Passing by reference options in C++

Question

I want to pass an object of Class A (call it a) by reference (in the broad sense, i.e. either by A& or by A*) to the constructor of another Class B. I do not want 'a' to be modified inside B ('a' is read only and it is big in size which is why I want to pass it by reference). I know of two options:

1) Pass 'a' as

const A & a_

2) Pass 'a' as

const A * a_

The disadvantage of option 1 is that I can mistakenly pass an r-value. The disadvantage of option 2 is that I can mistakenly pass a null pointer.

My questions are: 1) Am I right about the disadvantages above? 2) Is there another recommended way of passing 'a' by reference?

I saw the answers in the link below. But I am curious if there are other options than what is posted in that question. Are there benefits of passing by pointer over passing by reference in C++?

How probable is it that you will mistakenly pass an r-value, and how severe will the consequences of that action be? — Emil Laine, Oct 29 '15 at 18:37
If you have your worries, and you know the issues, you need error checking. Pass by reference and and do try/catch. If you do it by a pointer, you'll modify the original instance of your object. — David Pulse, Oct 29 '15 at 18:38
What is a rvalue? Is it an undefined equality? Like a lvalue? — David Pulse, Oct 29 '15 at 18:39
@DavidPulse http://en.cppreference.com/w/cpp/language/value_category — Emil Laine, Oct 29 '15 at 18:39
Very helpful answers. @David Pulse: Is it true that even if I pass by const pointer I might still modify the original instance as you mentioned above in your first comment? — Ned_the_Dolphin, Oct 29 '15 at 19:06
@DavidPulse: He may be referring to `const_cast` by that previous comment. Riju: C++ gives programmers safety from accidents, but virtually every safety has backdoors, just in case. There is no 100% protection, but `const A*` is more than enough safety for what you're doing. — Mooing Duck, Oct 29 '15 at 20:33

score 18 · Accepted Answer · answered Oct 29 '15 at 18:41

18

Pass by const A &.

You can prevent accidental passing of rvalues by declaring an rvalue overload of that function = delete. For example:

struct A { };

void func(const A & a) { }
void func(A && a) = delete;

int main()
{
    A a;
    func(a);   // compiles
    func(A()); // doesn't compile
}

answered Oct 29 '15 at 18:41

Emil Laine

41,598
9
101
157

1

There is one tricky issue that I bumped into when dealing with this: if you really want to disable *all* rvalues, it's better to use `void func(const A&&) = delete;` instead. Looks weird and dirty, but it does the job. That's because otherwise legacy code that returns by `const` value (not recommended, but unfortunately frequently used in factory-like functions) will bind to `func(const &);` instead. Related: http://stackoverflow.com/q/33244951/3093378 – vsoftco Oct 31 '15 at 17:07

score 4 · Answer 2 · answered Oct 29 '15 at 18:41

Take a look at std::reference_wrapper's constructors: http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper/reference_wrapper

It has an extra deleted constructor that takes rvalue reference, so if you try to pass an rvalue to it, that constructor wins overload resolution and you get an error.

You can do the same thing: declare both const A& and A&& constructors, and delete the latter. Then nobody will be able to pass in an rvalue.

David Pulse · Answer 3 · 2015-10-29T22:17:15.923

-3

The code:

#include <iostream>
using namespace std;

struct A
{ };

int func_one(A& var) {
    A * insertVal=new A;
    if (&var==nullptr)
        return 0;
    else
        return 1;
}

int main() {
    A refVar;
    int x;
    x=func_one(refVar);

    return 1;
}

edited Oct 29 '15 at 22:17

answered Oct 29 '15 at 18:49

David Pulse

90
7

2

This doesn't compile. – Emil Laine Oct 29 '15 at 20:35
2

Try to compile it, see what errors you get, and fix those first. A code-only answer with invalid code isn't very helpful. – Emil Laine Oct 29 '15 at 20:40
1

`func_one` doesn't return a value on all control paths and leaks memory. `new` does not return `nullptr` if it fails to allocate. – Neil Kirk Oct 29 '15 at 21:31
Thanks @Neilkirk , You out of all know I don't fit the perspective of these moreso coders here. I'm just trying my hardest. – David Pulse Oct 29 '15 at 22:07
@neilkirk Thanks again, I'm not very good at C++ but thanks for not downvoting me. – David Pulse Oct 29 '15 at 22:08

Passing by reference options in C++

3 Answers3