# Canonical Implementation of Move Semantics

I am trying to compose an illustrative example which shows how to implement move semantics on an object that will be stored in a vector.

Please consider the following code, which is my illustrative example so far. It is designed to be a canonical, pedantically correct implementation of an object that implements move semantics, does not implement copy semantics, and can be stored in a vector<> (Where T is Moveable, below). How did I do?

#include <string>
#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
#include <memory>
#include <iterator>
using namespace std;

class Moveable
{
public:
string foo_;
string bar_;

Moveable(Moveable&& rhs) : foo_(std::move(rhs.foo_)), bar_(std::move(rhs.bar_)) {}  // move construction
Moveable(const string& foo) : foo_(foo) {};             // convert construction
Moveable& operator=(Moveable&& rhs)     // move assignment
{
foo_ = std::move(rhs.foo_);
bar_ = std::move(rhs.bar_);
return * this;
}
private:
Moveable(const Moveable&);              // not defined, not copy-constructible
Moveable& operator=(const Moveable&);   // not defined, not copy-assignable
Moveable();                             // not defined, not default constructible
};

Moveable generate_it()
{
static string foo ;
if( foo.empty() || foo[0] == 'z' )
foo.insert(0, 1, 'a');
else
foo[0]++;
return foo;
}

int main()
{
typedef vector<Moveable> Moveables;
Moveables v;
generate_n(back_inserter(v), 1024, &generate_it);
cout << v.size() << endl;
string target = "zzz";
auto that = find_if(v.begin(), v.end(), [target](const Moveables::value_type& it) -> bool
{
return it.foo_ == target;
});

if( that != v.end() )
v.erase(that);

sort(v.begin(), v.end(), [](const Moveables::value_type& lhs, const Moveables::value_type& rhs) -> bool
{
return lhs.foo_ > rhs.foo_;
});
cout << v.size() << endl;
}

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## locked by Jamal♦Aug 9 at 23:58

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I had posted another question very similar to this, but deleted it and replaced it with this one. The other question had used unique_ptr which ultimately was orthogonal to what I was really going for. –  John Dibling Jun 22 '12 at 17:18

• One small error: Mark the move constructor as noexcept, otherwise there are situations where it won’t be used. The linked case is different since the class has a copy-constructor, yet I can imagine that there are still situations where it matters, especially since your copycon isn’t deleted, just private and undefined.

• Why is the destructor virtual? If the example is supposed to be minimal then this might be distracting.

• In generate_it:

return std::move(Moveable(foo));


The std::move here is redundant, since you are returning a temporary. What’s more, the explicit constructor call is redundant too, since the constructor isn’t marked as explicit. Just return foo; will do.

• The find_if call could be replaced by vanilla find, using temporary construction again:

auto that = find(v.begin(), v.end(), target);

• Finally, in the sort call, why aren’t the arguments declared const&? Granted, makes the line even longer as it stands this is inconsistent with the const-correctness illustrated in the find_if call.

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+1: various replies. 1) The destructor was an artifact of my testing durig development. Since its implementation was trivial, and the tor was public, I've removed it entirely. –  John Dibling Jun 26 '12 at 16:35
2) noexcept is not supported by MSVC10. Sad. –  John Dibling Jun 26 '12 at 16:38
3) Fixed generate_it 4) fixed sort call. These were simply part of the harness, but it's important that the whole thing is correct. –  John Dibling Jun 26 '12 at 16:40