问题出在哪里，如何解决？ 无需尝试挤压自定义分配器，乍一看，我的向量即可正常工作。 我也很乐意指出我的代码中的任何错误。或正确实现自定义矢量或其他容器的示例，这对我有帮助。 该代码不起作用：
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实现（应该没问题）：
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);
}
}
};
向量实现：
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);
}
};
}
答案 0 :(得分：4)
Vector<int> v(CustomAllocator<int>());
您被most vexing parse击中。 Vector<int> v(CustomAllocator<int>());可以解析为变量声明或函数声明，语法更喜欢后者。因此，编译器认为v是一个函数，这就是为什么您收到“表达式必须具有类类型”错误的原因-您只能对具有类类型的值调用方法，而v是一个功能。
即使使用以下选项之一解决了该错误，也是如此：
// C++03 solution (extra parens)
Vector<int> v((CustomAllocator<int>()));
// C++11 solution (uniform initialization)
Vector<int> v{CustomAllocator<int>{}};
您的代码虽然可以运行，但仍然无法满足您的期望。 Vector<int>与Vector<int, std::allocator>是同一件事，因此v仍会使用标准分配器。
为什么这不会导致编译错误？因为CustomAllocator<int>是继承 std::allocator<int>（不应该这样！），所以std::allocator<int>复制构造函数用于将您的自定义分配器切成{{1} }，然后程序使用标准分配器继续。您的std::allocator<int>临时元素基本上会转换为CustomAllocator<int>。
为说明起见，以上两个“固定的”示例在大致上均等效于此代码（如果我们忽略了一些与程序的可观察行为无关的值复制/移动）：
std::allocator<int>正确的解决方法是指定第二个// 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);
类型参数，然后甚至不需要构造函数参数，因为默认构造函数将做正确的事情：
Vector