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.