auto and const object

Question

#include <iostream> #include <boost/shared_ptr.hpp> using namespace std; class A { public: const shared_ptr<const int> getField () const { return field_; } private: shared_ptr<int> field_; }; void f(const A& a) { auto v = a.getField(); //why auto doesn't a const shared_ptr<const int> here ? v.reset(); //OK: no compile error } int main() { A a; f(a); std::cin.ignore(); }

In the above code why does the compiler deduce v's type as shared_ptr<int> and not as const shared_ptr<const int> the type returned by getField?

EDIT: MSVC2010

related: stackoverflow.com/questions/7138588/…

Nate Kohl
– Nate Kohl

2012-01-21 14:38:54 +00:00
Commented Jan 21, 2012 at 14:38 — Nate Kohl
– Nate Kohl, Commented Jan 21, 2012 at 14:38

fredoverflow · Accepted Answer · 2012-01-21 15:02:12Z

8

auto ignores references and top level consts. If you want them back, you have to say so:

const auto v = a.getField();

Note that getField returns a copy of field_. Are you sure you dont want a reference to const?

const shared_ptr<int>& getField () const; auto& v = a.getField();

answered Jan 21, 2012 at 15:02

fredoverflow

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5 Comments

Guillaume Paris Over a year ago

why don't you write: const auto& v = a.getField(); ?

fredoverflow Over a year ago

Because the const is automatically inferred in this case.

Lara Bailey Over a year ago

That could probably benefit from some clarification. Superificially if I take const int&, ignore the reference (const int) and the now top level const I get int.

Guillaume Paris Over a year ago

what is a const which it is not qualified "top level" ?

legends2k Over a year ago

Top-Level Const: stackoverflow.com/questions/7914444/…

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