Exception and Copy Constructor : C++

Question

Referring to http://en.wikipedia.org/wiki/Copy_elision

I run below 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.
}

The Output I got:

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ make clean
rm -f Trial.exe Trial.o

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ make
g++ -Wall Trial.cpp -o Trial

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ ./Trial
Hello World!
Hello World!

I understand that the compiler might have optimized the code with unnecessary copying, which it is not doing here.

But What I want to ask, How does two calls to the copy constructor is being made?

catch(C c) - Since we are passing by value, hence here the copy constructor is being called.

But at throw c how is copy constructor being called? Can someone explain?

Answer 1

throw c;     

Creates a temporary object and it is this temporary object that is thrown. The creation of the temporary might be through copy/move constructor. And yes this copy/move can be elided.

---

References:
C++11 15.1 Throwing an exception

§3:

A throw-expression initializes a temporary object, called the exception object, the type of which is determined by removing any top-level cv-qualifiers from the static type of the operand of throw and adjusting the type.........

§5:

When the thrown object is a class object, the copy/move constructor and the destructor shall be accessible, even if the copy/move operation is elided (12.8).