musteresel's blog

Swapping C++ objects done right


tagged: c++

How to swap two objects? For example in a sorting function which is written as a template function so that it works for any type (which is comparable and swappable).

The naive approach to swap a and b of type T & is:

T temporary = a;
a = b;
b = temporary;

But this means that T has to be copy constructible and copy assignable. These are pretty tough requirements, as it forces T to have such a constructor and operator defined. Moreover it creates a probably superfluous copy. One can possibly mitigate this later issue by adding std::move and requiring move construction and move assignment, but this still extremely limits the implementation of type T.

Use the standard library! That’s a good thought. There’s std::swap after all, and it hopefully get’s its job done, no? Well, yes. It requires the type to be move assignable and move constructible, just as above. That’s because it’s (at least in the GNU C++ library) implemented exactly like above (factoring in the moves, of course). But nonetheless, the following is a not how one should swap two objects:

// Wrong!
std::swap(a, b);

This hard-codes std::swap as the function to use for swapping, and thus forces the type T to (correctly) implement the constructor and operator; even if they’re not needed or reasonably practicable.

C++ also has a powerful (and sometimes confusing) mechanism to allow multiple namespaces to be searched for a function: argument dependent lookup (ADL). This requires the function to be called via an “unqualified name” (e.g. foo instead of ::foo or nspace::foo) and results in the compiler searching for foo not only in the current scope but also in the namespaces in which the types of the arguments of the function are defined. Thus correct swapping is done like this:

// Correct!
using std::swap;
swap(a, b);

In most cases, this will work exactly like the previous (wrong) code snippet: The compiler will find std::swap which was brought into scope with using std::swap. The type has to implement the discussed constructor and operator. But if there’s a reasonable implementation to swap objects of the type T, then the developer implementing T can write a specialized swap function in the same namespace in which T is defined. Calling swap (an unqualified name) will then invoke ADL and find and use this specialized swap function:

namespace nspace {
  struct foo {};
  void swap(foo &, foo &);

// somewhere in the code
nspace::foo a = get_some_foo();
nspace::foo b = get_some_foo();
swap(a, b); // will call nspace::foo through ADL

Thus to correctly swap two objects one first brings std::swap into scope as a kind of “fallback” (in case there’s no specialized swap function) and then calls swap unqualified, which enables ADL.