consteval is a modifier introduced in C++20, which informs the compiler that the function is immediate - it can only be calculated at compile time. It is similar to modifier constexpr but it enforces compile time evaluation, not only allows it.
Questions tagged [consteval]
79 questions
80
votes
1 answer
What is consteval?
Apparently, consteval is going to be a keyword in C++20. The cppreference page for it is currently blank. What is it going to be and how does it relate to constexpr?
KevinZ
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34
votes
1 answer
New-expression with consteval constructor in constexpr context
struct A {
consteval A() {};
};
constexpr bool g() {
auto a = new A;
delete a;
return true;
}
int main() {
static_assert(g());
}
https://godbolt.org/z/jsq35WxKs
GCC and MSVC reject the program, ICC and Clang accept…
user17732522
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24
votes
3 answers
Will consteval allow using static_assert on function arguments?
Currently you cannot use static_assert to verify parameters of a constexpr function, even if all calls to it are indeed constexpr.
That makes sense because the compiler still has to create a non-constexpr instantiation of this function in case some…
Piotr Siupa
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19
votes
1 answer
Why does a consteval function allow undefined behavior?
There is a very neat property of constant expressions in C++: their evaluation cannot have undefined behavior (7.7.4.7):
An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine…
Mikhail
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18
votes
1 answer
What is if consteval needed for?
C++23 is going to introduce if consteval. Where is this going to be used and how does it differ from constexpr if?
Claas Bontus
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18
votes
1 answer
consteval vs constexpr on variables
Is there any difference between constexpr and consteval ?
consteval int x1 = 2;
constexpr int x2 = 5;
Is it better to use constexpr than consteval?
Ionut Alexandru
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14
votes
2 answers
Is compiler allowed to call an immediate (consteval) function during runtime?
This might be a stupid question, but I am confused. I had a feeling that an immediate (consteval) function has to be executed during compile time and we simply cannot see its body in the binary.
This article clearly supports my feeling:
This has…
Mikhail
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13
votes
1 answer
Cannot use consteval functions inside initializer list on MSVC
Consider that minimized code snippet:
#include
class Bar
{
public:
constexpr Bar() {}
};
consteval Bar foo()
{
return Bar();
}
int main()
{
std::vector bars{ foo(), foo() };
}
This doesn't compile on latest MSVC…
prapin
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11
votes
1 answer
Can a class with consteval constructor be created on heap in C++?
In the following code struct A has immediate function default constructor, and an object of the struct is created in the dynamic memory be means of new A{}:
struct A {
consteval A() {}
};
int main() {
new A{};
}
Only Clang accepts…
Fedor
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10
votes
1 answer
Non-literal types and constant expressions
struct A {
~A() {}
consteval A() {}
consteval auto f() {}
};
int main() {
A{};
//A{}.f(); //1
}
https://godbolt.org/z/4KPY5P7o7
This program is accepted by ICC, GCC and Clang, but rejected by MSVC which complains that…
user17732522
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10
votes
2 answers
Can `consteval` delegating constructor be called from an ordinary constructor?
In the following struct definition, the constructor A(int) delegates its work to immediate-function constructor A():
struct A {
int i = 0;
consteval A() = default;
A(int) : A() {}
};
Clang accepts it, but not GCC…
Fedor
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10
votes
1 answer
Are `inline` and `noexcept` redundant in a consteval context?
I am working with some code in which constexpr functions are used which I currently refactor to be consteval whenever possible.
inline constexpr auto example() noexcept { /*...*/ }
As I understand it, the inline keyword above is already redundant…
mutableVoid
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8
votes
1 answer
Returning std::vector from an immediate function
Since C++20, std::vector can be used in constant expressions. And as far as I known, current C++ permits dynamically allocate memory under the condition that any such allocation is deallocated by the time the constant expression is "over".
However,…
Fedor
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8
votes
1 answer
Must consteval constructor initialize all data members?
In the next program struct B has immediate consteval default constructor, which does not initialize i field. Then this constructor is used to make a temporary and its i field remains untouched:
struct B {
bool b = true;
int i;
consteval…
Fedor
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8
votes
2 answers
Why can't we use compile-time 'variables' in consteval functions as template parameters?
I was testing this code (https://godbolt.org/z/fe6hhbeqW)...
// Returns the nth type in a parameter pack of types (ommited for clarity)
// template
// nth_type{}
template
struct Typelist{
…
ronaldo
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