How to properly Initialize vector<int> & vecRef? Throwing error C2758

Question

I have run into this situation where I need to constantly pass a vector around. The ideal idea is to pass it by reference so the program doesn't slow down from constantly copying the vector.

class Bar has the original vector.

Bar.h:

#ifndef Bar_h
#define Bar_h

#include <iostream>
#include <vector>

using namespace std;

class Bar{

public:

    Bar();
    vector<int> & getVec();

private:
    vector<int> vec;
};
#endif

Bar.cpp:

#include <iostream>
#include "Bar.h"

using namespace std;

Bar::Bar(){
    vec.push_back(1);
    vec.push_back(2);
    vec.push_back(3);
    vec.push_back(4);
    vec.push_back(5);
}

vector<int> & Bar::getVec(){
    return vec;
}

Class foo receives the vector.

foo.h:

#ifndef Foo_h
#define Foo_h

#include <iostream>
#include <vector>

using namespace std;

class Foo{

public:

    Foo();

private:
    vector<int> & vecRef;
};
#endif

foo.cpp:

#include <iostream>
#include "Foo.h"
#include "Bar.h"

using namespace std;

Foo::Foo(){

    Bar bar;



    vecRef = bar.getVec();
}

main:

#include <iostream>
#include "Foo.h"

using namespace std;

void main(){

    Foo foo();

}

The problem is that when compiling I get Error code C2758 which stats "a member of reference type must be initialized". It is referring to the foo class and vector<'int> & vecRef; not being initialized properly. My question is how would I go about properly initializing vecRef while keeping its declaration in foo.h? (i have done it successfully by declaring vecRef in foo.cpp, but that's not what i want if possible). OR is adding the '&' into vector<'int> & vecRef; not the way to go about this at all?

Foo(); constructor must initialize the reference vecRef in a member initializer list. It's not possible without Bar passed as a constructor parameter or being a member variable of Foo. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jul 29, 2015 at 15:31

Barry · Accepted Answer · 2015-07-29 15:35:21Z

0

References must be initialized in the member initializer list here:

Foo::Foo()
: vecRef(???) // <==
{ }

If a member variable isn't listed there, it will be default-initialized, and a reference cannot be default-initialized - hence the error. Furthermore, it's a good thing your code doesn't compile, because otherwise:

{
    Bar bar;
    vecRef = bar.getVec();
} // <== bar gets destroyed here
  // vecRef is now a dangling reference

What you likely want to do is pass a Bar into Foo's constructor, by reference, and then initialize vecRef like so:

Foo::Foo(Bar& bar)
: vecRef(bar.getVec())
{ }

Or just take the reference directly:

Foo::Foo(std::vector<int>& v)
: vecRef(v)
{ }

edited Jul 29, 2015 at 15:35

answered Jul 29, 2015 at 15:29

Barry

312k32 gold badges734 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Rob.drc Over a year ago

Thanks for your reply. I wasn't able to get your first suggestion working for what ever reason (I'm new to C++) but the 2nd one worked fine. Ill try to incorporate this method into my real program now. Thanks!

463035818_is_not_an_ai · Accepted Answer · 2015-07-29 15:46:49Z

0

A reference cannot reference nothing. Thus you have to initialize foos vecRef in the constructor. Actually this is explained already in detail in Barrys answer and I just wanted to point out the following:

Once you provide public access to a reference of a member variable, there is no point in making this member variable private. You could as well make vec itself public in Bar (and not need a getter method). It would have the same effect but with less superfluous typing needed from you.

answered Jul 29, 2015 at 15:46

463035818_is_not_an_ai

129k11 gold badges116 silver badges240 bronze badges

Collectives™ on Stack Overflow

How to properly Initialize vector<int> & vecRef? Throwing error C2758

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related