C++11 stateful allocators with std::basic_string<> g++ 6.3.0

Question

I'm trying to get C++11 allocators working with std::basic_string<>. My code looks something like this (this is a minimal example). The problem I'm having is that it works on Xcode, and something similar works on Visual Studio, but I can't get it to compile on g++. I'm using g++ 6.3.0 and I've tried with -D_GLIBCXX_USE_CXX11_ABI=1 and -D_GLIBCXX_USE_CXX11_ABI=0

#include <stdio.h>
#include <iostream>
#include <string>

template <class TYPE> class my_allocator
    {
    public:
        int instance;

    public:
        using value_type = TYPE;

        my_allocator(int val) : instance(val) { }
        my_allocator(const my_allocator<TYPE> &other) : instance(other.instance) { }

        bool operator==(const my_allocator<TYPE> &that) const { return instance == that.instance; }
        bool operator!=(const my_allocator<TYPE> &that) const { return instance != that.instance; }

        TYPE *allocate(const size_t number)
            {
            if (number == 0)
                return nullptr;

            if (number > (size_t)-1 / sizeof(TYPE))
                throw std::bad_array_new_length();

            TYPE *pointer = (TYPE *)::malloc(number * sizeof(TYPE));

            if (pointer == nullptr)
                throw std::bad_alloc();

            return pointer;
            }

        void deallocate(TYPE * const pointer, size_t number) const { free(pointer); }
    };

typedef std::basic_string<char, std::char_traits<char>, my_allocator<char>>my_string;

int main(void)
    {
    my_allocator<char> allocator(1);

    my_string str(allocator);
    str = "one";
    std::cout << str;

    return 0;
    }

What I don't understand is that the error starts:

In file included from /usr/include/c++/6/string:52:0,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from broken.cpp:6:
/usr/include/c++/6/bits/basic_string.h: In instantiation of ‘class std::basic_string<char, std::char_traits<char>, my_allocator<char> >’:
broken.cpp:49:25:   required from here
/usr/include/c++/6/bits/basic_string.h:2616:63: error: no class template named ‘rebind’ in ‘class my_allocator<char>’
       typedef typename _Alloc::template rebind<_CharT>::other _CharT_alloc_type;
                                                               ^~~~~~~~~~~~~~~~~

Which states that basic_string.h line 2616 is being compiled - but how can that be if compiled with the flag -D_GLIBCXX_USE_CXX11_ABI=1 when one of the first lines of basic_string.h checks _GLIBCXX_USE_CXX11_ABI and does not include line 2616 in the build phase if its true?

I build with a shell script:

g++-6 -std=c++11 -Wall -D_GLIBCXX_USE_CXX11_ABI=1 broken.cpp -o broken

The version of Ubuntu I'm using is:

Distributor ID: Ubuntu
Description:    Ubuntu 14.04.5 LTS
Release:    14.04
Codename:   trusty

[EDIT: Everything below this line]

g++-6 -v gives:

Using built-in specs.
COLLECT_GCC=g++-6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 6.3.0-18ubuntu2~14.04' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=gcc4-compatible --disable-libstdcxx-dual-abi --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170519 (Ubuntu/Linaro 6.3.0-18ubuntu2~14.04)

This is a fresh install of Ubuntu with only cmake, g++6 and git installed using sudo add-apt-repository ppa:ubuntu-toolchain-r/test and apt-get. I get the same problem on Travis CI which is why'm trying to do it locally on a fresh Ubuntu install.

Any help is appreciated.

Hey Andrew! I think you forgot a struct called rebind in your allocator class. Did you see this [question](https://stackoverflow.com/questions/12362363/why-is-allocatorrebind-necessary-when-we-have-template-template-parameters) — JTejedor, Jul 08 '17 at 07:54
@JTejedor You don't need `rebind` in C++11 if the default one from `allocator_traits` does the job. — T.C., Jul 08 '17 at 08:01
I can't reproduce this on Wandbox's GCC 6. What does `g++-6 -v` say for you? — T.C., Jul 08 '17 at 08:03
Thanks - I'll go looking for a g++ with the dial ABI and install that. — Andrew Trotman, Jul 10 '17 at 03:15

score 0 · Answer 1 · answered Jul 08 '17 at 08:12

There are several things an allocator has to implement to be usable in all contexts of the STL:

What you already have:

value_type to specify what kind of objects are allocated
allocate and deallocate
== and != to check if storage allocated through one allocator can be deallocated through the other.
Copy-constructor

What was implicitly generated:

copy-assignment operator
Move constructor, move-assignment operator

What is missing:

The rebind constructor: something like
```
template<typename OTHER_TYPE>
my_allocator(const my_allocator<OTHER_TYPE> &other) ...
```
to allow the STL to use custom node types to store your elements, while you only provided an allocator for the element type, e.g. in a set or similar data structures

the rebind struct itself is optional if a rebind constructor is provided and your template arguments are sufficiently 'simple' (see the Allocator concept and std::allocator_traits)

C++11 stateful allocators with std::basic_string<> g++ 6.3.0

1 Answers1