A custom C++ memory handler: Overloading new/delete for (1) a class, and (2) globally

memory_handler.cpp

// Source: C++ Game Development Primer - Chapter 1

#include <iostream>
using namespace std;

class DummyClass{
    double a,b;

public:

    DummyClass(){cout<<"Const\n";}
    virtual ~DummyClass(){cout<<"Dest\n";}
    void* operator new(size_t size)
    {
        cout << "Allocating "<<size<<" byte for a new object\n";
        //DummyClass* o = ::new DummyClass;
        void* o = malloc(size);
        return o;
    }

    void operator delete(void *mem)
    {
        cout << "Freeing ...\n";
        //::delete DummyClass;
        free(mem);
    }
};

struct MemoryAllocationHeader{
    void* start;
    void* nextFree;
    size_t size;

};



namespace MyMemoryManager
{
    char mem[1024*1024];
}


void* operator new(size_t size)
{
    MemoryAllocationHeader* newHeader = reinterpret_cast<MemoryAllocationHeader*>(MyMemoryManager::mem);

    // First empty slot when nextFree=0x00
    while(newHeader->nextFree!= nullptr)
    {
        newHeader = reinterpret_cast<MemoryAllocationHeader*>(newHeader->nextFree);
    }

    newHeader->size = size;
    newHeader->start = newHeader+sizeof(MemoryAllocationHeader);
    newHeader->nextFree = (char*)(newHeader->start) + size;

    return newHeader->start;
}


void operator delete(void* memory)
{
    MemoryAllocationHeader* current = reinterpret_cast<MemoryAllocationHeader*>(MyMemoryManager::mem);

    MemoryAllocationHeader *last = nullptr;

    while(current->start!=memory)
    {
        last = current;
        current = reinterpret_cast<MemoryAllocationHeader*>(current->nextFree);
    }

    if(last!=nullptr)
        last->nextFree = current->nextFree;
    current->size = 0;
    current->start = nullptr;
}

void PrintAllocations() {

    MemoryAllocationHeader* pHeader = reinterpret_cast<MemoryAllocationHeader*>(MyMemoryManager::mem);

    while (pHeader != nullptr) {

        std::cout << pHeader << std::endl;
        std::cout << pHeader->start << std::endl;
        std::cout << "NextHeader: "<<pHeader->nextFree << std::endl; std::cout << pHeader->size << std::endl;

        pHeader = reinterpret_cast<MemoryAllocationHeader*> (pHeader->nextFree);

        std::cout << std::endl << std::endl;

    }

}


int main()
{
    auto x1 = new DummyClass;
    auto x2 = new int;
    auto x3 = new DummyClass;
    PrintAllocations();
    cout<<"---------------------\n";

    delete x2;
    PrintAllocations();
    cout<<"---------------------\n";

    x2 = new int;
    PrintAllocations();
    cout<<"---------------------\n";

    delete x1;
    delete x2;
    delete x3;
    PrintAllocations();
    cout<<"---------------------\n";

    x2 = new int;
    PrintAllocations();
    cout<<"---------------------\n";

    delete x2;
    PrintAllocations();
    cout<<"---------------------\n";

}