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using pointers to get values back from a function (c++)

Answer 1

+5 A:

You're writing to memory in get_dimensions, but you're never actually allocating that memory. Instead, you are just passing NULL pointers.

One potential fix is to change your calling code to:

int Latsize, Lonsize;
vector<double> Latgrid;
vector<double> Longrid;
int res = asp->get_dimensions(&Latsize,&Lonsize,&Latgrid,&Longrid);

This allocates space for the variables automatically on the stack, then you pass the address of that allocated space to get_dimensions, which will modify the values.

Michael 2009-08-24 21:02:07

Answer 2

+1 A:

Where are 'latgrid' and 'longrid' defined? If they are local to ASPFile::get_dimensions, then their destructors will be called as the function returns, destroying them.

Mark P Neyer 2009-08-24 21:03:41

thanks for the hint, i would've forgotten that. however, the problem ẃas solved with the above answer by michael.

andreash 2009-08-24 21:07:22

Answer 3

+5 A:

Your question is being answered well by others, but I want to point out that since this is C++, not C, all of this confusion could be avoided if you just used references instead of pointers. This is generally what you should do in a case like this (if NULL return values are not needed)

int ASPfile::get_dimensions(int& Lat, int& Lon, 
        vector<double>& Latgrid, vector<double>& Longrid) {

    Lat = get_latsize();
    Lon = get_lonsize();

    Latgrid = read_latgrid();
    Longrid = read_longrid();

    return 0;
}

// Call like this

int Latsize;
int Lonsize;
vector<double> Latgrid;
vector<double> Longrid;
int res = asp->get_dimensions(Latsize,Lonsize,Latgrid,Longrid);

Tyler McHenry 2009-08-24 21:07:31

Aw, you beat me to it by a minute! +1

sbi 2009-08-24 21:09:34

Answer 4

+3 A:

The function's signature taking pointers to me signals I might just as well pass in NULL pointers, if I don't have a fitting object at hand or if I am not interested in a particular result. Your function, however, doesn't check for this and happily writes to address 0.

If proper objects must be passed into the function, then why doesn't it take references instead of pointers? They are meant to be used for what you want to do, are easier to understand, and harder to get wrong.

int ASPfile::get_dimensions( int& Lat
                           , int& Lon
                           , vector<double>& Latgrid
                           , vector<double>& Longrid );

sbi 2009-08-24 21:09:04

Answer 5

A:

Agree with all said about pointers. As a rule of the thumb - don't use them unless you really have to. In the meantime, the pattern used here looks messy and ugly and the need of get_dimensions function is entirely obscure. You are adding unnecessary level of complexity.

Instead of this, I suspect making those get_ and read_ functions you already using in your prototype function public on ASPFile would be enough:

int Latsize = asp->get_latsize();
int Lonsize = asp->get_lonsize();
vector<double> Latgrid = asp->read_latgrid();
vector<double> Longrid = asp->read_longrid();

Just keep it simple.

blinnov.com 2009-08-25 00:57:58

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