temporary object creation in copy constructor

I am reading about copy constru开发者_如何学Pythonctor in C++

It was mentioned that an object is passed usuing call by value and return by values constructs a temporary objects that are never seen by the user. My understanding is that when we call by val for example

myclass b;

void myfunc(myclass c) {}  //c(b) copy constructor is called.

Where temporary object is created?

Thanks!

The copy is created in the function scope of myfunc(). That is, c is in scope of the entire function, which includes (and is slightly larger than) the function body. The copy is destroyed when the function returns, i.e. at the semicolon of myfunc(b);.

In the case of pass-by-value, a copy is made into the argument (rather than the temporary), so in the particular code in your example, there will be two objects:

myclass b;
myfunc( b ); // b is copied to argument "c" by means of the copy constructor

In the case of return by value, things are a bit more complex, in the code:

type f() {
   type tmp;
   return tmp;     // copies into temporary (again, copy constructor)
}
int main() {
   type x = f();   // copies from temporary into x (copy constructor)
   x = f();        // copies from temporary into x (assignment operator)
}

there are in theory 3 objects of type type. The compiler will create space for a type object before calling f in a location that is defined by the calling convention (this will be where the temporary is created), then the function is called and tmp is created inside f, it gets copied to the temporary in the return statement. Finally the temporary is copied to the x variable in main.

In many cases, though, the compiler can optimize away the temporaries by carefully choosing the locations of the variables. Most compilers will be able to optimize 2 of the 3 objects away in the previous example.

For a longer description you can read these:

• http://definedbehavior.blogspot.com/2011/08/value-semantics-nrvo.html
• http://definedbehavior.blogspot.com/2011/08/value-semantics-copy-elision.html