prolonging the lifetime of temporaries

Question

What is the design rationale behind allowing this

const Foo& a = function_returning_Foo_by_value();

but not this

Foo&am         


        

Answer 1

The reason that non-const pointers don't prolong the lifetime of temporaries is that non-const references can't be bound to temporaries in the first place.

There are LOTS of reasons for that, I'll just show one classic example involving implicit widening conversions:

struct Foo {};
bool CreateFoo( Foo*& result ) { result = new Foo(); return true; }

struct SpecialFoo : Foo {};
SpecialFoo* p;
if (CreateFoo(p)) { /* DUDE, WHERE'S MY OBJECT! */ }

The rationale for allowing const references to bind temporaries is that it enables perfectly reasonable code like this:

bool validate_the_cat(const string&);

string thing[3];
validate_the_cat(thing[1] + thing[2]);

Note that no lifetime extension was needed in this case.