I have this piece of code:
#include <iostream>
#include <vector>
using namespace std;
class Foo{
public:
Foo() noexcept {cout << "ctor" << endl;}
Foo(const Foo&) noexcept {cout << "copy ctor" << endl;}
Foo(Foo&&) noexcept {cout << "move ctor" << endl;}
Foo& operator=(Foo&&) noexcept {cout << "move assn" << endl; return *this;}
Foo& operator=(const Foo&) noexcept {cout << "copy assn" << endl; return *this;}
~Foo() noexcept {cout << "dtor" << endl;}
};
int main()
{
Foo foo;
vector<Foo> v;
v.push_back(std::move(foo));
// comment the above 2 lines and replace by
// vector<Foo> v{std::move(foo)};
}
The output is what I expect (compiled with g++ -std=c++11 --no-elide-constructors
, same output without the flag)
ctor
move ctor
dtor
dtor
Now instead of using push_back
initialize directly the vector v
as
vector<Foo> v{std::move(foo)};
I do not understand why I get the outputs:
1) (without --no-elide-constructors
)
ctor
move ctor
copy ctor
dtor
dtor
dtor
2) (with --no-elide-constructors
)
ctor
move ctor
move ctor
copy ctor
dtor
dtor
dtor
dtor
In the first case, why is the copy ctor invoked? And in the second case, when the compiler does not perform elision, I have absolutely no idea why the move ctor is invoked twice. Any ideas?
Best Answer
Here you're calling the vector constructor that takes an
std::initializer_list
. An initializer list only allowsconst
access to its elements, so thevector
is going to have to copy each element from theinitializer_list
to its own storage. That's what causes the call to the copy constructor.From ยง8.5.4/5 [dcl.init.list]
See also https://tristanbrindle.com/posts/beware-copies-initializer-list
As for the extra move constructor call with
-fno-elide-constructors
, this was discussed in another answer a couple of days ago. It seems as though g++ takes a very literal approach to the example implementation of aninitializer_list
shown in the standard in same section I've quoted above.The same example, when compiled using clang, doesn't produce the extra move constructor call.