C++ constructor: garbage while initialization of const reference

Question

what is wrong with this code, why do I get wrong answer:

class X
{
private:
        const int a;
        const int& b;
public:
        X(): a(10) , b(20)
        {
        //      std::cout << "constructor : a " << a << std::endl;
        //      std::cout << "constructor : b " << b << std::endl;
        }

        void display()
        {
            std::cout << "display():a:" << a << std::endl;
            std::cout << "display():b:" << b << std::endl;

        }
};


int
main(void)
{
        X x;
        x.display();
return 0;
}

The above code will give me the result as

display():a:10
display():b:1104441332

But If I remove the commented 2 lines inside the default constructor it gives me proper result which is

constructor : a 10
constructor : b 20
display():a:10
display():b:20

please help, Thank you

score 24 · Accepted Answer · edited May 23 '17 at 10:27

You are initializing b as a reference to a temporary.

The value 20 is created and exists only for the scope of the constructor.

The behavior of the code after this is very interesting - on my machine, I get different values from the ones you posted, but the fundamental behavior is still nondeterministic.

This is because when the value to which the reference points falls out of scope, it begins to reference garbage memory instead, giving unpredictable behavior.

See Does a const reference prolong the life of a temporary?; the answer https://stackoverflow.com/a/2784304/383402 links to the relevant section of the C++ standard, specifically the below text:

A temporary bound to a reference member in a constructor’s ctor-initializer
(12.6.2) persists until the constructor exits.

This is why you always get the right value in the print within the constructor, and rarely (but possibly sometimes!) after. When the constructor exits, the reference dangles and all bets are off.

thanks for your answer. that explains the results. but may I know how to initialize the b ? — Vivek Basappa, Feb 04 '12 at 07:16
@VivekBasappa: have it reference some variable, like you intend it to. Or, just make it a value like `a`. — André Caron, Feb 04 '12 at 07:18
@VivekBasappa It's an `int`, so you probably don't want it to be a reference anyhow. 'snot like copying an `int` is expensive. — Borealid, Feb 04 '12 at 07:18

score 19 · Answer 2 · answered Feb 04 '12 at 07:02

19

I'll let my compiler answer this one:

$ g++ -std=c++98 -Wall -Wextra -pedantic test.cpp
test.cpp: In constructor 'X::X()':
test.cpp:9:26: warning: a temporary bound to 'X::b' only persists until the constructor exits [-Wextra]
$

You should turn on the warnings on your compiler as well.

answered Feb 04 '12 at 07:02

R. Martinho Fernandes

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Thanks for the answer, can you explain me why b is a temporary initialization? I am trying to initialize const values through initialization list – Vivek Basappa Feb 04 '12 at 07:11
1

@VivekBasappa: `X::b` is a const reference, not a const value. – Cat Plus Plus Feb 04 '12 at 07:16
1

@VivekBasappa The `const` isn't the problem, the problem is that it's a *reference* that has nothing to reference. – David Schwartz Feb 04 '12 at 07:21

justin · Answer 3 · 2012-02-04T07:21:01.140

4

b refers to a temporary. What you have read (when printing) is an invalid location by the time it is read since the temporary 20 has technically gone out of scope.

To explain inconsistent results:

It is undefined behavior. What you see may be different if you:

change your compiler
change your compiler settings
build for another architecture
change your class' member layout
add or remove things from the memory region near the instance of x
etc.

You should always always avoid undefined behavior.

But why would the value change? Your reference likely refers to a stack address which has been rewritten (e.g. reused) by the time it's printed.

edited Feb 04 '12 at 07:21

answered Feb 04 '12 at 06:58

justin

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I honestly can't make sense of the final "as for why it is different..." part. I'd appreciate it if you could point out exactly what you are talking about. Even the first "as for why:" isn't all that clear, although what follows after the ':' does make sense. – batbrat Feb 04 '12 at 07:06

Michael Burr · Answer 4 · 2012-02-04T07:04:48.230

You're binding the const& to a temporary, which doesn't live beyond the call to the constructor. The C++03 standard specfically says "a temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits" (12.2/5 "Temporary objects").

So your code has undefined behavior - you might get nonsense, or something that appears to be 'working'.

FWIW, MSVC 2010 gives the following warning on that code:

C:\temp\test.cpp(12) : warning C4413: 'X::b' : reference member is initialized to a temporary that doesn't persist after the constructor exits

C++ constructor: garbage while initialization of const reference

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