Manual Memory Management in D
Although D defaults to garbage-collected memory, it is fundamentally a systems language, and it gives you full access to manual allocation when the GC's nondeterministic pauses or heap overhead are unacceptable — for real-time audio, embedded targets, game engines with tight frame budgets, or code compiled with -betterC where the D runtime and GC are unavailable entirely. The most direct route is core.stdc.stdlib, which exposes the familiar C allocation functions malloc, calloc, realloc, and free directly, giving you raw untyped void* blocks that you are responsible for sizing, initializing, and releasing exactly once. Because these blocks are invisible to the GC by default, storing a D class reference or GC-allocated array inside manually allocated memory is dangerous unless you explicitly register that memory with GC.addRange, since otherwise the GC has no way to know your malloc'd block is keeping something reachable and may collect it out from under you.
Cricket analogy: Opting into manual memory management is like a franchise choosing to build and maintain its own private training facility instead of relying on the board's shared academy — full control, but you're now responsible for every maintenance detail yourself.
malloc, free, and emplace
Raw malloc gives you uninitialized memory, so constructing a D struct inside it correctly requires std.conv's emplace function, which runs the type's constructor logic on an already-allocated block: auto ptr = cast(MyStruct*) malloc(MyStruct.sizeof); emplace(ptr, arg1, arg2); constructs a fully initialized MyStruct in place without any GC involvement. Destruction is the mirror image: call destroy(*ptr) to run the struct's destructor deterministically, then free(ptr) to release the raw memory back to the C allocator — forgetting either step leaks either the resource the destructor would have released or the memory block itself, and calling free on memory the GC allocated (or vice versa) corrupts the heap because the two allocators use incompatible bookkeeping. This pattern is the standard way to get a class-like heap object with struct-style deterministic control: allocate raw memory manually, use emplace to run construction logic, and pair every allocation with an explicit destroy-then-free at the point where the object's lifetime should end.
Cricket analogy: emplace running a constructor on raw malloc'd memory is like a groundstaff crew marking out and preparing a brand-new, unmarked practice pitch to exact match specifications before any team is allowed to use it.
std.experimental.allocator
Rather than calling malloc and free directly everywhere, D's std.experimental.allocator package provides a composable allocator framework: Mallocator.instance wraps the C allocator behind a uniform interface, GCAllocator.instance exposes the GC heap through the same interface, and you can build custom composed allocators — a region allocator that bump-allocates from a fixed arena, or a FreeList that recycles fixed-size blocks — all satisfying the same allocate/deallocate contract, which means code written against the interface can be retargeted to a different allocation strategy without being rewritten. The framework's make and dispose helpers mirror emplace and destroy-then-free but generically for any allocator: auto obj = make!MyStruct(Mallocator.instance, arg1, arg2); allocates and constructs in one call, and dispose(Mallocator.instance, obj); destroys and deallocates in one call, which removes most of the manual bookkeeping risk of pairing raw malloc/emplace/destroy/free by hand.
Cricket analogy: Swapping Mallocator.instance for a custom region allocator without rewriting client code is like a league letting any ground swap its turf supplier without changing a single rule of how matches are scheduled or played.
Deterministic Cleanup with Structs and scope(exit)
Unlike a class's ~this(), a struct's destructor runs deterministically the instant a stack-allocated instance goes out of scope, which is exactly the RAII (Resource Acquisition Is Initialization) pattern: wrap a manually allocated resource — a malloc'd buffer, a file descriptor, a mutex lock — inside a struct whose destructor releases it, and the compiler guarantees the release happens at scope exit regardless of how control leaves the scope, including via an exception. For cases where you need guaranteed cleanup without writing a whole struct, D's scope(exit) statement registers a block of code to run unconditionally when the enclosing scope ends — scope(failure) runs only if the scope exits via an exception, and scope(success) runs only if it exits normally — giving you fine-grained, ref-counting-free control that composes naturally with manual allocation. Combining these two patterns — RAII structs for reusable resource wrappers, scope(exit) for one-off cleanup at a call site — is the idiomatic D way to get C++-like deterministic destruction without opting into the GC for that particular resource.
Cricket analogy: A struct's deterministic destructor releasing a resource at scope exit is like a groundstaff rule that covers must come off the pitch the instant the umpires call play resumed, no matter why the rain delay ended — guaranteed timing, not best-effort.
import core.stdc.stdlib : malloc, free;
import std.conv : emplace;
import std.experimental.allocator.mallocator : Mallocator;
import std.experimental.allocator : make, dispose;
struct Buffer
{
private ubyte* data;
private size_t len;
this(size_t n)
{
data = cast(ubyte*) malloc(n);
len = n;
}
~this()
{
if (data !is null)
{
free(data); // deterministic release at scope exit
data = null;
}
}
@disable this(this); // prevent accidental shallow copies of the pointer
}
struct Counter
{
int value;
this(int start) { value = start; }
}
void main()
{
// Raw malloc + emplace + destroy + free, done manually
auto raw = cast(Counter*) malloc(Counter.sizeof);
emplace(raw, 10);
scope(exit) { destroy(*raw); free(raw); }
raw.value += 5;
// std.experimental.allocator: make/dispose bundle the same steps
auto c2 = make!Counter(Mallocator.instance, 100);
scope(exit) dispose(Mallocator.instance, c2);
// RAII struct: destructor runs automatically at scope exit
{
auto buf = Buffer(1024);
// use buf.data here
} // buf's ~this() releases the malloc'd memory here, guaranteed
}Struct destructors run deterministically at scope exit for stack-allocated instances — this is the opposite of class destructors, which run at a GC-chosen, nondeterministic time. Wrapping a manually allocated resource in a struct is D's standard RAII pattern.
Never free() memory that the GC allocated, and never pass a malloc'd pointer to the GC to free — the two allocators maintain separate, incompatible bookkeeping, and mismatching them corrupts the heap or leaks memory. If GC-managed references are stored inside malloc'd memory, register that block with GC.addRange so the collector can see them, or keep GC and manual allocations strictly separate.
- core.stdc.stdlib exposes malloc, calloc, realloc, and free for raw, GC-invisible memory blocks.
- std.conv.emplace runs a type's constructor logic inside already-allocated raw memory; destroy() then free() is the matching cleanup pair.
- std.experimental.allocator provides a composable allocator interface (Mallocator, GCAllocator, region/FreeList allocators) with make()/dispose() helpers.
- Struct destructors run deterministically at scope exit, unlike GC-driven class destructors, making structs the natural RAII wrapper for manual resources.
- scope(exit), scope(failure), and scope(success) register cleanup code that runs unconditionally, only on exception, or only on normal exit, respectively.
- Never mix allocators: free() memory obtained from malloc, and let the GC reclaim memory it allocated — never cross the two.
- @nogc and -betterC builds rely entirely on manual memory management since the GC and full D runtime are unavailable or disallowed.
Practice what you learned
1. What does std.conv.emplace do when given a pointer to raw malloc'd memory?
2. What is the correct cleanup sequence for an object constructed with emplace inside malloc'd memory?
3. Why is it dangerous to call free() on memory that was allocated by the garbage collector?
4. What advantage does std.experimental.allocator's make()/dispose() provide over raw malloc/emplace/destroy/free?
5. How does a struct's destructor timing differ from a class's destructor timing in D?
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