Exception and Copy Constructor : C++

Question

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

I run below code:

#include 

struct C {
  C() {}
  C(const C&) { std::cout         


        

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).

Answer 2

Copy & Move constructor while throwing user-defined type object

struct demo
{
    demo() = default;
    demo(demo &&) = delete;
    demo(const demo &) = delete;
};

int main()
{
    throw demo{};
    return 0;
}

• Upon throw expression, a copy of the exception object always needs to be created as the original object goes out of the scope during the stack unwinding process.
• During that initialization, we may expect copy elision (see this) – omits copy or move constructors (object constructed directly into the storage of the target object).
• But even though copy elision may or may not be applied you should provide proper copy constructor and/or move constructor which is what C++ standard mandates(see 15.1). See below compilation error for reference.

error: call to deleted constructor of 'demo'
    throw demo{};
          ^~~~~~
note: 'demo' has been explicitly marked deleted here
    demo(demo &&) = delete;
    ^
1 error generated.
compiler exit status 1

• If we catch an exception by value, we may also expect copy elision(compilers are permitted to do so, but it is not mandatory). The exception object is an lvalue argument when initializing catch clause parameters.

From: 7 best practices for exception handling in C++