How to set std::array size via parameter pack arguments?
I have an N-dimensional Matrix class that has a constructor with a parameter pack. Is it possible to set the size of the std::array member variable depending on the values in the parameter pack? As far as I understand the values in the parameter pack should be known at compile time.
template<size_t N>
class Matrix {
public:
template<typename... Exts>
Matrix(Exts... exts) : dimSizes{exts...} { }
private:
std::array<size_t, N> dimSizes;
std::array<float, N> data;
// e.g something like this: std::array<float, dimSizes[0]> data;
};
int main(void) {
Matrix<3> mat(2, 3, 2);
return 0;
}
c++ c++11 variadic-templates variadic-functions stdarray
edited Nov 25 '18 at 18:43
max66
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
|
show 3 more comments
I have an N-dimensional Matrix class that has a constructor with a parameter pack. Is it possible to set the size of the std::array member variable depending on the values in the parameter pack? As far as I understand the values in the parameter pack should be known at compile time.
template<size_t N>
class Matrix {
public:
template<typename... Exts>
Matrix(Exts... exts) : dimSizes{exts...} { }
private:
std::array<size_t, N> dimSizes;
std::array<float, N> data;
// e.g something like this: std::array<float, dimSizes[0]> data;
};
int main(void) {
Matrix<3> mat(2, 3, 2);
return 0;
}
c++ c++11 variadic-templates variadic-functions stdarray
edited Nov 25 '18 at 18:43
max66
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
3
You can't havesizeof(Matrx<3>)magically vary depending on how an instance was constructed.
– Igor Tandetnik
Nov 25 '18 at 18:14
@JesperJuhl I don't think so. I have to specify the size of thestd::arrayvia a template argument. So I have to specify the size as soon as I declare it.
– SPA
Nov 25 '18 at 18:15
@SPA what stops you from usingstd::vector<float>(dimSizes[0])?
– Piotr Skotnicki
Nov 25 '18 at 18:23
|
show 3 more comments
I have an N-dimensional Matrix class that has a constructor with a parameter pack. Is it possible to set the size of the std::array member variable depending on the values in the parameter pack? As far as I understand the values in the parameter pack should be known at compile time.
template<size_t N>
class Matrix {
public:
template<typename... Exts>
Matrix(Exts... exts) : dimSizes{exts...} { }
private:
std::array<size_t, N> dimSizes;
std::array<float, N> data;
// e.g something like this: std::array<float, dimSizes[0]> data;
};
int main(void) {
Matrix<3> mat(2, 3, 2);
return 0;
}
c++ c++11 variadic-templates variadic-functions stdarray
edited Nov 25 '18 at 18:43
max66
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
I have an N-dimensional Matrix class that has a constructor with a parameter pack. Is it possible to set the size of the std::array member variable depending on the values in the parameter pack? As far as I understand the values in the parameter pack should be known at compile time.
template<size_t N>
class Matrix {
public:
template<typename... Exts>
Matrix(Exts... exts) : dimSizes{exts...} { }
private:
std::array<size_t, N> dimSizes;
std::array<float, N> data;
// e.g something like this: std::array<float, dimSizes[0]> data;
};
int main(void) {
Matrix<3> mat(2, 3, 2);
return 0;
}
c++ c++11 variadic-templates variadic-functions stdarray
c++ c++11 variadic-templates variadic-functions stdarray
edited Nov 25 '18 at 18:43
max66
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
edited Nov 25 '18 at 18:43
max66
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
edited Nov 25 '18 at 18:43
max66
36.1k74165
edited Nov 25 '18 at 18:43
max66
36.1k74165
edited Nov 25 '18 at 18:43
max66
36.1k74165
36.1k74165
asked Nov 25 '18 at 18:10
SPASPA
154
asked Nov 25 '18 at 18:10
SPASPA
154
asked Nov 25 '18 at 18:10
SPASPA
154
154
Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
3
You can't havesizeof(Matrx<3>)magically vary depending on how an instance was constructed.
– Igor Tandetnik
Nov 25 '18 at 18:14
@JesperJuhl I don't think so. I have to specify the size of thestd::arrayvia a template argument. So I have to specify the size as soon as I declare it.
– SPA
Nov 25 '18 at 18:15
@SPA what stops you from usingstd::vector<float>(dimSizes[0])?
– Piotr Skotnicki
Nov 25 '18 at 18:23
|
show 3 more comments
Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
3
You can't havesizeof(Matrx<3>)magically vary depending on how an instance was constructed.
– Igor Tandetnik
Nov 25 '18 at 18:14
@JesperJuhl I don't think so. I have to specify the size of thestd::arrayvia a template argument. So I have to specify the size as soon as I declare it.
– SPA
Nov 25 '18 at 18:15
@SPA what stops you from usingstd::vector<float>(dimSizes[0])?
– Piotr Skotnicki
Nov 25 '18 at 18:23
Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
3
3
You can't have
sizeof(Matrx<3>) magically vary depending on how an instance was constructed.– Igor Tandetnik
Nov 25 '18 at 18:14
You can't have
sizeof(Matrx<3>) magically vary depending on how an instance was constructed.– Igor Tandetnik
Nov 25 '18 at 18:14
@JesperJuhl I don't think so. I have to specify the size of the
std::array via a template argument. So I have to specify the size as soon as I declare it.– SPA
Nov 25 '18 at 18:15
@JesperJuhl I don't think so. I have to specify the size of the
std::array via a template argument. So I have to specify the size as soon as I declare it.– SPA
Nov 25 '18 at 18:15
@SPA what stops you from using
std::vector<float>(dimSizes[0]) ?– Piotr Skotnicki
Nov 25 '18 at 18:23
@SPA what stops you from using
std::vector<float>(dimSizes[0]) ?– Piotr Skotnicki
Nov 25 '18 at 18:23
|
show 3 more comments
1 Answer
1
active
oldest
votes
Is it possible to set the size of the
std::arraymember variable depending on the values in the parameter pack?
// e.g something like this:
std::array<float, dimSizes[0]> data;
No, as far I know is impossible exactly as you want.
Because, this way, different instances of the same class would contain members with same name but different types. Strictly forbidden in a strongly typed language as C++.
If you want a std::array with different size, you have to differentiate the types; so the dimension for the second std::array has to be a template parameter.
Obviously you can substitute the std::array with a container that doesn't depend from the size; as suggested by Piotr Skotnicki a possible solution is std::vector
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
add a comment |
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1 Answer
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1 Answer
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Is it possible to set the size of the
std::arraymember variable depending on the values in the parameter pack?
// e.g something like this:
std::array<float, dimSizes[0]> data;
No, as far I know is impossible exactly as you want.
Because, this way, different instances of the same class would contain members with same name but different types. Strictly forbidden in a strongly typed language as C++.
If you want a std::array with different size, you have to differentiate the types; so the dimension for the second std::array has to be a template parameter.
Obviously you can substitute the std::array with a container that doesn't depend from the size; as suggested by Piotr Skotnicki a possible solution is std::vector
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
add a comment |
Is it possible to set the size of the
std::arraymember variable depending on the values in the parameter pack?
// e.g something like this:
std::array<float, dimSizes[0]> data;
No, as far I know is impossible exactly as you want.
Because, this way, different instances of the same class would contain members with same name but different types. Strictly forbidden in a strongly typed language as C++.
If you want a std::array with different size, you have to differentiate the types; so the dimension for the second std::array has to be a template parameter.
Obviously you can substitute the std::array with a container that doesn't depend from the size; as suggested by Piotr Skotnicki a possible solution is std::vector
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
add a comment |
Is it possible to set the size of the
std::arraymember variable depending on the values in the parameter pack?
// e.g something like this:
std::array<float, dimSizes[0]> data;
No, as far I know is impossible exactly as you want.
Because, this way, different instances of the same class would contain members with same name but different types. Strictly forbidden in a strongly typed language as C++.
If you want a std::array with different size, you have to differentiate the types; so the dimension for the second std::array has to be a template parameter.
Obviously you can substitute the std::array with a container that doesn't depend from the size; as suggested by Piotr Skotnicki a possible solution is std::vector
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
Is it possible to set the size of the
std::arraymember variable depending on the values in the parameter pack?
// e.g something like this:
std::array<float, dimSizes[0]> data;
No, as far I know is impossible exactly as you want.
Because, this way, different instances of the same class would contain members with same name but different types. Strictly forbidden in a strongly typed language as C++.
If you want a std::array with different size, you have to differentiate the types; so the dimension for the second std::array has to be a template parameter.
Obviously you can substitute the std::array with a container that doesn't depend from the size; as suggested by Piotr Skotnicki a possible solution is std::vector
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
edited Nov 25 '18 at 18:35
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
answered Nov 25 '18 at 18:29
max66max66
36.1k74165
36.1k74165
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
add a comment |
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
2
2
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
"Because, this way, different instances of the same class would contain members with same name but different types." Good argument. I completly missed that.
– SPA
Nov 25 '18 at 18:39
add a comment |
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Can't you use the initialisation list of the constructor?
– Jesper Juhl
Nov 25 '18 at 18:11
Sort of in c++17, no (mostly) before. Unless I warp your question.
– Yakk - Adam Nevraumont
Nov 25 '18 at 18:13
3
You can't have
sizeof(Matrx<3>)magically vary depending on how an instance was constructed.– Igor Tandetnik
Nov 25 '18 at 18:14
@JesperJuhl I don't think so. I have to specify the size of the
std::arrayvia a template argument. So I have to specify the size as soon as I declare it.– SPA
Nov 25 '18 at 18:15
@SPA what stops you from using
std::vector<float>(dimSizes[0])?– Piotr Skotnicki
Nov 25 '18 at 18:23