Copy elision

In C++ computer programming, copy elision refers to a compiler optimization technique that eliminates unnecessary copying of objects. The C++ language standard generally allows implementations to perform any optimization, provided the resulting program's observable behavior is the same as if, i.e. pretending, the program was executed exactly as mandated by the standard.

The standard also describes a few situations where copying can be eliminated even if this would alter the program's behavior, the most common being the return value optimization. Another widely implemented optimization, described in the C++ standard, is when a temporary object of class type is copied to an object of the same type.^[1] As a result, copy-initialization is usually equivalent to direct-initialization in terms of performance, but not in semantics; copy-initialization still requires an accessible copy constructor.^[2] The optimization can not be applied to a temporary object that has been bound to a reference. Example:

#include <iostream>

int n = 0;
struct C {
  explicit C(int) {}
  C(const C&) { ++n; } // the copy constructor has a visible side effect
};                     // it modifies an object with static storage duration

int main() {
  C c1(42); // direct-initialization, calls C::C(42)
  C c2 = C(42); // copy-initialization, calls C::C( C(42) )
  
  std::cout << n << std::endl; // prints 0 if the copy was elided, 1 otherwise
  return 0;
}

According to the standard a similar optimization may be applied to objects being thrown and caught,^[3][4] but it is unclear whether the optimization applies to both the copy from the thrown object to the exception object, and the copy from the exception object to the object declared in the exception-declaration of the catch clause. It is also unclear whether this optimization only applies to temporary objects, or named objects as well.^[5] Given the following source code:

#include <iostream>

struct C {
  C() {}
  C(const C&) { std::cout << "Hello World!\n"; }
};

void f() {
  C c;
  throw c; // copying the named object c into the exception object.
}          // It is unclear whether this copy may be elided.

int main() {
  try {
    f();
  }
  catch(C c) {  // copying the exception object into the temporary in the exception declaration.
  }             // It is also unclear whether this copy may be elided.
}

A conforming compiler should therefore produce a program which prints "Hello World!" twice. In the current revision of the C++ standard (C++11), the issues have been addressed, essentially allowing both the copy from the named object to the exception object, and the copy into the object declared in the exception handler to be elided.^[5]

GCC provides the -fno-elide-constructors option to disable copy-elision. This option is useful to observe (or not observe!) the effects of Return Value Optimization or other optimizations where copies are elided. It is generally not recommended to disable this important optimization.

References

1. ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §12.8 Copying class objects [class.copy] para. 15
2. Sutter, Herb (2001). More Exceptional C++. Addison-Wesley.
3. ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §15.1 Throwing an exception [except.throw] para. 5
4. ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §15.3 Handling an exception [except.handle] para. 17
5. "C++ Standard Core Language Defect Reports". WG21. Retrieved 2009-03-27.