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
4
votes
3 answers
Cleaner way to specify type to get from a std::variant?
I've got code that can be simplified to
std::variant v[2] = foo();
int a = std::get(v[0]);
float b = std::get(v[1]);
Obviously this can go throw if foo() returns the wrong variants, but that's not my problem…
MSalters
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4
votes
1 answer
How to wrap C++17 std::variant with SWIG for use in python?
I am trying to wrap in Python with SWIG some C++17 code that uses std::variant.
I've found this answer (https://stackoverflow.com/a/58513139) about wrapping boost::variant and I managed to adapt the code so that it works with std::variant instead.…
A.Manfreda
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4
votes
1 answer
g++ std::variant seems can't support user class with std::atomic/std::mutex variable member (with detail/code)
If I have a class with std::atomic_bool or std::mutex member for example, and if I put this class inside std::variant, my g++ will complain with "no matching function for call to std::variant<....>". Now I have to declare my std::mutex member to be…
MingC
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4
votes
2 answers
How to use alternative types of std::variant polymorphically
I am writing a parser and am using a std::variant to represent the nodes of the AST. For basic expressions, I have something like:
struct Add;
struct Sub;
struct Mul;
struct Div;
typedef std::variant Node;
(The actual structure…
Willis Blackburn
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4
votes
5 answers
Predefined type list passed to a std::variant
Is there any way to create a pre-defined type list, and use those types in a std::variant in c++ 17? Here is what I'm trying to do, it compiles, but doesn't work as I was hoping:
template < class ... Types > struct type_list {};
using valid_types =…
David Massat
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3
votes
2 answers
How do you write a function template that determines if two arbitrary variants are holding the same type?
Consider the following function for determining if variables of identical variant types are holding the same type:
#include
#include
template
bool hold_same_types(const std::variant& v1, const…
jwezorek
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3
votes
2 answers
Why does std::variant behave differently on GCC 8.5 and GCC 12.1 in respect to a `const char *` literal?
#include
#include
#include
int main()
{
std::variant v{ "hasta la vista" };
std::cout << std::boolalpha << std::holds_alternative(v) << ' ' << std::holds_alternative(v) <<…
user7610
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3
votes
1 answer
Techniques for cutting down on verbosity when do polymorphism via std::variant rather than inheritance
Say you have entities in a 2D game framework or something similar -- e.g. a GUI framework -- where there are various types of entities that share common properties like position and rotation but where some of these properties must be handled on a…
jwezorek
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3
votes
2 answers
C++17 std::variant is slower than dynamic polymorphism?
I was following this blog and was trying to replace a dynamic polymorphism code into using std::variant and std::visit. But I can not get std::variant + std::visit to work better than a virtual struct impl. Its about 1.3-1.5x slower! (GCC 10.3 -O3…
n1r44
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3
votes
1 answer
Observing the state of a variant during construction
Consider the following code:
#include
#include
struct foo {
foo() noexcept;
foo(const foo&) noexcept = default;
foo(foo&&) noexcept = default;
foo& operator=(const foo&) noexcept = default;
foo& operator=(foo&&)…
Jean-Michaël Celerier
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3
votes
1 answer
inconsistent behavior in regard to passing a pointer to a C++17 Standard Variant to function expecting const reference
I'm seeing some odd behavior in regard to passing a pointer into a function that expects a constant reference. Obviously C++ expects me to de reference the pointer before I pass it into the function expecting the reference. I'm looking for an error…
Timothy John Laird
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3
votes
2 answers
Why does assigning an int to a std::variant fail?
I feel like I'm missing something obvious about int type promotion when assigning to a variant.
On gcc version 9.3.0 (Ubuntu 9.3.0-11ubuntu0~18.04.1), compiling with -std=c++17, the following code fails to compile:
#include
#include…
Joe Schober
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3
votes
1 answer
return value from possible type in std::variant through std::visit
I am trying to wrap my head around std::variant and std::visit and I am trying to come up with a method to specify a couple of types that I would like my variable to hold (which would go into my std::variant) and then retrieve stored data through a…
tom
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3
votes
2 answers
std::visit a std::variant with overloaded free-function instead of function-object
In C++17 is there a simple way to std::visit a variant with an overloaded free-function or must I use an object with an overloaded call operator?
In other words, is it possible to add something simple to make the following //ERROR! line compile to…
Timtro
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3
votes
1 answer
Compile error when using std::variant with gmock 1.8 object
Let's have this code using gmock 1.8:
#include "gtest/gtest.h"
#include "gmock/gmock.h"
#include
class Obj {
public:
MOCK_METHOD0( mock, void() );//<-!!!
};
using Variant = std::variant;
TEST(Test, Test) {
Obj obj;
…
AshleyWilkes
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