Bulldozer00's Blog

Since I have the privilege of using C++11/14 on my current project, I’ve been using the new language idioms as fast as I can discover and learn them. For example, instead of writing risky, exception-unsafe, naked “new“, code like this:

I’ve been writing code like this instead:

By using std::unique_ptr instead of a naked pointer, I don’t have to veer away from the local code I’m writing to write matching delete statements in destructors or in catch() exception clauses to prevent inadvertent memory leaks.

I could’ve used a std::shared_ptr (which can be copied instead of “moved“) in place of the std::unique_ptr, but std::shared_ptr is required to maintain a fatter internal state in the form of strong and weak owner counters. Unless I really need shared ownership of a dynamically allocated object, which I haven’t so far, I stick to the slimmer and more performant std::unique_ptr.

When I first wrote the std::unique_ptr code above, I was concerned that using the std::move() function to transfer encapsulated memory ownership into the safeTgtList vector would add some runtime overhead to the code (relative to the C++98/03 style of simply copying the naked pointer into the scaryTgtList vector). It is, after all, a function, so I thought it must insert some code into my own code.

However, after digging a little deeper into my concern, I discovered (via Stroustrup, Sutter, and Meyers) that std::move() adds zero runtime overhead to the code. Its use is equivalent to performing a static_cast on its argument – which is evaluated at compile time.

As Scott Meyers stated at GoingNative13, std::move() doesn’t really move anything. It simply prepares for a subsequent real move by casting its argument from an lvalue to an rvalue – which is required for movement of an object’s innards. In the previous code, the move is actually performed within the std::vector::emplace_back() function.

Quoting Scott Meyers: “think of std::move() as an rvalue_cast“. I’m not sure why the ISO C++ committee didn’t define a new rvalue_cast keyword instead of std::move() to drive home the point that no runtime overhead is imposed, but I’d speculate that the issue was debated. Perhaps they thought rvalue_cast was too technical a term for most users?

Update 10/25/13

As I said early in the post, the code example is “like” the code I’ve been writing. The real code that triggered this post is as shown here:

Since each “entry” CfarDetState object must be uniquely intialized form it’s associated CfarCrossing object, I can’t simply insert estd::make_unique<CfarDetState>() into the emplace_back() function. All of the members  of each “entry” must be initialized first. Regardless of whether I use emplace_back() or push_back(), std::move(entry) must be used as the argument of the chosen function.