8.03 new / delete Lifecycles, Dangling Pointers & Heap Fragmentation in Microcontrollers

1. Heap Allocation with new and delete

The new operator requests memory from the free store (heap), invokes the object's constructor, and returns a pointer. The delete operator invokes the destructor and releases the memory back to the heap.

2. Dangling Pointers and Double-Free Bugs

After calling delete ptr;, the pointer variable still holds the original address (a dangling pointer). Dereferencing it is Undefined Behavior. Calling delete twice on the same pointer corrupts the heap metadata. Always set ptr = nullptr; immediately after deletion.

1. Why Dynamic Heap Allocation is Banned in Embedded Safety Systems

• Heap Fragmentation: Repeated allocations and deallocations of varying sizes create tiny free holes across SRAM. Eventually, a small allocation request fails (std::bad_alloc) even if total free RAM exceeds the requested size.
• Non-Deterministic Latency: Heap allocators (malloc) search free lists. The time required varies wildly based on fragmentation state, violating real-time deadlines.
• Stack-Heap Collision: In bare-metal linker scripts, the stack grows downwards while the heap grows upwards. A heap overflow silently overwrites the active stack, corrupting return addresses.

Here is how embedded engineers replace heap new with static storage buffers (zero heap allocation):

#include <cstdint>
#include <cstddef>
#include <new>

// Statically allocated memory slot (Zero Heap Overhead)
template <typename T>
class StaticSlot {
    alignas(T) std::byte storage_[sizeof(T)];
    bool occupied_{false};

public:
    template <typename... Args>
    T* construct(Args&&... args) noexcept {
        if (occupied_) return nullptr;
        T* obj = new (storage_) T(std::forward<Args>(args)...); // Placement new
        occupied_ = true;
        return obj;
    }

    void destroy() noexcept {
        if (occupied_) {
            reinterpret_cast<T*>(storage_)->~T(); // Explicit destructor call
            occupied_ = false;
        }
    }
};