Custom Polymorphic Memory Resource (std::pmr) Implementation

#include <iostream>
#include <memory_resource>
#include <vector>
 
class TrackingMemoryResource : public std::pmr::memory_resource {
private:
    std::pmr::memory_resource* upstream_;
    size_t total_allocated_{0};
 
protected:
    void* do_allocate(size_t bytes, size_t alignment) override {
        total_allocated_ += bytes;
        std::cout << "[PMR Alloc] " << bytes << " bytes (Total: " << total_allocated_ << ")\n";
        return upstream_->allocate(bytes, alignment);
    }
 
    void do_deallocate(void* p, size_t bytes, size_t alignment) override {
        std::cout << "[PMR Dealloc] Freeing " << bytes << " bytes\n";
        upstream_->deallocate(p, bytes, alignment);
    }
 
    bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {
        return this == &other;
    }
 
public:
    explicit TrackingMemoryResource(std::pmr::memory_resource* upstream = std::pmr::get_default_resource())
        : upstream_(upstream) {}
};
 
int main() {
    TrackingMemoryResource tracker;
    std::pmr::vector<int> numbers(&tracker);
    numbers.reserve(4);
    numbers.push_back(42);
}