Is there a way to get a given number of inputs where the number is given by a template in compile time in...
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 3 hours ago
user5876164
798
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 3 hours ago
user5876164
798
With a for loop.
– tkausl
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
1
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
2 hours ago
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 3 hours ago
user5876164
798
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 3 hours ago
user5876164
798
asked 3 hours ago
user5876164
798
edited 3 hours ago
asked 3 hours ago
user5876164
798
asked 3 hours ago
user5876164
798
asked 3 hours ago
user5876164
798
798
With a for loop.
– tkausl
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
1
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
2 hours ago
add a comment |
With a for loop.
– tkausl
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
1
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
2 hours ago
With a for loop.
– tkausl
3 hours ago
With a for loop.
– tkausl
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
1
1
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
2 hours ago
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
2 hours ago
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>{}(std::tuple(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 2 hours ago
Jans
7,39422233
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
decltype(std::make_index_sequence<N>{})can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
52 mins ago
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
21 mins ago
|
show 2 more comments
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1 Answer
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active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
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>{}(std::tuple(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 2 hours ago
Jans
7,39422233
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
decltype(std::make_index_sequence<N>{})can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
52 mins ago
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
21 mins ago
|
show 2 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>{}(std::tuple(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 2 hours ago
Jans
7,39422233
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
decltype(std::make_index_sequence<N>{})can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
52 mins ago
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
21 mins ago
|
show 2 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>{}(std::tuple(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 2 hours ago
Jans
7,39422233
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>{}(std::tuple(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 2 hours ago
Jans
7,39422233
edited 28 mins ago
answered 2 hours ago
Jans
7,39422233
answered 2 hours ago
Jans
7,39422233
answered 2 hours ago
Jans
7,39422233
7,39422233
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
decltype(std::make_index_sequence<N>{})can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
52 mins ago
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
21 mins ago
|
show 2 more comments
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
decltype(std::make_index_sequence<N>{})can be written concisely asstd::make_index_sequence<N>. Less noise is better.
– StoryTeller
52 mins ago
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
21 mins ago
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
Okay that's pretty neat
– Lightness Races in Orbit
2 hours ago
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
2 hours ago
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
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
@user5876164 I edited the answer with some explanation, hope you understand now.
– Jans
2 hours ago
1
1
decltype(std::make_index_sequence<N>{}) can be written concisely as std::make_index_sequence<N>. Less noise is better.– StoryTeller
52 mins ago
decltype(std::make_index_sequence<N>{}) can be written concisely as std::make_index_sequence<N>. Less noise is better.– StoryTeller
52 mins ago
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
21 mins ago
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
21 mins ago
|
show 2 more comments
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With a for loop.
– tkausl
3 hours ago
@tkausl Can you give an example of how that would look?
– Lightness Races in Orbit
3 hours ago
1
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
2 hours ago