Initializing reference member variable with literal

Question

In the following code , I am initializing a reference variable with a literal.

class ABC { public: const int& a; ABC():a(43) { } void newfoo() { printf("NEWFOO %d",a); } }; int main() { ABC obj; obj.newfoo(); }

The output of this program is NEWFOO 32767 which seems illogical when I know that the following code works just fine.

int main() { const int& b=3; printf("%d",b); }

What is happening here ? If compiler declares some temp variable during initializing of the reference variable , then isn't the scope of that variable will be inside main since the class is in global scope ?

Don't initialize member references with literals. I believe it is undefined behavior. — Basile Starynkevitch
– Basile Starynkevitch, Commented Jan 31, 2014 at 13:41
Why was it standardized this way? It doesn't make sense to let this compile. — Karoly Horvath
– Karoly Horvath, Commented Jan 31, 2014 at 13:55

Shafik Yaghmour · Accepted Answer · 2014-01-31 14:17:22Z

Well clang produces the following warning for this code even without any flags (see it live):

warning: binding reference member 'a' to a temporary value [-Wdangling-field] ABC():a(43) { } ^~

gcc on the other hand requires either -Wall or -Wextra.

and if we check out this reference initialization reference it says:

a temporary bound to a reference member in a constructor initializer list persists only until the constructor exits, not as long as the object exists.

This can be found in the draft C++ standard section 12.2 Temporary objects paragraph 5 which includes the following bullet

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

Vaughn Cato · Accepted Answer · 2014-01-31 14:14:33Z

A temporary will be created and the reference is bound to that temporary (C++03 8.5.3.5). The temporary will be destroyed at the end of the constructor call, leaving a dangling reference. This is specified in C++03 12.2.5:

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

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