The class template std::variant represents a type-safe union. An instance of std::variant at any given time either holds a value of one of its alternative types, or in the case of error - no value
Questions tagged [std-variant]
139 questions
92
votes
4 answers
`std::variant` vs. inheritance vs. other ways (performance)
I'm wondering about std::variant performance. When should I not use it? It seems like virtual functions are still much better than using std::visit which surprised me!
In "A Tour of C++" Bjarne Stroustrup says this about pattern checking after…
The Moisrex
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votes
2 answers
Why is my struct destructed twice with `std::variant` and `std::monostate`?
I am trying to learn std::variant. I do not understand why in this example, where I prefer not to initialize ab yet, and I use std::monostate for that, the class A gets constructed once, but destructed twice. What is happening?
#include…
Chiel
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4 answers
How can I code something like a switch for std::variant?
I have some var = std::variant when a, b, c is some types.
How, at runtime, do I check what type var contains?
In the official documentation I found information that if var contains a type and I write std::get(var) I get…
dasfex
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1 answer
May I change the held type in a std::variant from within a call to std::visit
Does the following code invoke undefined behaviour?
std::variant v = ...;
std::visit([&v](auto& e){
if constexpr (std::is_same_v,A>)
e.some_modifying_operation_on_A();
else {
int i =…
burnpanck
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3 answers
Split a given std::variant type by a given criteria
How to by a given variant type
using V = std::variant>;
declare two variant types
using V1 = std::variant;
using V2 = std::variant
Alexey Starinsky
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15
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1 answer
std::variant behaves differently in MSVC and gcc
Update: This is a C++ standard defect, which is fixed in C++20 (P0608R3). Also, VS 2019 16.10 has fixed this bug with /std:c++20.
MSVC 19.28 rejects the following code but gcc 10.2 accepts it and outputs true false
#include
#include…
El Mismo Sol
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How is std::optional never "valueless by exception"?
std::variant can enter a state called "valueless by exception".
As I understand, the common cause of this is if a move assignment throws an exception. The variant's old value isn't guaranteed to be present anymore, and neither is the intended new…
Drew Dormann
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3 answers
In C++, how to make a variant that can contain a vector of of same variant?
I a trying to make a std::variant that can contain a vector of the same variant:
class ScriptParameter;
using ScriptParameter = std::variant >;
I am getting ScriptParameter redefinition.…
JeffV
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Why isn't std::variant allowed to equal compare with one of its alternative types?
For example, it should be very helpful to equal compare a std::variant with a T1 or T2. So far we can only compare with the same variant type.
wanghan02
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Why is sizeof( std::variant< char > ) == 8 when using libc++ and not 2 (like with MSVC's STL and libstdc++)?
Consider this example on Compiler explorer.
Basically, we have this code snippet:
#include
#include
enum class Enum1 : std::uint8_t { A, B };
enum class Enum2 : std::uint8_t { C, D };
using Var = std::variant< Enum1, Enum2…
DoDo
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3 answers
Is there any practical reason why std::get_if (std::variant) takes a variant argument by pointer instead of by value/&/const&?
I've never used std::get_if, and since its name is different from std::get, I don't see a reason why its argument should be a pointer¹ (whereas std::get has a by-reference parameter).
¹If it was named std::get too, then overload resolution would be…
Enlico
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What should I use instead of void as one of the alternative types in an variant?
I want to have a variant which may contain type Foo, (disjoint) type Bar, or nothing. Well, naturally, I was thinking of using std::variant - but this doesn't seem to work. That is, you can define this type, but if you try to…
einpoklum
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4 answers
How to improve compiler error messages when using C++ std::visit?
I am using C++17's std::visit() function on a variant with many alternatives, and the error messages produced by the compiler whenever I forget one or more of the alternatives in my visitor are quite difficult to understand.
e.g.
template
jinscoe123
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Why do std::variant implementations take more than 1 byte for variant of empty types?
This is mostly trivia question, as I doubt that I will ever need this space saving.
While playing around on godbolt I noticed that both libstdc++ and libc++ implementations of std::variant require more than 1 byte to store variant of empty…
NoSenseEtAl
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std::bind to a std::variant containing multiple std::function types
I'm playing around with callback functions and wish to register multiple functions via std::bind that differ in signatures (altough they all return void). Assigning the result of the std::bind to the std::variant yields in a "conversion to…
Tmplt
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