不要使用 static 进行算术转换（cpp-core-guidelines）

Question

msvc's code analyzer for the cpp core guidelines tells me

Warning C26472 Don't use a static_cast for arithmetic conversions. Use brace initialization, gsl::narrow_cast or gsl::narrow (type.1).

for this snippet

static_cast<IntType>(static_cast<unsigned long long>(hexValue(digit)) << (digitIdx * 4));

Why shouldn't I use static_cast here?

Also, with brace init this would look like this

IntType{unsigned long long{hexValue(digit)} << (digitIdx * 4)};

which doesn't look any better imo. This looks more like a function style cast than anything else.

I cannot use gsl and I think gsl::narrow is a wrapper around static_cast itself, so is this purely a readability issue here?

Answer 1

so is this purely a readability issue here?

Nope. Brace initialization prohibits narrowing conversions, and will cause a diagnostic. The guideline's purpose is to help protect code from unintended narrowing. A static_cast will allow a narrowing conversion through silently.

You seem to be dealing in integers, so short of using an extended integer type that is larger than unsigned long long , you'll probably not hit any narrowing.

But the guideline is there for the general case, and it's better to write code consistently, even when there is no actual risk.

Answer 2

Here is a link to Microsoft documentation:

[https://docs.microsoft.com/en-us/cpp/code-quality/c26472?view=vs-2019][1]

• gsl::narrow ensures lossless conversion and causes run-time crash if it is not possible.
• gsl::narrow_cast clearly states that conversion can lose data and it is acceptable.

static_cast doesn't perform these checks, so it is safer to explicitly state your intentions.