c++ - trivial allocator aware container? -

i studying/playing allocators trying understand how works. run problems trying implement trivial container accepts allocator. ended this:

template<class t, class allocator =std::allocator<t>> class container { public:     using allocator_type    = allocator;     using value_type        = t;     using pointer           = typename std::allocator_traits<allocator_type>::pointer;     using reference         = value_type&;     using size_type         = std::size_t;      container( size_type n =0 , const allocator_type& allocator =allocator_type() ){         std::cout << "ctor" << std::endl;         allocator.allocate(n);     }; };  int main(int argc, const char* argv[]){     container<int> c {5};     return 0; } 

it gives me error member function 'allocate' not viable: 'this' argument has type 'const allocator_type' (aka 'const std::__1::allocator<int>'), function not marked const

how fix error, please? missing ? intend use traits later make work using old way first.

your line

allocator.allocate(n); 

attempts call allocate method of allocator, not defined const method. if look, though, type of allocator const allocator_type&, is, const reference allocator_type.

how can use then? 1 thing can const object (or reference one) construct different non-const object it. this, example, builds:

allocator_type(allocator).allocate(n); 

as sergeya correctly notes in comments, common not construct temporary ad-hoc allocator_type, rather make such member:

    allocator_type m_alloc; // should private      container( size_type n =0 , const allocator_type& allocator =allocator_type() ) :              m_alloc{allocator}{         std::cout << "ctor" << std::endl;         m_alloc.allocate(n);     };