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
3
votes
0 answers
No template named 'variant' in namespace 'std'
In XCode 10.3 I am getting the error message when I try to build -> No template named 'variant' in namespace 'std'
for the lines of code ->
#include
typedef std::variant…
Adam Freeman
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3
votes
2 answers
Why does a std::variant compile with begin and end iterators?
It seems that the compiler should be able to catch the fact that std::variant doesn't have iterator methods, but it seems that my code compiles with no problem (even if I randomly make up methods or member variables for the variant), but it crashes…
mriera
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3
votes
1 answer
How to get a reference to variant’s value?
I have std::variant where all classes are derived from the same base. I want to cast variant to base.
return std::visit( []( const Base& b ) { return b; }, v );
This compiles but gives warning C4172: returning address of local variable or…
Soonts
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3
votes
3 answers
how to return a specific type from a variant using a visitor?
I have the code below and why visitor1 and visitor2 gives errors?
Does that mean the visitor cannot return one type within the variant?
#include
#include
struct Visitor1
{
template
T operator()(const T & t)…
shelper
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3
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1 answer
Why `std:variant`'s `operator=(T&& t)`'s noexcept spec doesn't depend on inner types's destructor's noexcept spec?
Long title: Why std:variant's operator=(T&& t)'s noexcept specification doesn't depend on inner types's destructor's noexcept specification?
I can see on cppreference that
template variant& operator=(T&& t) noexcept(/* see below…
Mate059
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2
votes
1 answer
Is it okay to inherit from an std::variant in order to make it recursive?
This question comes out of a reddit thread about an Advent of Code problem from last year.
I had mentioned that the only way to make an std::variant recursive was to wrap it in a struct so you can forward declare the struct as there is no way to…
jwezorek
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2
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2 answers
std::variant behavior leads to an error when calling std::get inside std::visit: Is it intended?
I'm currently writing a method in C++17 where a std::variant is visited through std::visit. The variant is embedded in a object that is passed by reference to the lambda function used in std::visit, and this object is then used in the function to…
RaymoAisla
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2
votes
1 answer
Recursive typedef definition using std::variant
I want to define a std::variant that can store a vector of pairs, with strings and Values.
I want to create a structure like below:
typedef std::variant>> Value;
How can I do it in C++17?
Stuart
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2
votes
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struct without constructors containing std::variant containing array cannot be returned by value?
In C++ I want to return a struct by value which contains a variant with an array inside. The struct has no explicit constructors. Is this not possible?
In the following I get the compiler error (gcc):
"error: use of deleted function…
Ernie Mur
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2
votes
2 answers
How to efficiently initialize a std::variant data member in a class template
Consider the following class template, that can hold either a value of type T or an instance of some ErrorInfo class, using a std::variant data member:
template
class ValueOrError
{
private:
std::variant…
Mr.C64
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2
votes
2 answers
Unable to visit a variant if one of its alternatives does not have a specific field
I am trying to visit a variant containing several classes. One of them does not have a specific field value but I handle it with constexpr, however, the compiler still fails to compile.
#include
#include
struct A {};
struct B…
fsquirrel
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2
votes
1 answer
Is it safe to call std::visit on classes with virtual functions?
I'd like to create a hierarchy of visitors to handle instances of std::variant. I'd like to override some of the methods in subclasses for more specialized behavior. My code looks like this:
#include
#include
#include…
Presto
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2
votes
1 answer
C++ variant containing a map of itself
I would like to be able to create a variant that contains a std::map as one of its cases. The ideal would be able to write something like
using MyVariant = std::variant
user17789309
2
votes
4 answers
"increment" `std::variant` alternative
I want to increment / decrement a std::variant's type alternative, essentially like so:
using var_t = std::variant;
var_t var;
var.emplace< (var.index()+1) % std::variant_size >(); // "increment" case, wrapping for good measure
The…
nonchip
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2
votes
1 answer
Extract a c++ variant type from a list of type names
I am designing a pipeline class that needs to extract a std::variant from a list of distinct types in a filter class. For example:
template
struct Filter {
using type = T;
// ...
}
template
struct Pipeline {
…
Jes
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