generating functions at compile time
0
I am trying to generate functions at compile time using boost hana. Here is the code I wrote
#include <boost/hana/transform.hpp>
#include <array>
template<int i>
double f(double x)
{
return x * i;
}
int main()
{
constexpr std::array arr = {1,5,10,100,500};
constexpr auto functions = hana::transform(arr,
(const int a) -> double (*)(double)
{
return f<a>;
}
);
}
when compiling I get the error that f is not convertible to type double (*)(double).
I think the problem is that a is not constexpr(which is not possible since it is a function argument). Is there a way to make this working?
c++ templates metaprogramming
asked Nov 12 '18 at 21:08
user3726947
304210
add a comment |
0
I am trying to generate functions at compile time using boost hana. Here is the code I wrote
#include <boost/hana/transform.hpp>
#include <array>
template<int i>
double f(double x)
{
return x * i;
}
int main()
{
constexpr std::array arr = {1,5,10,100,500};
constexpr auto functions = hana::transform(arr,
(const int a) -> double (*)(double)
{
return f<a>;
}
);
}
when compiling I get the error that f is not convertible to type double (*)(double).
I think the problem is that a is not constexpr(which is not possible since it is a function argument). Is there a way to make this working?
c++ templates metaprogramming
asked Nov 12 '18 at 21:08
user3726947
304210
No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
Well it would be interesting if you wrote aconstexprfunction and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo ishana::transformconstexpr? Probably not...
– Galik
Nov 12 '18 at 21:46
@galik - "That might work" - Have you tried? If theavalue (the value used forf<a>) is passed through a template parameter, should works,constexpror notconstexpr. But ifais passed as normal parameter, shouldn't works, also inside aconstexprfunction, because aconstexprfunction can be called also run-time.
– max66
Nov 12 '18 at 22:48
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35
add a comment |
0
0
0
1
I am trying to generate functions at compile time using boost hana. Here is the code I wrote
#include <boost/hana/transform.hpp>
#include <array>
template<int i>
double f(double x)
{
return x * i;
}
int main()
{
constexpr std::array arr = {1,5,10,100,500};
constexpr auto functions = hana::transform(arr,
(const int a) -> double (*)(double)
{
return f<a>;
}
);
}
when compiling I get the error that f is not convertible to type double (*)(double).
I think the problem is that a is not constexpr(which is not possible since it is a function argument). Is there a way to make this working?
c++ templates metaprogramming
asked Nov 12 '18 at 21:08
user3726947
304210
I am trying to generate functions at compile time using boost hana. Here is the code I wrote
#include <boost/hana/transform.hpp>
#include <array>
template<int i>
double f(double x)
{
return x * i;
}
int main()
{
constexpr std::array arr = {1,5,10,100,500};
constexpr auto functions = hana::transform(arr,
(const int a) -> double (*)(double)
{
return f<a>;
}
);
}
when compiling I get the error that f is not convertible to type double (*)(double).
I think the problem is that a is not constexpr(which is not possible since it is a function argument). Is there a way to make this working?
c++ templates metaprogramming
c++ templates metaprogramming
asked Nov 12 '18 at 21:08
user3726947
304210
asked Nov 12 '18 at 21:08
user3726947
304210
edited Nov 12 '18 at 21:18
asked Nov 12 '18 at 21:08
user3726947
304210
asked Nov 12 '18 at 21:08
user3726947
304210
asked Nov 12 '18 at 21:08
user3726947
304210
304210
No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
Well it would be interesting if you wrote aconstexprfunction and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo ishana::transformconstexpr? Probably not...
– Galik
Nov 12 '18 at 21:46
@galik - "That might work" - Have you tried? If theavalue (the value used forf<a>) is passed through a template parameter, should works,constexpror notconstexpr. But ifais passed as normal parameter, shouldn't works, also inside aconstexprfunction, because aconstexprfunction can be called also run-time.
– max66
Nov 12 '18 at 22:48
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35
add a comment |
No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
Well it would be interesting if you wrote aconstexprfunction and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo ishana::transformconstexpr? Probably not...
– Galik
Nov 12 '18 at 21:46
@galik - "That might work" - Have you tried? If theavalue (the value used forf<a>) is passed through a template parameter, should works,constexpror notconstexpr. But ifais passed as normal parameter, shouldn't works, also inside aconstexprfunction, because aconstexprfunction can be called also run-time.
– max66
Nov 12 '18 at 22:48
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35
No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
Well it would be interesting if you wrote a
constexpr function and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo is hana::transform constexpr? Probably not...– Galik
Nov 12 '18 at 21:46
Well it would be interesting if you wrote a
constexpr function and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo is hana::transform constexpr? Probably not...– Galik
Nov 12 '18 at 21:46
@galik - "That might work" - Have you tried? If the
a value (the value used for f<a>) is passed through a template parameter, should works, constexpr or not constexpr. But if a is passed as normal parameter, shouldn't works, also inside a constexpr function, because a constexpr function can be called also run-time.– max66
Nov 12 '18 at 22:48
@galik - "That might work" - Have you tried? If the
a value (the value used for f<a>) is passed through a template parameter, should works, constexpr or not constexpr. But if a is passed as normal parameter, shouldn't works, also inside a constexpr function, because a constexpr function can be called also run-time.– max66
Nov 12 '18 at 22:48
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35
add a comment |
1 Answer
1
active
oldest
votes
-1
Is there a way to make this working?
Not this way.
Look at your lambda
(const int a) -> double (*)(double)
{
return f<a>;
}
You're using the argument a, that is potentially known run-time in a lambda, as template parameter, that must be known compile type.
Can't work.
answered Nov 12 '18 at 21:19
max66
34.4k63762
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
@user3726947 - I don't see a way withstd::array. Is good for you if you usestd::integer_sequence; something asstd::integer_sequence<int, 1, 5, 10, 100, 500>?
– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
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active
oldest
votes
-1
Is there a way to make this working?
Not this way.
Look at your lambda
(const int a) -> double (*)(double)
{
return f<a>;
}
You're using the argument a, that is potentially known run-time in a lambda, as template parameter, that must be known compile type.
Can't work.
answered Nov 12 '18 at 21:19
max66
34.4k63762
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
@user3726947 - I don't see a way withstd::array. Is good for you if you usestd::integer_sequence; something asstd::integer_sequence<int, 1, 5, 10, 100, 500>?
– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
add a comment |
-1
Is there a way to make this working?
Not this way.
Look at your lambda
(const int a) -> double (*)(double)
{
return f<a>;
}
You're using the argument a, that is potentially known run-time in a lambda, as template parameter, that must be known compile type.
Can't work.
answered Nov 12 '18 at 21:19
max66
34.4k63762
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
@user3726947 - I don't see a way withstd::array. Is good for you if you usestd::integer_sequence; something asstd::integer_sequence<int, 1, 5, 10, 100, 500>?
– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
add a comment |
-1
-1
-1
Is there a way to make this working?
Not this way.
Look at your lambda
(const int a) -> double (*)(double)
{
return f<a>;
}
You're using the argument a, that is potentially known run-time in a lambda, as template parameter, that must be known compile type.
Can't work.
answered Nov 12 '18 at 21:19
max66
34.4k63762
Is there a way to make this working?
Not this way.
Look at your lambda
(const int a) -> double (*)(double)
{
return f<a>;
}
You're using the argument a, that is potentially known run-time in a lambda, as template parameter, that must be known compile type.
Can't work.
answered Nov 12 '18 at 21:19
max66
34.4k63762
answered Nov 12 '18 at 21:19
max66
34.4k63762
answered Nov 12 '18 at 21:19
max66
34.4k63762
answered Nov 12 '18 at 21:19
max66
34.4k63762
34.4k63762
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
@user3726947 - I don't see a way withstd::array. Is good for you if you usestd::integer_sequence; something asstd::integer_sequence<int, 1, 5, 10, 100, 500>?
– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
add a comment |
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
@user3726947 - I don't see a way withstd::array. Is good for you if you usestd::integer_sequence; something asstd::integer_sequence<int, 1, 5, 10, 100, 500>?
– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
so how can I make it work?
– user3726947
Nov 12 '18 at 21:20
1
1
@user3726947 - I don't see a way with
std::array. Is good for you if you use std::integer_sequence; something as std::integer_sequence<int, 1, 5, 10, 100, 500>?– max66
Nov 12 '18 at 21:22
@user3726947 - I don't see a way with
std::array. Is good for you if you use std::integer_sequence; something as std::integer_sequence<int, 1, 5, 10, 100, 500>?– max66
Nov 12 '18 at 21:22
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
ok i'll look into that, thanks!
– user3726947
Nov 12 '18 at 21:24
add a comment |
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No, there is no way to instantiate a template with a runtime value.
– Galik
Nov 12 '18 at 21:18
but the array is known at compile time...
– user3726947
Nov 12 '18 at 21:22
Well it would be interesting if you wrote a
constexprfunction and used that instead of your lambda. That might work (I don't know how far constexpr is willing to introspect). Aslo ishana::transformconstexpr? Probably not...– Galik
Nov 12 '18 at 21:46
@galik - "That might work" - Have you tried? If the
avalue (the value used forf<a>) is passed through a template parameter, should works,constexpror notconstexpr. But ifais passed as normal parameter, shouldn't works, also inside aconstexprfunction, because aconstexprfunction can be called also run-time.– max66
Nov 12 '18 at 22:48
Also Lambdas can be constexpr so this makes no diffrence
– user3726947
Nov 13 '18 at 23:35