Is there a way to get a given number of inputs where the number is given by a template in compile time in c++?
Clash Royale CLAN TAG#URR8PPP
For example, suppose I make a class like below:
template <unsigned int INPUT_SIZE>
class A
public:
int operator()(int input, ...) // get INPUT_SIZE-many inputs
// return sum;
;
I want to get input as many as INPUT_SIZE, not more or less. How can I achieve that?
Also, I am using c++11, but if there is a better way in c++14 or above, I would also like to know.
c++ templates
asked Dec 23 '18 at 4:29
user5876164
848
add a comment |
For example, suppose I make a class like below:
template <unsigned int INPUT_SIZE>
class A
public:
int operator()(int input, ...) // get INPUT_SIZE-many inputs
// return sum;
;
I want to get input as many as INPUT_SIZE, not more or less. How can I achieve that?
Also, I am using c++11, but if there is a better way in c++14 or above, I would also like to know.
c++ templates
asked Dec 23 '18 at 4:29
user5876164
848
With a for loop.
– tkausl
Dec 23 '18 at 4:30
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
3
Does it need to be SFINAE-friendly? If not, I'd use astatic_assertinside the body ofoperator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put thesizeof...in arequiresclause.
– Daniel H
Dec 23 '18 at 4:50
1
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27
add a comment |
For example, suppose I make a class like below:
template <unsigned int INPUT_SIZE>
class A
public:
int operator()(int input, ...) // get INPUT_SIZE-many inputs
// return sum;
;
I want to get input as many as INPUT_SIZE, not more or less. How can I achieve that?
Also, I am using c++11, but if there is a better way in c++14 or above, I would also like to know.
c++ templates
asked Dec 23 '18 at 4:29
user5876164
848
For example, suppose I make a class like below:
template <unsigned int INPUT_SIZE>
class A
public:
int operator()(int input, ...) // get INPUT_SIZE-many inputs
// return sum;
;
I want to get input as many as INPUT_SIZE, not more or less. How can I achieve that?
Also, I am using c++11, but if there is a better way in c++14 or above, I would also like to know.
c++ templates
c++ templates
asked Dec 23 '18 at 4:29
user5876164
848
asked Dec 23 '18 at 4:29
user5876164
848
edited Dec 23 '18 at 4:32
asked Dec 23 '18 at 4:29
user5876164
848
asked Dec 23 '18 at 4:29
user5876164
848
asked Dec 23 '18 at 4:29
user5876164
848
848
With a for loop.
– tkausl
Dec 23 '18 at 4:30
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
3
Does it need to be SFINAE-friendly? If not, I'd use astatic_assertinside the body ofoperator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put thesizeof...in arequiresclause.
– Daniel H
Dec 23 '18 at 4:50
1
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27
add a comment |
With a for loop.
– tkausl
Dec 23 '18 at 4:30
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
3
Does it need to be SFINAE-friendly? If not, I'd use astatic_assertinside the body ofoperator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put thesizeof...in arequiresclause.
– Daniel H
Dec 23 '18 at 4:50
1
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27
With a for loop.
– tkausl
Dec 23 '18 at 4:30
With a for loop.
– tkausl
Dec 23 '18 at 4:30
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
3
3
Does it need to be SFINAE-friendly? If not, I'd use a
static_assert inside the body of operator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put the sizeof... in a requires clause.– Daniel H
Dec 23 '18 at 4:50
Does it need to be SFINAE-friendly? If not, I'd use a
static_assert inside the body of operator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put the sizeof... in a requires clause.– Daniel H
Dec 23 '18 at 4:50
1
1
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27
add a comment |
1 Answer
1
active
oldest
votes
Live demo 1
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
int operator ()(always_t<T, Is>...)
return 0;
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
A<2>(1, 2); // fine
A<2>(1, 2, 3); // fail
and this is a version that allows you to compute the sum of the parameters:
Live demo 2
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
constexpr int operator ()(std::tuple<always_t<T, Is>...>&& t)
auto adder = (auto... ts)
return (0 + ... + ts);
;
return std::apply(adder, std::move(t));
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
constexpr int sum = A<3>(1, 4, 5);
static_assert(sum == 10);
The trick is to use a parameter pack with length N so that we can use it to expand as N times a specific type into the parameter list of A_impl::operator().
A parameter pack can expand into N repetition of the pattern that (usually) precede ...
Consider a function like:
template<class... T>
void foo(T...);
T... indicate in simple terms that it can be replaced by successive types into the parameter list of foo, one possible expansion could be foo(int, int, double, char), also notice that what preside ... is an identifier that comes from class... T.
Returning to the code, we need to generate a parameter pack, we did that through std::make_index_sequence<N>, that generate the sequence 0..(N-1) which is captured by std::size_t... Is, then we use this pack to expand the pattern always_t<T, Is> that is just an alias to T=int, this end up repeating T=int as many times as elements Is contains.
Note: ellipsis parameter ... is not the same as parameter pack.
answered Dec 23 '18 at 4:45
Jans
8,40522635
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
Is it okay if I ask how this code works? I have never used...in practice with templates so it is hard for me to understand.
– user5876164
Dec 23 '18 at 4:58
1
decltype(std::make_index_sequence<N>)can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
Dec 23 '18 at 6:47
1
always_tis just a an alias template, is used to allows for parameter packIsappear on it and be used as a pack pattern.e.galways_t<int, 0> == int
– Jans
Dec 23 '18 at 7:18
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
|
show 3 more comments
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Live demo 1
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
int operator ()(always_t<T, Is>...)
return 0;
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
A<2>(1, 2); // fine
A<2>(1, 2, 3); // fail
and this is a version that allows you to compute the sum of the parameters:
Live demo 2
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
constexpr int operator ()(std::tuple<always_t<T, Is>...>&& t)
auto adder = (auto... ts)
return (0 + ... + ts);
;
return std::apply(adder, std::move(t));
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
constexpr int sum = A<3>(1, 4, 5);
static_assert(sum == 10);
The trick is to use a parameter pack with length N so that we can use it to expand as N times a specific type into the parameter list of A_impl::operator().
A parameter pack can expand into N repetition of the pattern that (usually) precede ...
Consider a function like:
template<class... T>
void foo(T...);
T... indicate in simple terms that it can be replaced by successive types into the parameter list of foo, one possible expansion could be foo(int, int, double, char), also notice that what preside ... is an identifier that comes from class... T.
Returning to the code, we need to generate a parameter pack, we did that through std::make_index_sequence<N>, that generate the sequence 0..(N-1) which is captured by std::size_t... Is, then we use this pack to expand the pattern always_t<T, Is> that is just an alias to T=int, this end up repeating T=int as many times as elements Is contains.
Note: ellipsis parameter ... is not the same as parameter pack.
answered Dec 23 '18 at 4:45
Jans
8,40522635
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
Is it okay if I ask how this code works? I have never used...in practice with templates so it is hard for me to understand.
– user5876164
Dec 23 '18 at 4:58
1
decltype(std::make_index_sequence<N>)can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
Dec 23 '18 at 6:47
1
always_tis just a an alias template, is used to allows for parameter packIsappear on it and be used as a pack pattern.e.galways_t<int, 0> == int
– Jans
Dec 23 '18 at 7:18
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
|
show 3 more comments
Live demo 1
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
int operator ()(always_t<T, Is>...)
return 0;
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
A<2>(1, 2); // fine
A<2>(1, 2, 3); // fail
and this is a version that allows you to compute the sum of the parameters:
Live demo 2
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
constexpr int operator ()(std::tuple<always_t<T, Is>...>&& t)
auto adder = (auto... ts)
return (0 + ... + ts);
;
return std::apply(adder, std::move(t));
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
constexpr int sum = A<3>(1, 4, 5);
static_assert(sum == 10);
The trick is to use a parameter pack with length N so that we can use it to expand as N times a specific type into the parameter list of A_impl::operator().
A parameter pack can expand into N repetition of the pattern that (usually) precede ...
Consider a function like:
template<class... T>
void foo(T...);
T... indicate in simple terms that it can be replaced by successive types into the parameter list of foo, one possible expansion could be foo(int, int, double, char), also notice that what preside ... is an identifier that comes from class... T.
Returning to the code, we need to generate a parameter pack, we did that through std::make_index_sequence<N>, that generate the sequence 0..(N-1) which is captured by std::size_t... Is, then we use this pack to expand the pattern always_t<T, Is> that is just an alias to T=int, this end up repeating T=int as many times as elements Is contains.
Note: ellipsis parameter ... is not the same as parameter pack.
answered Dec 23 '18 at 4:45
Jans
8,40522635
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
Is it okay if I ask how this code works? I have never used...in practice with templates so it is hard for me to understand.
– user5876164
Dec 23 '18 at 4:58
1
decltype(std::make_index_sequence<N>)can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
Dec 23 '18 at 6:47
1
always_tis just a an alias template, is used to allows for parameter packIsappear on it and be used as a pack pattern.e.galways_t<int, 0> == int
– Jans
Dec 23 '18 at 7:18
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
|
show 3 more comments
Live demo 1
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
int operator ()(always_t<T, Is>...)
return 0;
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
A<2>(1, 2); // fine
A<2>(1, 2, 3); // fail
and this is a version that allows you to compute the sum of the parameters:
Live demo 2
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
constexpr int operator ()(std::tuple<always_t<T, Is>...>&& t)
auto adder = (auto... ts)
return (0 + ... + ts);
;
return std::apply(adder, std::move(t));
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
constexpr int sum = A<3>(1, 4, 5);
static_assert(sum == 10);
The trick is to use a parameter pack with length N so that we can use it to expand as N times a specific type into the parameter list of A_impl::operator().
A parameter pack can expand into N repetition of the pattern that (usually) precede ...
Consider a function like:
template<class... T>
void foo(T...);
T... indicate in simple terms that it can be replaced by successive types into the parameter list of foo, one possible expansion could be foo(int, int, double, char), also notice that what preside ... is an identifier that comes from class... T.
Returning to the code, we need to generate a parameter pack, we did that through std::make_index_sequence<N>, that generate the sequence 0..(N-1) which is captured by std::size_t... Is, then we use this pack to expand the pattern always_t<T, Is> that is just an alias to T=int, this end up repeating T=int as many times as elements Is contains.
Note: ellipsis parameter ... is not the same as parameter pack.
answered Dec 23 '18 at 4:45
Jans
8,40522635
Live demo 1
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
int operator ()(always_t<T, Is>...)
return 0;
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
A<2>(1, 2); // fine
A<2>(1, 2, 3); // fail
and this is a version that allows you to compute the sum of the parameters:
Live demo 2
template <class T, auto> using always_t = T;
template <class T, class Arity>
struct A_impl;
template <class T, std::size_t... Is>
struct A_impl<T, std::index_sequence<Is...>>
constexpr int operator ()(std::tuple<always_t<T, Is>...>&& t)
auto adder = (auto... ts)
return (0 + ... + ts);
;
return std::apply(adder, std::move(t));
;
template <std::size_t N>
struct A : A_impl<int, std::make_index_sequence<N>>
;
constexpr int sum = A<3>(1, 4, 5);
static_assert(sum == 10);
The trick is to use a parameter pack with length N so that we can use it to expand as N times a specific type into the parameter list of A_impl::operator().
A parameter pack can expand into N repetition of the pattern that (usually) precede ...
Consider a function like:
template<class... T>
void foo(T...);
T... indicate in simple terms that it can be replaced by successive types into the parameter list of foo, one possible expansion could be foo(int, int, double, char), also notice that what preside ... is an identifier that comes from class... T.
Returning to the code, we need to generate a parameter pack, we did that through std::make_index_sequence<N>, that generate the sequence 0..(N-1) which is captured by std::size_t... Is, then we use this pack to expand the pattern always_t<T, Is> that is just an alias to T=int, this end up repeating T=int as many times as elements Is contains.
Note: ellipsis parameter ... is not the same as parameter pack.
answered Dec 23 '18 at 4:45
Jans
8,40522635
edited Dec 23 '18 at 15:39
answered Dec 23 '18 at 4:45
Jans
8,40522635
answered Dec 23 '18 at 4:45
Jans
8,40522635
answered Dec 23 '18 at 4:45
Jans
8,40522635
8,40522635
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
Is it okay if I ask how this code works? I have never used...in practice with templates so it is hard for me to understand.
– user5876164
Dec 23 '18 at 4:58
1
decltype(std::make_index_sequence<N>)can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
Dec 23 '18 at 6:47
1
always_tis just a an alias template, is used to allows for parameter packIsappear on it and be used as a pack pattern.e.galways_t<int, 0> == int
– Jans
Dec 23 '18 at 7:18
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
|
show 3 more comments
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
Is it okay if I ask how this code works? I have never used...in practice with templates so it is hard for me to understand.
– user5876164
Dec 23 '18 at 4:58
1
decltype(std::make_index_sequence<N>)can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
Dec 23 '18 at 6:47
1
always_tis just a an alias template, is used to allows for parameter packIsappear on it and be used as a pack pattern.e.galways_t<int, 0> == int
– Jans
Dec 23 '18 at 7:18
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
Okay that's pretty neat
– Lightness Races in Orbit
Dec 23 '18 at 4:47
1
1
Is it okay if I ask how this code works? I have never used
... in practice with templates so it is hard for me to understand.– user5876164
Dec 23 '18 at 4:58
Is it okay if I ask how this code works? I have never used
... in practice with templates so it is hard for me to understand.– user5876164
Dec 23 '18 at 4:58
1
1
decltype(std::make_index_sequence<N>) can be written concisely as std::make_index_sequence<N>. Less noise is better.– StoryTeller
Dec 23 '18 at 6:47
decltype(std::make_index_sequence<N>) can be written concisely as std::make_index_sequence<N>. Less noise is better.– StoryTeller
Dec 23 '18 at 6:47
1
1
always_t is just a an alias template, is used to allows for parameter pack Is appear on it and be used as a pack pattern.e.g always_t<int, 0> == int– Jans
Dec 23 '18 at 7:18
always_t is just a an alias template, is used to allows for parameter pack Is appear on it and be used as a pack pattern.e.g always_t<int, 0> == int– Jans
Dec 23 '18 at 7:18
1
1
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
Note that all that examples use C++17, but can be rewritten in C++11 with relative small changes.
– Jarod42
Dec 23 '18 at 8:25
|
show 3 more comments
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With a for loop.
– tkausl
Dec 23 '18 at 4:30
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
Dec 23 '18 at 4:34
3
Does it need to be SFINAE-friendly? If not, I'd use a
static_assertinside the body ofoperator(); if so, I'd go with Jans's answer until C++20, at which point I think you can put thesizeof...in arequiresclause.– Daniel H
Dec 23 '18 at 4:50
1
See also: Number of Variadic Template Function Parameters?
– user202729
Dec 23 '18 at 9:27