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: http://stackoverflow.com/questions/7138588/c0x-auto-what-if-it-gets-a-constant-reference — Nate Kohl, Jan 21 '12 at 14:38

score 8 · Accepted Answer · answered Jan 21 '12 at 15:02

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 '12 at 15:02

fredoverflow

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why don't you write: const auto& v = a.getField(); ? – Guillaume Paris Jan 21 '12 at 15:08
Because the `const` is automatically inferred in this case. – fredoverflow Jan 21 '12 at 15:13
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`. – CB Bailey Jan 21 '12 at 15:39
what is a const which it is not qualified "top level" ? – Guillaume Paris Jan 21 '12 at 16:22
**Top-Level Const:** http://stackoverflow.com/questions/7914444/what-are-top-level-const-qualifiers – legends2k Jun 21 '13 at 07:13

score 0 · Answer 2 · answered Jan 21 '12 at 15:07

0

In the new C++11 standard I think the auto keyword used in this context is substituted by the compiler for whatever type a.getField() returns. It's a shortcut for the programmer.

See http://en.wikipedia.org/wiki/C%2B%2B11#Type_inference

answered Jan 21 '12 at 15:07

Jim Lahey

84
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auto and const object

2 Answers2