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
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Getting the value of a constexpr std::variant via constexpr function
This is a reduced example of a bigger function. The core issue is that at one point I try to get the value out of a constexpr std::variant via a constexpr/consteval function. This fails and I do not know why.
Code:
#include
#include…
std_unordered_map
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2 answers
Function accepting a reference to std::variant
I'm trying to pass values to a function accepting a std::variant.
I noticed I can use a function accepting a const reference to a variant value, but not a reference alone. Consider this code
#include
#include
#include…
Moia
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1 answer
Invoke a method for each alternative in a list of std::variant without macros
I have a set of Writer classes each with a different implementation.
I have a list of writers that offers the same interface. Calling a method on the list should invoke the same method on each of the elements in the list (composite design pattern).…
doxygen
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1 answer
Generically wrap member function of object to modify return type
Version restriction: C++17.
I'm trying to create a type capable of accepting a callable object of any type and wrapping one of its member functions (in this case, operator()) to take the same arguments, but modify (cast) the return type. An example…
user4520
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How is it possible to unite multiple types in the operator function called by std::visit in C++?
I am using std::variant and std::visit to call operator functions. I have a lot of variants (which mostly inherit from one superclass), but most of the operator functions should return the same value. Is there a way to have one operator function,…
wittn
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2 answers
Can't call function with variant parameter using initializer_list
I'm not so skilled in using std::variant and couldn't completely understand all constructor cases.
using map_type = std::multimap;
using union_type = std::variant;
void foo(union_type)
{
...
}
void…
mouse_00
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Accessing the struct members inside a std::variant
I've got a std::variant, which has as alternatives two different structs.
Now I need to fill it with data for one alternative (which i know at runtime), but it has to be in a way, that it's possible to write the elements in the struct…
Gabriel
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1 answer
std::map with values of different function-types
I want to use a std::map with certain types of functions (certain, specific parameter types) as possible values. However, the following (minimal) example does not compile.
Why is that and how can I allow FunctionType and FunctionType as…
Simon Fromme
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std::variant as last argument of template function taking multiple variadic arguments
I have the following code:
#include
#include
#include
template
struct overloaded : Ts...
{
using Ts::operator()...;
};
template
overloaded(Ts...) ->…
NoSenseEtAl
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dealing with variants containing move-only types
Consider the following code:
#include
#include
#include
template struct overloaded : Ts... { using Ts::operator()...; };
template overloaded(Ts...)->overloaded;
struct foo {
int…
jwezorek
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2 answers
vector of std::variant types to std::tuple
I am working on vector of std::variant types. Is there a way to convert it to std::tuple of the values holded by given std::variants ?
typedef std::variant a_union;
std::vector vec;
For example, I would like to have tuple…
bring112
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4 answers
Default-construct all the types in a std::variant and put them in a std::vector
How would I make a std::vector that contains a default-constructed instance of all the types contained in a std::variant?
using TaskVariant = std::variant;
std::vector variants;
// I'd like to do this in a…
GlassBeaver
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1 answer
std::bad_variant_access error when trying to store value to std::variant in map
I'm parsing a file into a map with std::variant (to support two types of values) and I can't figure out the right way to store the value into the map.
I'm getting an:
std::bad_variant_access
what(): unexpected index
This is the map…
Niki
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Why can't std::variant find operator<() when not in same namespace as compared classes
I was trying to provide a custom operator< for a class from an external library. This class is within that library's namespace, however, the operator I wanted to define is not. Now if I define a std::variant and want to use it in a std::set, the…
StefanKssmr
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1 answer
std::variant visitor where the visitor modifies the variant
Is it safe to modify a variant inside the visitor function?
struct visitor {
visitor(std::variant & var) : var(var){}
void operator()(int i) {
var = std::to_string(i);
}
void operator()(const std::string &…
jo-art
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