Cannot convert from T to T&

[JUANDENT]

Cannot convert from T to T& as in the following:

auto f(const map<string, int>& mp)
{
	auto p = find_if(mp.begin(),mp.end(), Greater_than{ 42 });
	return p;
}

void usePredicates()
{
	map<string, int> myMap
	{
		{"Jose", 30},
		{ "Javier", 62}
	};
	auto& pointer = f(myMap);  // ERROR cannot convert from '_InIt' to '_InIt&'
}

'
But when I change 'pointer' type to auto&& the error goes away!

Why?



Is there any harm in using auto&&? does f return an rvalue? is it a temporary object?

[seeplus]

pointer is a ref and you can't assign the temp value from f to a ref. && will work when the return value is either a ref or a temp.

[Peter87]

It would have worked if you used const auto& on line 14.

auto& can sometimes deduce to a const reference:

const int ci = 5;
auto& r = ci; // OK 

But for literals and other rvalues it doesn't seem to be allowed:

auto& r = 5; // Error 

[JUANDENT]

Is there any harm in using auto&&? does f return an rvalue? is it a temporary object? is it destroyed upon line ending?

[Peter87]

Is there any harm in using auto&&?

No, I don't think so.

does f return an rvalue? is it a temporary object?

Yes.

is it destroyed upon line ending?

No, references that bind directly to temporaries extend the lifetime of the temporary so that it is not destroyed until the reference goes out of scope. This is true for all reference types (assuming the code compiles).

If the function had returned a reference the answer would have been different.