CS253 Copy Elision

The word “elision” means “omission”, like how the two-syllable phrase “do not” becomes the one-syllable word “don’t”.

The Loud Class

The following examples use our class Loud, which displays a message for every method invoked.

#include "Loud.h"

int main() {
    Loud alpha;
}

Loud::Loud()
Loud::~Loud()

No surprise, here. alpha got created & destroyed.

#include "Loud.h"

int main() {
    Loud beta, gamma;
}

Loud::Loud()
Loud::Loud()
Loud::~Loud()
Loud::~Loud()

Sure.

#include "Loud.h"

int main() {
    Loud delta;
    Loud epsilon(delta);
    delta=epsilon;
}

Loud::Loud()
Loud::Loud(const Loud &)
Loud::operator=(const Loud &)
Loud::~Loud()
Loud::~Loud()

As expected.

#include "Loud.h"

Loud foo() {
    Loud zeta;
    return zeta;
}

int main() {
    Loud eta(foo());
}

Loud::Loud()
Loud::~Loud()

#include "Loud.h"

int main() {
    Loud theta = Loud(Loud(Loud(Loud(Loud(Loud(Loud(Loud())))))));
}

Loud::Loud()
Loud::~Loud()

There should be more than that.

#include "Loud.h"
Loud foo() {
    Loud iota;
    return iota;
}

int main() {
    Loud kappa(foo());
}

Loud::Loud()
Loud::~Loud()

The compiler is allowed (sometimes required) to elide (suppress, omit) ctors, copy ctors, and dtors, even if the methods have side effects, like the Loud class.
Another way to view it: the compiler can assume that ctors, copy ctors, and dtors only do creation/copying/destruction, even if they don’t.
This is efficient. Copying takes time & space.
There is no variable iota. Instead, the ctor in foo() constructs directly in kappa. No copying; it’s built in the right place.

For this to make sense, you have to understand how parameters & results are passed to & from functions.
How does an int get passed as an argument?
- On the stack, or in a register?
- A double *?
- A std::string?
- A std::string *?
- A std::string &?
- A const std::string &?
How does a a function return an int?
- A std::string?