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Questions tagged [void-t]

In C++17, a utility metafunction that maps a sequence of any types to the type void.

19 questions
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C++ Templates - The Complete Guide: Understanding footnote comment about decltype and return type

The 2nd edition of C++ Templates - The Complete Guide features the following code at page 435 #include #include template struct HasBeginT : std::false_type {}; template struct…
Enlico
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SFINAE `std::void_t` class template specialisation

Using SFINAE, I need to detect some properties of a container. For example: #include #include template struct Trait {}; template struct Trait< Container, …
BiagioF
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SFINAE-based detection using void_t and protected nest classes

I encountered some divergent behavior between clang and gcc recently regarding void_t property detection and protected/private class information. Consider a type trait defined as follows: #include template
apmccartney
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Do C++20 Concepts replace other forms of constraints?

C++20 has landed, bringing with it Concepts. If a project were to start now and target only C++20 and later standards, would it be appropriate to say previous forms of constraints are now superseded by Concepts and the requires clause? Are there any…
Mark A. Ropper
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Mixing void_t and variadic templates?

Consider the following code: template > struct is_invokable : std::false_type {}; template struct is_invokable>> :…
Vincent
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Combining void_t and enable_if?

In C++17, void_t allow to easily do SFINAE with class/struct templates: template struct test { static constexpr auto text = "general case"; }; template struct test
Vincent
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Compile time error on my void_t wrapper intead of fallback

Lately i tried to write wrapper arount void_t idiom simple as following: namespace detail { template struct applicable : std::false_type {}; template struct applicable,…
David
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Why does this custom type trait not compile

I'm not sure what I've done wrong below. I'm trying to write a trait can_visit and using the standard pattern of std::void_t and sfinae but it just won't compile and keeps telling me there is an error in the decltype statement containing the…
bradgonesurfing
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SFINAE without void_t (maybe a template specialization question)

Sorry for the title, I am not sure about the category of my question. I am trying to do an is_incrementable with SFINAE. This works fine, however when I try to understand it more deeply, and when I remove the void_t then the code snippet does not…
thamas
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C++ void_t SFINAE false_type true_type can't get specialization

It all seemed simple. Shouldn't has_member_type::value be true for T=vector in my code below? Sorry if it had already be answered. I looked for a while, but couldn't find an answer. #include #include //…
user9987379
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Reliable way of ordering mulitple std::void_t partial specializations for type traits

I want to classify the "level of feature support" for a type. The type can contain aliases red and blue. It can contain just one, both , or neither, and I want to classify each of these cases with an enum class: enum class holder_type { none, …
Jan Schultke
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What is the purpose of "int[]" here: "std::void_t(-1) < static_cast(0)]>;"

This is from an example from a tutorial on std::enable_if. Here is more context: // handle signed types template auto incr1(Int& target, Int amount) -> std::void_t(-1) < static_cast(0)]>; From Shouldn't…
Octo
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Why std::void_t is required to detect ill-formed types in SFINAE context?

I have is_callable trait as below: template struct is_callable: std::false_type {}; template struct is_callable>: std::true_type {}; // (A) There I have to wrap…
oliora
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Why does this use of the detection idiom result in different compilation errors for Clang and GCC and none for MSVC

While playing around with the detection idiom and void_t I found that expressions where the subtraction operator and void* are involved lead to different errors in the tested compilers. GCC 10.2 and below, no matter if C++11,14,17 or 20 is used,…
jesses
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Beside concepts are there any other void_t replacements in C++20?

Assuming I do not want to write a concept (or my compiler does not support them) and there is no matching type trait is there C++20 non void_t way to check if I can construct A with arguments B and C? Note: this is a toy example, real question is is…
NoSenseEtAl
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