Fold expression: iterate over variadic template type parameter to check compile-time conditions on the...
What I want to do is simple: in a variadic class template, I want to check some compile-time condition on the types. In this case, I want to find out if a certain type is in the pack or not. This is what the code might have looked like with C++17's fold expressions, but obviously that's not valid syntax. How to implement it?
#include <type_traits>
template <class... Types>
struct TypesPack
{
template <typename T>
static constexpr bool hasType() {
return std::is_same<T, Types>::value || ... || false;
}
};
c++ templates c++17
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
add a comment |
What I want to do is simple: in a variadic class template, I want to check some compile-time condition on the types. In this case, I want to find out if a certain type is in the pack or not. This is what the code might have looked like with C++17's fold expressions, but obviously that's not valid syntax. How to implement it?
#include <type_traits>
template <class... Types>
struct TypesPack
{
template <typename T>
static constexpr bool hasType() {
return std::is_same<T, Types>::value || ... || false;
}
};
c++ templates c++17
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
1
Since you're using C++17, you can take advantage of the convenient_vadditions to traits:std::is_same_v<T, U>==std::is_same<T, U>::value.
– chris
Nov 11 at 23:01
add a comment |
What I want to do is simple: in a variadic class template, I want to check some compile-time condition on the types. In this case, I want to find out if a certain type is in the pack or not. This is what the code might have looked like with C++17's fold expressions, but obviously that's not valid syntax. How to implement it?
#include <type_traits>
template <class... Types>
struct TypesPack
{
template <typename T>
static constexpr bool hasType() {
return std::is_same<T, Types>::value || ... || false;
}
};
c++ templates c++17
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
What I want to do is simple: in a variadic class template, I want to check some compile-time condition on the types. In this case, I want to find out if a certain type is in the pack or not. This is what the code might have looked like with C++17's fold expressions, but obviously that's not valid syntax. How to implement it?
#include <type_traits>
template <class... Types>
struct TypesPack
{
template <typename T>
static constexpr bool hasType() {
return std::is_same<T, Types>::value || ... || false;
}
};
c++ templates c++17
c++ templates c++17
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
edited Nov 12 at 16:06
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
asked Nov 11 at 22:50
Violet Giraffe
14.3k26131240
14.3k26131240
1
Since you're using C++17, you can take advantage of the convenient_vadditions to traits:std::is_same_v<T, U>==std::is_same<T, U>::value.
– chris
Nov 11 at 23:01
add a comment |
1
Since you're using C++17, you can take advantage of the convenient_vadditions to traits:std::is_same_v<T, U>==std::is_same<T, U>::value.
– chris
Nov 11 at 23:01
1
1
Since you're using C++17, you can take advantage of the convenient
_v additions to traits: std::is_same_v<T, U> == std::is_same<T, U>::value.– chris
Nov 11 at 23:01
Since you're using C++17, you can take advantage of the convenient
_v additions to traits: std::is_same_v<T, U> == std::is_same<T, U>::value.– chris
Nov 11 at 23:01
add a comment |
1 Answer
1
active
oldest
votes
static constexpr bool hasType() {
return (std::is_same<T, Types>::value || ...);
}
A fold-expression must be parenthesized, and you're allowed to omit the false when using || as the operator.
answered Nov 11 at 22:52
Brian
63.9k794179
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
static constexpr bool hasType() {
return (std::is_same<T, Types>::value || ...);
}
A fold-expression must be parenthesized, and you're allowed to omit the false when using || as the operator.
answered Nov 11 at 22:52
Brian
63.9k794179
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
add a comment |
static constexpr bool hasType() {
return (std::is_same<T, Types>::value || ...);
}
A fold-expression must be parenthesized, and you're allowed to omit the false when using || as the operator.
answered Nov 11 at 22:52
Brian
63.9k794179
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
add a comment |
static constexpr bool hasType() {
return (std::is_same<T, Types>::value || ...);
}
A fold-expression must be parenthesized, and you're allowed to omit the false when using || as the operator.
answered Nov 11 at 22:52
Brian
63.9k794179
static constexpr bool hasType() {
return (std::is_same<T, Types>::value || ...);
}
A fold-expression must be parenthesized, and you're allowed to omit the false when using || as the operator.
answered Nov 11 at 22:52
Brian
63.9k794179
answered Nov 11 at 22:52
Brian
63.9k794179
answered Nov 11 at 22:52
Brian
63.9k794179
answered Nov 11 at 22:52
Brian
63.9k794179
63.9k794179
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
add a comment |
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
Will it also work if there's only one type in the pack?
– Violet Giraffe
Nov 12 at 5:45
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
@VioletGiraffe Yes, it will even work if there are zero.
– Brian
Nov 12 at 5:59
add a comment |
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1
Since you're using C++17, you can take advantage of the convenient
_vadditions to traits:std::is_same_v<T, U>==std::is_same<T, U>::value.– chris
Nov 11 at 23:01