mfbt — Mozilla Framework-Based Templates

mfbt/ is Mozilla's in-house C++ utility library: the bits that look like the C++ standard library but predate it (or that the team prefers to control directly). Header-only and used everywhere in the tree.

What's inside

Header	What it provides
RefPtr.h	Refcounting smart pointer
UniquePtr.h	Mozilla's std::unique_ptr
WeakPtr.h	Weak references
Maybe.h	Optional ("Maybe", Some(...), Nothing())
Result.h	mozilla::Result<T, E> and MOZ_TRY
Span.h	View over a range
Variant.h	Tagged union
Atomics.h	Atomic primitives
Logging.h	The MOZ_LOG macros
Assertions.h	MOZ_ASSERT, MOZ_RELEASE_ASSERT, MOZ_DIAGNOSTIC_ASSERT
EnumSet.h, EnumeratedArray.h	Strongly-typed enum collections
Tuple.h, Pair.h	Tuple/pair
Range.h, RangedPtr.h	Bounds-checked pointer arithmetic
HashFunctions.h	Mozilla's hashing
MemoryReporting.h	MallocSizeOf + memory accounting
Vector.h	Mozilla's std::vector
Buffer.h, BufferList.h	Byte buffers
TimeStamp.h, TimeDuration.h	Monotonic time
MathAlgorithms.h	min/max/clamp/IsPowerOfTwo/RoundUp

Why not std?

mfbt predates many C++17/20 standard-library facilities, and parts deliberately diverge:

• Stricter assertions and bounds checks.
• Memory-reporting hooks (so about:memory can size objects).
• Static-analysis annotations.
• Refcounting integration with nsISupports.

New code that has the choice is encouraged to use mfbt types where they exist (e.g., mozilla::Maybe over std::optional) for consistency.

Related

• XPCOM
• Patterns and conventions