C++ Creating new objects in functions [duplicate]

Question

I am at a loss as to why I cannot

Declare an object pointer set to 0
Pass the object pointer to a function that will create the object
The function sets the passed pointer to the newly allocated object

Example

#include <iostream>

using namespace std;

class Unit {
  public:
  Unit();
  ~Unit();
  string name;
};

Unit::Unit(){}
Unit::~Unit(){}

void DoFoo(Unit *unit);

int main()
{
    Unit *unit = 0;
    DoFoo(unit);
    cout << unit->name;

    return 0;
}

void DoFoo(Unit *unit){
    unit = new Unit();
    unit->name = "hi";
}

Result is Segment fault...

In my head I am simply declaring a pointer, passing the address to the function, and then the function is setting the address to the address of the allocated object...

Because there's nothing special about pointers. It's exactly the same as void foo(int x) { x = 12; } int main() { int y = 0; foo(y); /* y is still 0 */}. — molbdnilo
– molbdnilo, Commented Aug 7, 2018 at 13:17
Or return an object - thereby avoiding pointers and this kind of trouble — doctorlove
– doctorlove, Commented Aug 7, 2018 at 13:21
You did "passing the address to the function" - you pass zero, which gets copied, by value. And then set to something else inside the function. Meanwhile, but at the call site... things haven't changed. So unit->name is sitll 0->name i.e. goes bang — doctorlove
– doctorlove, Commented Aug 7, 2018 at 13:25
Missing a delete-statement. Try using std::shared_ptr and std::unique_ptr instead of new. That's the modern C++ approach. — JHBonarius
– JHBonarius, Commented Aug 7, 2018 at 13:27

Lex Sergeev · Accepted Answer · 2018-08-07 13:19:17Z

4

You pass a copy of the pointer and allocate an object there. Use a references:

void DoFoo(Unit *&unit);

void DoFoo(Unit *&unit){
    unit = new Unit();
    unit->name = "hi";
}

answered Aug 7, 2018 at 13:19

Lex Sergeev

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