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How can I use a constexpr value in a lambda?


How can I use a constexpr value in a lambda?

By : Don Klayman
Date : November 21 2020, 09:01 AM
help you fix your problem I would want to use a constexpr value in a lambda. Reading the answer to Using lambda captured constexpr value as an array dimension, I assumed the following should work: ,
Should this be considered a bug in clang 3.8?
code :


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C++ VS2015 constexpr compile error with constexpr constructor call to constexpr member function

C++ VS2015 constexpr compile error with constexpr constructor call to constexpr member function


By : Jared Hobbs
Date : March 29 2020, 07:55 AM
will be helpful for those in need Your class is not a literal because you have a non-trivial destructor.
According to the C++ standard
code :
~HashedString() = default;
Using a lambda in a `constexpr` function in a non-`constexpr` context: clang vs gcc

Using a lambda in a `constexpr` function in a non-`constexpr` context: clang vs gcc


By : Joaquin000
Date : March 29 2020, 07:55 AM
I wish did fix the issue. Both of the compilers are in agreement that get_x() can't be used in a constant expression. You can tell by changing auto res = get_x(); to constexpr auto res = get_x();, where GCC will reject it equally.
As for detecting it at function definition time like clang does rather than at function use like gcc does, both are allowed: (emphasis mine)
Is there a way to pass a `constexpr` value into lambda so that it remains `constexpr` inside that lambda?

Is there a way to pass a `constexpr` value into lambda so that it remains `constexpr` inside that lambda?


By : user2125746
Date : March 29 2020, 07:55 AM
Does that help Use a generic lambda and a constexpr conversion operator :
code :
template <typename... ValueTypes>
void traverse(ValueTypes... values)
{
    static_for<0, sizeof...(ValueTypes)>([&](auto I)
    //                                       ~~~^
    {
        auto v = value_by_index<I>(values...);
        //                     ~^~
        std::cout << v << std::endl;
    });
}
template <typename... ValueTypes>
void traverse(ValueTypes... values)
{
    static_for<0, sizeof...(ValueTypes)>([&]<int I>(std::integral_constant<int, I>)
    //                                       ~~~~^                             ~^~
    {
        auto v = value_by_index<I>(values...);
        //                     ~^~
        std::cout << v << std::endl;
    });
}
Constexpr variable captured inside lambda loses its constexpr-ness

Constexpr variable captured inside lambda loses its constexpr-ness


By : Jeff
Date : March 29 2020, 07:55 AM
I wish this helpful for you Gcc is right. b (as constexpr variable) doesn't need to be captured in fact.
code :
template<class T> void f(){
    constexpr static bool b=std::is_same_v<T,int>;
    auto func_x=[](){
        if constexpr(b){
        }else{
        }
    };
    func_x();
}
Clang claims constexpr member of generic lambda argument is not constexpr

Clang claims constexpr member of generic lambda argument is not constexpr


By : user2964701
Date : March 29 2020, 07:55 AM
wish helps you Since this_r is a static member you can always access it without dereferencing (non constexpr) reference to object instance to make clang or other compilers happy:
code :
int f(std::variant<S<0>, S<1>, S<2> > v) {
    return std::visit([](auto const& arg) {
        if constexpr(::std::remove_reference_t<decltype(arg)>::this_r == 0) { return 42; }
        else { return ::std::remove_reference_t<decltype(arg)>::this_r; }
    }, v);
}
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