problems with retrieving the value of a class pointer object member allocated with operator new inside a function

Leonardo Miquelutti Published at Dev

Leonardo Miquelutti

I'm having problem with the following code. I try to fill in a member of an object called station inside a function, but I cannot retrieve it in main().

This is my header foo.h

class DirectoryProperties
{
public:
    size_t      numberOfFolders;

    void        initialize_stations( StationBase *station );

private:
    void        fill_station_names( StationBase *station );
};

class StationInfo
{
public:
    std::string     name;
};

And this is my foo.cpp

#include "foo.h"

void DirectoryProperties::fill_station_names( StationBase *station )
{
    station[0].name = "dummy";
}

void DirectoryProperties::initialize_stations( StationBase *station )
{
    size_t N = 1;

    station = new StationBase[ N ];

    this->fill_station_names( station );

    // this works
    std::cout << station[0].stationName << std::endl;
}

int main()
{
    DirectoryProperties dirInfo;

    StationBase *station = NULL;

    dirInfo.initialize_stations( station );

    // breaks here
    std::cout << station[0].stationName << std::endl;

    return 0;
}

So, I can correcly print station[0].name inside DirectoryProperties::initialize_stations( StationBase *station ), but no in main().

It also breaks if I try

int main()
{
    DirectoryProperties dirInfo;

    StationBase *station = NULL;

    dirInfo.initialize_stations( station );

    // breaks here
    station[0].stationName = "dummy";

    return 0;
}

So I assume that object pointer station has no memory allocated in main.

juanchopanza

station is a local variable in initialize_stations, so no changes to it have an effect outside of the function.

An immediate fix is to have the function return a pointer to the newed array.

StationBase* DirectoryProperties::initialize_stations(  )
{
  ....
  StationBase* station = new StationBase[ N ];
  ....
  return station;
}

A better solution is to return an std::vector<StationBase>. Or just a single StationBase, since you're only allocating one object anyway.

std::vector<stationBase> DirectoryProperties::initialize_stations()
{
    size_t N = 1;
    std::vector<stationBase> station(N);
    fill_station_names( station );
    return station;
}

and fix fill_station_names accordingly.

Collected from the Internet

Please contact [email protected] to delete if infringement.

edited at2021-02-17

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