The C++ class std::expected provides a way to store either either an expected value, or an unexpected value, which is often an error code or error message. Functions that can fail often return std::expected to either return a result, or error information.
Questions tagged [std-expected]
10 questions
28
votes
1 answer
What is std::expected in C++?
In one of the most respected stackoverflow answer I found an example of std::expected template class usages:
What are coroutines in C++20?
At the same time I cannot find any mentioning of this class on cppreference.com. Could you please explain what…
Fedor
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votes
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When to use std::expected instead of exceptions
When should I use std::expected and when should I use exceptions? Take this function for example:
int parse_int(std::string_view str) {
if (str.empty()) {
throw std::invalid_argument("string must not be empty");
}
/* ... */
…
Jan Schultke
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1 answer
Why are there no monadic operations in std::expected?
In C++23, monadic operations in std::optional was adopted, and later on std::expected. Why were monadic operations like and_then, transform, and or_else not included in the main paper?
Desmond Gold
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1 answer
std::unexpected constructor constraint
I am reading the cpp 23 standard and I stumbled upon std::unexpected.
Section expected.un.cons defines
template
constexpr explicit unexpected(Err&& e);
with the following costraint (among others)
is_same_v,…
NotMe
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1 answer
How to avoid calling the destructor twice when using a factory function with a std::expected return without runtime costs
It's about embedded C++. Let's assume I have a struct Timer_t.
To create a new object
the constructor is private
we have a factory-function as a public member makeTimer()
We can't use exceptions on the device and to intiailize the Timer we can get…
Sickeroni
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2 answers
C++ is there an implementation of std::expected that doesn't consider a default constructed `expected` to have a value?
I've been looking for a good way to improve error handling in a C++ library, with the goal of reducing the risk of bug-prone code while maintaining efficiency.
I stumbled across Andrei Alexandrescu's Expect the Expected talk on YouTube and am very…
Tom
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Is that possible to upcast erased type without RTTI?
I have a custom implementation of std::expected, but with a type-erased error. It makes the expected type looks similar to exceptions. We just can use the next code:
Expected V = 123;
V.SetError(std::string("Error occurred"));
And when we try…
Artem Selivanov
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vote
1 answer
How to use std::expected in conjunction with std::transform?
I have a flatbuffer. This flatbuffer has a vector that I want to transform using std::transform into a std::vector.
It is possible that T can contain values that are not valid - because they are a plain integer converted to an enum. In this…
Raildex
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std::expected, references, and std::reference_wrapper
This question is related to std::expected and reference return type
I try to get my head around std::expected (respectively https://github.com/TartanLlama/expected) as an alternative error handling methodology, and I like the concept. However,…
NeitherNor
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error: expected initializer before 'std' and other errors after adding one line to working code
The following program worked already before I added line 12, the line reading:
" std::cout<<" Natürlicher Logarithmus von ";std::cin>>X; ".
Now I get a lot of errors (see below)!
Please Help
// Berechnung des natürlichen Logarithmus (Grenzwert…
fribir
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