This code is a small building block extracted from a larger register / field interaction framework in some driver code I recently wrote for Invitrometrix.
The focus here is not on templates or language features for their own sake, but on clean, intention-revealing usage when interacting with hardware registers.
I come from the hardware side, where bits, fields, and widths are explicit and easily accessed. This code is an attempt to make register-level C++ feel a little friendlier to that world.
The primary goal is to make register manipulation read closer to how you think about the underlying hardware.
Instead of repeating bitwise operations and masks at every call site, that information is defined exactly once and used expressively:
auto mymask = Mask<uint32_t>::of_bits(6, 7);
// clear masked field, then insert field from new_value
value = mymask.merge(new_value, value);For comparison, the traditional approach typically looks something like this:
#define MASK 0x1F80
value = (new_value & MASK) | (value & ~MASK);The problem with the traditional form is not correctness — it’s cognitive load. Every use site requires thinking about bitwise operations and type-related hazards. Some use sites must re-derive and re-validate the same knowledge about bit widths and offsets. Once masks become named, validated objects instead of raw numbers, usage becomes mechanical and much harder to get wrong.
Under the hood, masks are generated using a small, reusable helper function evaluated entirely at compile time.
template<typename T>
consteval T mask_of_bits(unsigned bits,
unsigned lowest_bit = 0);This function has the luxury of doing terribly inefficient things in service to generality and clarity -- because it's entirely compile time.
This function validates:
- the target type
- the width of the mask
- the starting bit position
- and the final shifted width
Invalid configurations fail during compilation rather than becoming latent runtime bugs.
Wrapped in a small Mask<T> reusable class with consteval constructors and
factories, and some constexpr convenience functions, this gives me:
- compile-time sanity checking
- zero runtime overhead
- and much clearer intent at call sites
If you're into that kind of thing, dive deeper into the exact details of the generated machine code at Godbolt's Compiler Explorer. Something tells me you're into that kind of thing; you've read this far...
One interesting part of this exercise was noticing the pull of the
traditional approach.
In conversations with my AI “thinking buddy,” even when presented with my
answer, I got guidance pushing me back to using something like: ( 1<<bits - 1 ).
When pressed, the list of special cases and exceptions started getting added.
Those suggestions aren’t wrong. They’re just optimized for familiarity.
I wanted usage that was readable.
I wanted mask generation that worked correctly for every type, of every width -- signed or unsigned.
I wanted to do this once and be done. At least until the next new C++ feature.
A deliberate design goal was ensuring that all mask construction happens at compile time. If a mask definition is invalid — wrong width, wrong offset, or incompatible type — the error appears during compilation rather than after deployment.
This mirrors how hardware descriptions behave: invalid configurations should not simulate or synthesize.
Another interesting side note: the error reporting here relies on what is
admittedly a bit of a hack (throw inside consteval).
This has the desirable end effect: compile-time error and pointer to the area in code.
However, the compiler error message is misleading: about the use of exceptions in this context. Fortunately, it looks like future C++ standards are improving diagnostics in this area so compile-time error reporting will become clearer.
I'm sure I'll be tweaking this approach as I put it to more real-world use.
This is a small example abstraction, but it reflects a broader trend: modern C++ continues to get better for low-level, hardware-adjacent work. The language keeps providing better tools. The challenge — and the opportunity — is learning when to stop doing things the old way.