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POHandling smart pointers in stl container
 

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    <p>I've a class <code>Foo&lt;T&gt;</code> which has a vector of smart pointers to <code>Shape</code> derived classes. I'm trying to implement an <code>at(index)</code> member function. Here's what I would to do intuitively: </p> <pre><code>Foo&lt;float&gt; myfoo; std::unique_ptr&lt;Shape&lt;float&gt;&gt; shape_ptr = myfoo.at(i); shape_ptr-&gt;doSomething(param1, param2, ...); </code></pre> <p>When defining the <code>at(index)</code> function, I'm getting a compiler error message. Note that the move constructor was defined and that the Shape base class is abstract. Below, I'm giving some code for illustration purposes.</p> <p>Furthermore, I found recently on the web an example on how to overload the assignment operator using <code>std::move</code>. I usually follow the Copy-Swap idiom. Which of those two ways for overloading the mentioned operator makes sense for my case? Below, I'm also illustrating the function's definition.</p> <pre><code>template &lt; typename T &gt; class Foo{ public: Foo(); Foo( Foo &amp;&amp; ); ~Foo(); void swap(Foo&lt;T&gt; &amp;); //Foo&lt;T&gt; &amp; operator =( Foo&lt;T&gt; ); Foo&lt;T&gt; &amp; operator =( Foo&lt;T&gt; &amp;&amp; ); std::unique_ptr&lt;Shape&lt;T&gt; &gt; at ( int ) const; // error here! int size() const; private: std::vector&lt; std::unique_ptr&lt;Shape&lt;T&gt; &gt; &gt; m_Bank; }; template &lt; typename T &gt; Foo&lt;T&gt;::Foo( Foo &amp;&amp; other) :m_Bank(std::move(other.m_Bank)) { } /*template &lt; typename T &gt; void Filterbank&lt;T&gt;::swap(Filterbank&lt;T&gt; &amp; refBank ){ using std::swap; swap(m_Bank, refBank.m_Bank); } template &lt; typename T &gt; Foo&lt;T&gt; &amp; Filterbank&lt;T&gt;::operator =( Foo&lt;T&gt; bank ){ bank.swap(*this); return (*this); }*/ template &lt; typename T &gt; Foo&lt;T&gt; &amp; Foo&lt;T&gt;::operator =( Foo&lt;T&gt; &amp;&amp; bank ){ //bank.swap(*this); m_Bank = std::move(bank.m_Bank); return (*this); } template &lt; typename T &gt; std::unique_ptr&lt;Shape&lt;T&gt; &gt; Foo&lt;T&gt;::at( int index ) const{ return m_Bank[index]; // Error here! =&gt; error C2248: 'std::unique_ptr&lt;_Ty&gt;::unique_ptr' : cannot access private member declared in class 'std::unique_ptr&lt;_Ty&gt;' } </code></pre>
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    1. USHoward Hinnant
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    1. COUnique pointers have **unique ownership semantics**, so it makes no sense to make a copy of one. What do you want to happen? Do you want a deep copy of the object?
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      1. USKerrek SB
    2. CO@KerrekSB, no, i don't want a deep copy of the object. The idea is that i first fill the `Shape` m_Bank vector. Once it's filled, I just want to call the corresponding `doSomething` function (e.g triangle->doSomething(), square->doSomething), sth. like: `myfoo.m_Bank.at(i)->doSomething(param1, param2, ...);` but the m_Bank is a private class member, therefore, I need a member function that allows me to access the element at index `i`
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    3. COSuch an interface would be extremely surprising (i.e. "bad"), because you could only ever call `at(i)` *once*, with `std::move`, and it would destroy the vector's element value. Do you want a shared pointer instead? If you really just want the vector as a staging area, then I think you need to reconsider the design.
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