydat.m;
        u = mydat.u;
        f = mydat.f;
        
        return *this;
    }
};

struct Example
{
    vector <Data> m;
    string k;
    
    Example (string info, Data* data)
    {
        m.push_back(Data(2,-9.32322,-324332));
        k = info; *data = m.back();
    }
    
    ~Example()
    {
        cout << "Deleted!\n";
    }
};

int main()
{
    Data* flv = new Data(0,0,0);
    Example doesnotwork("", flv);
    return 0;
}
Last edited on
ne555 (10692)
¿what are you trying to show?
rgork (14)
Ah, got it. Of course by doing it the simple way:

Example doesnotwork = {"", &this.at(0)};

instead of

Example doesnotwork = {"", 0};
doesnotwork.data = this.at(0);
naraku9333 (2163)
@Smac89
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typedef union Data{
    int m;
    double u;
    long f;
    
    Data(int kj, double lh, long fg)
    {
        m = kj;
        u = lh;
        f = fg;
    }
    
    Data& operator= (const Data& mydat)
    {
        m = mydat.m;
        u = mydat.u;
        f = mydat.f;
        
        return *this;
    }
};
Why are you using a union, isn't this undefined behavior?
Catfish3 (666)
I have to say, I've never seen a union written that way.

But it looks like undefined behavior to me as well, because for a union you can only access one "member" at a time, after you've previously written to it. If you write a member then read another, it might blow up.
naraku9333 (2163)
Smac89 (1727)
uhh...union? Found it being used somewhere, decided to experiment. I wouldn't do it that way ofc; that is just an example

E: Why would it not work the way I showed?
Last edited on
naraku9333 (2163)
Check the output in the links I posted.
Smac89 (1727)
Yep, just read about union. Thanks for doing that ^
LB (13399)
You can't have a typedef with no alias.
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typedef union Data1 {/**/} /*alias*/;
typedef struct Data2 {/**/} /*alias*/;
typedef class Data3 {/**/} /*alias*/;
Recall from C:
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typedef struct tagData
{
    //...
} Data, *PData;
Data and PData are types, not variables. In C++ you could write tagData and it would have the same meaning as Data.
Last edited on
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