"Temporary object" warning - is it me or the compiler?

Question

The following snippet gives the warning:

[C++ Warning] foo.cpp(70): W8030 Temporary used for parameter '_Val' in call to 'std::vector<Base *,std::allocator<Base *> >::push_back(Base * const &)'

.. on the indicated line.

class Base
{
};

class Derived: public Base
{
public:
 Derived()   // << warning disappears if constructor is removed!
 {
 };
};

std::vector<Base*> list1;
list1.push_back(new Base);
list1.push_back(new Derived);  // << Warning on this line!

Compiler is Codegear C++Builder 2007.

Oddly, if the constructor for Derived is deleted, the warning goes away... Is it me or the compiler?

EDIT: The only way I've found to remove the warning is to something similar to this:

Derived * d;
list1.push_back(d = new Derived);  // << No warning now...

Answer 1

Simple try:

list1.push_back(new Derived());

I am afraid there is something about POD (with trivial constructors) vs non-POD going on here.

EDIT:

Given that the code compiles fine with gcc.3.4.2 (--pedantic) I would say it's a compiler quirk. I am leaning toward MarkB explanation, ie the compiler creating a temporary even though I don't understand why it would be required and then complaining when assigning it to the const&... but I'm still perplex.

Answer 2

Since list1 is a vector of Base*, the push_back function in the list is going to expect a parameter of type Base* const&, while your new is providing Derived*. In order to pass by reference (as needed in push_back) the compiler needs an actual object of the reference type, in this case Base*. There is an available implicit conversion from Derived* to Base* that the compiler uses to create a temporary object of type Base* to pass into push_back, and the compiler is warning you that it's creating this temporary.

The reason it works when you assign it to a variable is that there's no longer an implicit temporary needed: It can implicitly convert the named variable to Base* implicitly and pass that reference in.

I think you can silence this warning by telling push_back to treat the pointer as a Base*:

list1.push_back(static_cast<Base*>(new Derived));