Given the following source code:
#include <memory>
#include <iostream>
using namespace std;
struct concept
{
virtual void perform() = 0;
};
struct model : concept, enable_shared_from_this<model>
{
void perform() override {
cout << "my pointer is " << shared_from_this().get() << endl;
}
};
int main(int argc, const char * argv[])
{
// shared_ptr<concept> concept_ptr = make_shared<model>();
shared_ptr<concept> concept_ptr { new model };
concept_ptr->perform();
return 0;
}
Compiling under gcc, this code compiles and associates the internal weak_ptr with the address of model.
Under clang the code will not compile (error message included at the end)
If you replace the initialisation of concept_ptr with shared_ptr<concept> concept_ptr = make_shared<model>(); it will compile on both.
Which is correct?
edit:
My version of clang is the one that ships with Xcode:
$ clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix
edit2:
Just wanted to say thanks to everyone for contributing.
If you're interested, the reason I want to do this is that I want an opaque interface to an implementation with shared-handle semantics. Some implementations (async ones) require that callback objects ensure that the implementation object still exists (argues for shared_from_this and weak_ptr::lock). Other implementations do not require this. I wanted to avoid encumbering the concept (public interface) with the enable_shared_from_this<> base class, since that couples implementation with interface - a known evil.
In most cases, it's reasonable to use make_shared to create the implementation object. In rarer cases that require a custom destructor, the following seems portable:
auto concept_ptr = static_pointer_cast<concept>(shared_ptr<model> {
new model ,
[](model* self) {
// some_deletion_operation on self;
} });
appendix: error message on clang:
In file included from /Users/richardh/Documents/dev/Scratchpad/tryit/tryit/try2.cpp:1:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/memory:4013:35: error: no viable overloaded '='
__e->__weak_this_ = *this;
~~~~~~~~~~~~~~~~~ ^ ~~~~~
...etc...
