I did not understand the meaning of this argument default shared_ptr?

Question

Take a look at this:

#include <iostream>
#include <memory>

using Foo = int;
using FooPtr = std::shared_ptr<Foo>;

FooPtr makeFoo()
{
    FooPtr f{
        new Foo(),
        [](Foo* ptr) {
            delete ptr;

            std::cerr << "!\n";
        }
    };

    return f;
}

void bar(FooPtr p = {})
{
    p = makeFoo();
}

int main()
{
    bar();
}

// Expected output: '!'
// Failure case: no output (deleter not invoked?)

I expected the debugger to shared_ptrbe called when it bar()returns, and on my 64-bit CentOS 7 system using GCC 4.8.5 this will happen.

However, on my 32-bit CentOS 6 system using GCC 4.8.2 under devtoolset-2 (also I think in gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_linux, in my Raspberry Pi dashboard), this is not the case.

Taking a look at the code, and given the experimental nature of C ++ 11 in 4.8, this smells like a compiler error. But I could also fall into the UB trap somewhere (or just don't understand at all how this should work).

Who's guilty? And how to fix it?

---

Works for

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-initfini-array --disable-libgcj --with-isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/cloog-install --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.5 20150623 (Red Hat 4.8.5-16) (GCC)

Does not work

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/opt/rh/devtoolset-2/root/usr/libexec/gcc/i686-redhat-linux/4.8.2/lto-wrapper
Target: i686-redhat-linux
Configured with: ../configure --prefix=/opt/rh/devtoolset-2/root/usr --mandir=/opt/rh/devtoolset-2/root/usr/share/man --infodir=/opt/rh/devtoolset-2/root/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --enable-languages=c,c++,fortran,lto --enable-plugin --with-linker-hash-style=gnu --enable-initfini-array --disable-libgcj --with-isl=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-i686-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-i686-redhat-linux/cloog-install --with-mpc=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-i686-redhat-linux/mpc-install --with-tune=generic --with-arch=i686 --build=i686-redhat-linux
Thread model: posix
gcc version 4.8.2 20140120 (Red Hat 4.8.2-15) (GCC)

Answer 1

, .

, , , GCC libstd++, 60367, , , , GCC 4.8.5.

= {} = FooPtr{} .

---

, 7.2 "8.0" , ( !).

Answer 2

bar, , bar.

[expr.call]/4 ( ++ 17),

, (11.3.5) (11.6, 15.8, 15.1) . [...] , , , , . [...]

, p FooPtr , MakeFoo, ( , ) bar bar main.