What are unevaluated operands in C++?
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What are unevaluated operands in C++?
1–10 of 20 posts
Re: What are unevaluated operands in C++?
#2Poor man's lazy evaluation. I'm sure they'll try to shoehorn monads in somehow with C++23
Re: What are unevaluated operands in C++?
#3Poor man's lazy evaluation. I'm sure they'll try to shoehorn monads in somehow with C++23
I'm not sure what lazy evaluation had to do with it... but it's something that will be much easier when concepts are in the language with C++20.
Re: What are unevaluated operands in C++?
#4I thought this would be about the arm of a conditional not taken but as soon as he listed the operators it all became clear.
Re: What are unevaluated operands in C++?
#5Poor man's lazy evaluation. I'm sure they'll try to shoehorn monads in somehow with C++23
It's not lazy evaluation at all. Lazy evaluation is a runtime thing, where you delay evaluating an expression until you actually need to.
This concept is operators that will never evaluate the expressions they operate on.
sizeof is the oldschool obvious one, sizeof(foo()) never runs the function foo. It's just a way of querying the static properties of an expression, asking specific things about its type in a way.
They're not new things in C++. sizeof has been around since the beginning.
Re: What are unevaluated operands in C++?
#6A fun (or maybe horrifying) bit of trivia is that sizeof in C can end up evaluating its operand in certain cases; namely, when you use it on a VLA: https://godbolt.org/z/-UX2KQ
Re: What are unevaluated operands in C++?
#7This seems like a potential solution to the problem of how to #define MY_ASSERT(x) in release mode such that it fails to compile if "x" is not a valid expression.
Previous I have used this idiom:
#define MY_ASSERT(expr) do {} while (false && (expr))
Another possible alternative might be:
#define MY_ASSERT(expr) sizeof(expr)
Re: What are unevaluated operands in C++?
#8This seems like a potential solution to the problem of how to #define MY_ASSERT(x) in release mode such that it fails to compile if "x" is not a valid expression. Previous I have used this idiom: #define MY_ASSERT(expr) do {} while (false && (expr)) Another possible alternative might be: #define MY_ASSERT(expr) sizeof(expr)
> #define MY_ASSERT(expr) sizeof(expr)
sizeof accepts a type or an expression as its input, so it would compile successfully if expr were a type but not a valid expression (say, int).
Re: What are unevaluated operands in C++?
#9This seems like a potential solution to the problem of how to #define MY_ASSERT(x) in release mode such that it fails to compile if "x" is not a valid expression. Previous I have used this idiom: #define MY_ASSERT(expr) do {} while (false && (expr)) Another possible alternative might be: #define MY_ASSERT(expr) sizeof(expr)
> #define MY_ASSERT(expr) sizeof(expr) sizeof accepts a type or an expression as its input, so it would compile successfully if expr were a type but not a valid expression (say, int).
You could do sizeof((expr)?1:2) instead, which would also force expr to be contextually convertible to a bool.
Re: What are unevaluated operands in C++?
#10This kind of tag dispatch to disambiguate is really clever. I never thought of disambiguating based on the conversion of the tag.
Edit: I have probably seen it in concepts lite though, I just forgot about it.