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Questions tagged [variable-templates]

Use this tag for anything related to C++14 variable templates.

Variable templates are a feature of C++ as of C++14. They allow variables to be declared as templates, just like functions and classes:

template<class T>
constexpr T pi = T(3.1415926535897932385);

template<class T>
    T circular_area(T r) {
    return pi<T> * r * r;
}
68 questions
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Is it possible to pass variable template to function through lambda?

As I know how to pass template function as template argument, I'm now struggling to pass variable template in a similar manner. Here is minimal example of what I tried: #define PASS_VARIABLE_TEMPLATE(name) [dummy=nullptr](auto&&...args) \ …
xinaiz
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Variable template fails to link

The following code: template constexpr T foo { 1.2345 }; template T fun(T x) { return -foo * x; } int main() { fun(2.0); } compiled using gcc version 5.1.0 on Linux g++ gcc-bug.cpp -std=c++14 fails during the…
Escualo
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Simulating nan/inf in a constexpr variable template

Is there a way to simulate nan/inf in a constant expression and without! using the C macros HUGE_VAL and INFINITY or any other for that matter! Plus, even with them, it still isn't constexpr. I do not wish to use any standard function that the C++…
DeiDei
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Define a global variable template?

Usually, we do declare but not define a global variable in the header file. However, we define templates in it. Then the issue arises: is it possible to define a global variable template? template uint8_t BitMask =…
埃博拉酱
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C++ Type Traits if_v (automatic type deduction + ensure same type)

Consider the following code #include template constexpr T if_v = std::conditional_t, …
Phil-ZXX
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Float constant evaluates to 0 when utilizing variable template

In the following source, why does the DEG_TO_RAD_B constant evaluate to 0? /// @file Main.cpp // Includes #include #include // Constant definitions template const T PI = std::acos(-T(1)); const float…
S. McPharlin
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Why does const affect linkage of an instantiation of a global variable template in gcc?

I'm taking addresses of the following instantiated variable templates from two translation units: template bool b = true; template const bool cb = true; template inline const bool icb = true; I'm printing addresses of…
Mikhail
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Visual Studio no template variables defined in type_traits

When I tried use std::extent_v template it was underscored with red and I got error "namespace std doesn't contain member extent_v" in IDE, but it compiles without an error ( is of course included). When I found its definition in…
aaalex88
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Spirit-X3 parsers stored in variable template specializations not working on Clang

I have a working Spirit-X3 parser, that can parse two closely related grammars for setting up draughts and checkers positions. I define two variable templates specializations as parsers for the two dialects of the grammar: // general variable…
TemplateRex
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variable templates and std::enable_if

Can I use enable_if (or is there some alternative technique available) with template variables. e.g. typedef float Float; typedef double Double; template constexpr Bool IsFloat = std::is_same_v; template constexpr Bool…
user673679
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How to introduce static_assert into template variable definition

How to introduce static_assert into template variable definition? My attemption is to use lambda function: #include #include #include namespace { #pragma clang diagnostic push #pragma clang diagnostic ignored…
Tomilov Anatoliy
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Variable template at class scope

Using N3651 as a basis, A variable template at class scope is a static data member template. The example given is: struct matrix_constants { template using pauli = hermitian_matrix; Yet all of the following…
user1508519
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Recursive variable template specialization (and MSVC behaviour)

Given the code: template struct list{}; template list<> foo{}; template list>)> foo>{}; what is the type of foo>? I'm asking this…
rm -rf
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Detecting if a generic lambda with certain arguments is invocable

I am experimenting with some lambda detection functionality. What I am trying to achieve is to be able to detect if I can call a lambda F with an argument of type const C& returning a result convertible to bool. The code below looks close to what I…
Dmitriy Kumshayev
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Why is there no variable template template parameter?

I'm planning to create a variable template that takes (variable) template-template parameter and one typename: template