Question

I have the following code (as much simplified as possible):

#include <vector>
#include <algorithm>

using std::vector;

enum class Foo {
    BAR, BAZ
};

void print_to_file(const vector<const vector<Foo> >& sequences) {
    std::for_each(sequences.begin(), sequences.end(), [](const vector<Foo>& sequence) {
    });
}

int main(int argc, char **argv) {
    return EXIT_SUCCESS;
}

when compiling with g++ (SUSE Linux) 4.7.1 20120723 [gcc-4_7-branch revision 189773] as g++ --std=c++11 test.cpp I get the following error message:

In file included from /usr/include/c++/4.7/x86_64-suse-linux/bits/c++allocator.h:34:0,
                 from /usr/include/c++/4.7/bits/allocator.h:48,
                 from /usr/include/c++/4.7/vector:62,
                 from test.cpp:1:
/usr/include/c++/4.7/ext/new_allocator.h: In instantiation of ‘struct __gnu_cxx::new_allocator<const std::vector<Foo> >’:
/usr/include/c++/4.7/bits/allocator.h:89:11:   required from ‘class std::allocator<const std::vector<Foo> >’
/usr/include/c++/4.7/bits/alloc_traits.h:89:43:   required from ‘struct std::allocator_traits<std::allocator<const std::vector<Foo> > >’
/usr/include/c++/4.7/ext/alloc_traits.h:109:10:   required from ‘struct __gnu_cxx::__alloc_traits<std::allocator<const std::vector<Foo> > >’
/usr/include/c++/4.7/bits/stl_vector.h:76:28:   required from ‘struct std::_Vector_base<const std::vector<Foo>, std::allocator<const std::vector<Foo> > >’
/usr/include/c++/4.7/bits/stl_vector.h:208:11:   required from ‘class std::vector<const std::vector<Foo> >’
test.cpp:11:25:   required from here
/usr/include/c++/4.7/ext/new_allocator.h:83:7: error: ‘const _Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::const_reference) const [with _Tp = const std::vector<Foo>; __gnu_cxx::new_allocator<_Tp>::const_pointer = const std::vector<Foo>*; __gnu_cxx::new_allocator<_Tp>::const_reference = const std::vector<Foo>&]’ cannot be overloaded
/usr/include/c++/4.7/ext/new_allocator.h:79:7: error: with ‘_Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::reference) const [with _Tp = const std::vector<Foo>; __gnu_cxx::new_allocator<_Tp>::pointer = const std::vector<Foo>*; __gnu_cxx::new_allocator<_Tp>::reference = const std::vector<Foo>&]’

the same code compiles fine with VS2012, but fails badly under g++ and I’ve not the slightest idea how to interpret the error message.

Answer 1

I’m surprised you don’t get problems elsewhere, as this vector<const vector<Foo> > is not a possible data type.

The value_type of a std::vector must at minimum be copy constructible (C++03) or movable (C++11). If it is const, you cannot assign or move it.