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
9
votes
2 answers
What is the best way to disable implicit conversion from pointer types to bool when constructing an std::variant?
Consider the following:
struct foo {
};
struct bar {
};
int main()
{
foo f;
bar b;
std::variant v;
v = &b; // compiles in Visual Studio 19 v16.7.3
}
As discussed in comments, I believe the above is legal C++17. There…
jwezorek
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9
votes
1 answer
Is there boost::visit like std::visit, for boost::variant?
With C++14, I'm using boost::variant as a way of compile-time polymorphism:
using MyType = boost::variant;
Both classes have a method sayHello(). I'd like to call:
MyType obj = ...; // either A() or B()
boost::visit([](auto&& o) {…
The Quantum Physicist
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8
votes
1 answer
Compare std::variant with int using C++20 <=> is not a constant expression
Since the std::variant is not allowed to compare with one of its alternative types in the standard library, I am implementing the compare function using C++20 <=> operator:
template
constexpr auto operator<=>(const…
康桓瑋
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8
votes
1 answer
In C++, can I move something out of a std::variant?
I have a resource-wrapper class which is noncopyable, but is movable. Something like this (pseudeocode)
class Wrapper {
SomeResource* m_handle = nullptr;
public:
Wrapper(const Wrapper&) = delete;
Wrapper& operator=(const Wrapper&) = delete;
…
Orion Edwards
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7
votes
1 answer
Can I make lambdas deduce the variant return type?
This mostly theoretical since I can always spell out the return type, but I wonder if there is a way for me to tell lambda that return type should be union(std::variant) of all returns in lambda body.
#include
#include
struct…
NoSenseEtAl
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7
votes
1 answer
Why does clang handle exceptions for this trivial std::variant code?
If we have code like this:
#include
int main(){
using V = std::variant;
V a = 5;
V b = 5.6;
a.swap(b);
}
https://gcc.godbolt.org/z/oqGiHs
If you compile with clang, it emits code that handle exception during…
Nick
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6
votes
1 answer
compiler error with std::variant - use of deleted function error
I have a class C that contains a struct S and a std::variant as members. The struct S has an int member a that is initialized to 0. Here is the code:
#include
class C
{
struct S {
int a = 0;
};
std::variant…
Sir2B
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6
votes
1 answer
well-known overloads for std::visit does not work with reference_wrapper
Here's a sample code: http://coliru.stacked-crooked.com/a/5f630d2d65cd983e
#include
#include
template struct overloads : Ts... { using Ts::operator()...; };
template overloads(Ts &&...) ->…
slyx
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6
votes
1 answer
std::variant of std::string inside any class in constexpr context fails to compile
The following code
#include
#include
#include
constexpr bool USE_VARIANT = 1;
using T = std::conditional_t, std::string>;
struct S
{
T t;
};
constexpr int func()
{
S…
LHLaurini
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6
votes
3 answers
How can classes with `std::variant` members be copied safely?
The following sample builds and runs properly with the line Container container2(container1); removed. It appears the copy constructor for std::variant itself is deleted, which makes my Container's copy constructor implicitly deleted.
Effectively,…
Ευάριστος
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6
votes
2 answers
get currently held typeid of std::variant (like boost::variant type())
I've migrated from boost::variant to std::variant, and hit a snag.
I was using a nice function in boost 'type()' that lets you get the currently held typeid. See https://www.boost.org/doc/libs/1_48_0/doc/html/boost/variant.html#id1752388-bb
How can…
GreekFire
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5
votes
2 answers
How do I tell if a std::variant holds any value at all?
I already know how to use std::variant fairly well with std::get_if(), std::get() and std::visit(). But, what if I just want a simple way to tell if a variant has ever been initialized to any value? That is, I don't care what the value is, I just…
Joe
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5
votes
1 answer
Assignment operator of std::variant of custom type with deleted special member functions?
Consider:
#include
struct A {
A() = default;
A(A&&) = delete;
};
struct B {
B() = delete;
B(A&&) {};
};
int main() {
std::variant v{};
v = A{};
}
MSVC accepted it, while GCC and Clang rejected it with the same error…
康桓瑋
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5
votes
2 answers
is there a way to use std::variant with arbitrarily many defined types?
I am just learning to use std::variant and I would like to declare a type list consisting of (in principle) arbitrarily many of my user defined types. I.e., something like
template
struct MyType{
T x;
};
template
user366202
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5
votes
1 answer
StateMachine with std::variant, getting the custom template deduction right
With the following code how do I write the custom template deduction correctly?
template struct visitor : Ts... { using Ts::operator()...; };
template visitor(State,…
schultz
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