如何正确初始化向量 <int> ＆vecRef？ 抛出错误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?

Answer 1

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)
{ }

Answer 2

A reference cannot reference nothing. Thus you have to initialize foo s 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.