Copy-swap
The copy-swap idiom provides exception-safe copying. It requires that a correct copy ctor and swap are implemented.
struct String {
String(String const& other);
String& operator=(String copy) { // passed by value
copy.swap(*this); // nothrow swap
return *this; // old resources now in copy, released in its dtor
}
void swap(String& other) throw() {
using std::swap; // enable ADL, defaulting to std::swap
swap(data_members, other.data_members);
}
private:
Various data_members;
};
void swap(String& a, String& b) { // provide non-member for ADL
a.swap(b);
}
You can also implement the swap method with ADL (Argument Dependent Lookup) directly.
This idiom is important because it handles self-assignment[1], makes the strong exception guarantee[2], and is often very easy to write.
[1] Even though self-assignment isn't handled as efficiently as possible, it's supposed to be rare, so if it never happens, this is actually faster.
[2] If any exception is thrown, the state of the object (*this) is not modified.
