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CS253 Copy Elision

Elision

The word “elision” means “omission”, like how the two-syllable “do not” becomes the one-syllable “don’t”, or Captain Kirk never seems to brush his teeth. It’s not necessary to show everything.

The Loud Class

The following examples use my 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.

More Construction

#include "Loud.h"

int main() {
    Loud beta, gamma;
}

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

Sure.

Copy ctor & assignment

#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.

More copying

#include "Loud.h"

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

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

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

• Hey, wait. Where’s the copy ctor?
• Two distinct objects, zeta and eta, should have been created.
• They weren’t.

Ludicrous copying

#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.

Elision

#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.

Parameter Passing

• 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?

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