Quick Q: What are copy elision and return value optimization?

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Quick A: they are common optimizations that a compiler can do behind the scenes to avoid copying in certain cases.

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What are copy elision and return value optimization?

Copy elision is an optimization implemented by most compilers to prevent extra (potentially expensive) copies in certain situations. It makes returning by value or pass-by-value feasible in practice (restrictions apply).

It's the only form of optimization that elides (ha!) the as-if rule - copy elision can be applied even if copying/moving the object has side-effects.

The following example taken from Wikipedia:

struct C {
  C() {}
  C(const C&) { std::cout << "A copy was made.\n"; }
};

C f() {
  return C();
}

int main() {
  std::cout << "Hello World!\n";
  C obj = f();
}

Depending on the compiler & settings, the following outputs are all valid:

Hello World!
A copy was made.
A copy was made.
Hello World!
A copy was made.
Hello World!

This also means fewer objects can be created, so you also can't rely on a specific number of destructors being called. You shouldn't have critical logic inside copy/move-constructors or destructors, as you can't rely on them being called.

If a call to a copy or move constructor is elided, that constructor must still exist and must be accessible. This ensures that copy elision does not allow copying objects which are not normally copyable, e.g. because they have a private or deleted copy/move constructor.

C++17: As of C++17, Copy Elision is guaranteed when an object is returned directly:

struct C {
  C() {}
  C(const C&) { std::cout << "A copy was made.\n"; }
};

C f() {
  return C(); //Definitely performs copy elision
}
C g() {
    C c;
    return c; //Maybe performs copy elision
}

int main() {
  std::cout << "Hello World!\n";
  C obj = f(); //Copy constructor isn't called
}

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