Why my code doesn't work with custom allocator? - c++

Question

What is the problem and how to fix it?
Without trying to squeeze a custom allocator, my vector, at first glance, works correctly.
I will also be happy to point out any mistakes in my code. Or an example of the correct implementation of a custom vector or another container, that would help me.
This code doesn't work:
using MyLib::Vector;
int main()
{
//Vector<int> v; //Works fine
Vector<int> v(CustomAllocator<int>());
for (size_t i = 0; i < 256; i++) {
v.push_back(i); //Error: "expression must have class type"
}
}
CustomAllocator implementation (it should be fine):
template <typename T>
class CustomAllocator : public std::allocator<T>
{
private:
using Base = std::allocator<T>;
public:
T* allocate(size_t count){
std::cout << ">> Allocating " << count << " elements" << std::endl;
return Base::allocate(count);
}
T* allocate(size_t count, const void* p)
{
std::cout << ">> Allocating " << count << " elements" << std::endl;
return Base::allocate(count, p);
}
void deallocate(T* p, size_t count)
{
if (p != nullptr)
{
std::cout << ">> Deallocating " << count << " elements" << std::endl;
Base::deallocate(p, count);
}
}
};
Vector implementation:
namespace MyLib
{
template <typename T,
template <typename Y> class Allocator = std::allocator>
class Vector
{
private:
std::size_t capacityV;
std::size_t sizeV;
Allocator<T> alloc;
T* arr;
public:
typedef Allocator<T> AllocatorType;
typedef Vector<T, Allocator> VectorType;
using AllocTraits = std::allocator_traits<Allocator<T>>;
public:
explicit Vector(const AllocatorType& allocator = AllocatorType()) {
capacityV = 0;
sizeV = 0;
alloc = allocator;
arr = nullptr;
}
Vector(const std::initializer_list<T>& values,
const AllocatorType& allocator = AllocatorType()) {
sizeV = values.size();
alloc = allocator;
if (sizeV < 128)
capacityV = 128;
else
capacityV = (sizeV / 128) * 256; //that makes sense
arr = AllocTraits::allocate(alloc, capacityV);
AllocTraits::construct(alloc, arr, capacityV);
std::copy(values.begin(), values.end(), arr);
}
~Vector() {
if (arr)
AllocTraits::deallocate(alloc, arr, capacityV);
}
Vector(const Vector& rhs) {
capacityV = rhs.capacityV;
sizeV = rhs.sizeV;
arr = AllocTraits::allocate(alloc, capacityV);
std::copy(rhs.arr, rhs.arr + rhs.sizeV, arr);
}
Vector(Vector&& rhs) noexcept {
capacityV = std::move(rhs.capacityV);
sizeV = std::move(rhs.sizeV);
arr = std::move(rhs.arr);
}
Vector& operator = (const Vector& rhs) {
capacityV = rhs.capacityV;
sizeV = rhs.sizeV;
arr = AllocTraits::allocate(alloc, capacityV);
std::copy(rhs.arr, rhs.arr + rhs.sizeV, arr);
}
Vector& operator = (Vector&& rhs) {
capacityV = std::move(rhs.capacityV);
sizeV = std::move(rhs.sizeV);
arr = std::move(rhs.arr);
}
T& operator [](std::size_t i) noexcept {
if (i < sizeV)
return arr[i];
else
throw std::out_of_range("Wrong index!");
}
const T& operator [](std::size_t i) const noexcept {
if (i < sizeV)
return arr[i];
else
throw std::out_of_range("Wrong index!");
}
T* data() noexcept {
return arr;
}
const T* data() const noexcept {
return arr;
}
void push_back(const T& value) {
++sizeV;
if (!arr) {
if (!capacityV)
capacityV = 128;
arr = AllocTraits::allocate(alloc, capacityV);
}
if (sizeV > capacityV) {
if(capacityV > UINT32_MAX - 256)
throw std::runtime_error("Vector overflowed!");
size_t tmpCap = capacityV;
capacityV = (sizeV / 128) * 256; //Увеличим capacityV
T* buf = AllocTraits::allocate(alloc, capacityV);
std::move(arr, arr + sizeV - 1, buf);
AllocTraits::deallocate(alloc, arr, capacityV); //sizeof
arr = buf;
}
arr[sizeV - 1] = value;
}
void push_back(T&& value) {
++sizeV;
if (!arr) {
if (!capacityV)
capacityV = 128;
arr = AllocTraits::allocate(alloc, capacityV);
}
if (sizeV > capacityV) {
if (capacityV > UINT32_MAX - 256)
throw std::runtime_error("Vector overflowed!");
size_t tmpCap = capacityV;
capacityV = (sizeV / 128) * 256; //Увеличим capacityV
T* buf = AllocTraits::allocate(alloc, capacityV);
std::move(arr, arr + sizeV - 1, buf);
AllocTraits::deallocate(alloc, arr, capacityV); //sizeof
arr = buf;
}
arr[sizeV - 1] = std::move(value);
}
void pop_back() {
--sizeV;
}
void resize(std::size_t size) {
if (this->sizeV == size)
return;
if (this->sizeV > size) {
this->sizeV = size;
}
else {
size_t tmpSize = size;
if (capacityV >= size) {
this->sizeV = size;
for (size_t i = tmpSize - 1; i < this->sizeV; i++)
arr[i] = 0;
}
else {
size_t tmpCap = capacityV;
capacityV = (size / 128) * 256; //that makes sense
T* buf = AllocTraits::allocate(alloc, capacityV);
std::move(arr, arr + sizeV - 1, buf);
AllocTraits::deallocate(alloc, arr, capacityV); //sizeof
arr = buf;
this->sizeV = size;
for (size_t i = tmpSize - 1; i < this->sizeV; i++)
arr[i] = 0;
}
}
}
void reserve(std::size_t capacity) {
if (capacity > this->capacityV)
{
size_t tmpCap = capacity;
this->capacityV = capacity;
T* buf = AllocTraits::allocate(alloc, capacityV);
std::move(arr, arr + sizeV - 1, buf);
AllocTraits::deallocate(alloc, arr, capacityV); //sizeof
arr = buf;
}
}
std::size_t size() const noexcept {
return sizeV;
}
std::size_t capacity() const noexcept {
return capacityV;
}
bool empty() const noexcept {
return (sizeV == 0);
}
};
}

Answer 1

Vector<int> v(CustomAllocator<int>());
You got hit by the most vexing parse. Vector<int> v(CustomAllocator<int>()); can be parsed as a variable declaration or a function declaration and the grammar prefers the latter. Therefore, the compiler thinks that v is a function and this it why you get the "expression must have class type" error -- you can only invoke methods on values with a class type, but v is a function.
Even if you fixed that error using one of these options:
// C++03 solution (extra parens)
Vector<int> v((CustomAllocator<int>()));
// C++11 solution (uniform initialization)
Vector<int> v{CustomAllocator<int>{}};
Your code still wouldn't do what you expected, though it would run. Vector<int> is the same thing as Vector<int, std::allocator> and so v will still use the standard allocator.
Why doesn't this cause a compilation error? Because CustomAllocator<int> inherits std::allocator<int> (which it shouldn't!), so the std::allocator<int> copy constructor is used to slice your custom allocator into an std::allocator<int> and then the program proceeds using the standard allocator. Your CustomAllocator<int> temporary is basically converted into an std::allocator<int>.
To illustrate, both of the above "fixed" examples are roughly equivalent to this code (if we disregard some value copies/moves that are irrelevant to the program's observable behavior):
// Creates a custom allocator value.
CustomAllocator<int> a;
// Custom allocator is converted to std::allocator<int>.
std::allocator<int> b(a);
// std::allocator<int> is provided to the Vector constructor.
Vector<int> v(b);
The correct fix is to specify the second Vector type parameter and then the constructor arguments aren't even needed since the default constructor will do the right thing:
Vector<int, CustomAllocator> v;