Mixing
How to get template function to use back_inserter over inserter when appropriate
2
How do I create a function that adds the contents of one collection to another, using std::back_inserter() if possible for efficiency? I don't see an obvious trait for push_back() and I'm not an expert with std::enable_if, but I'm hoping some combination will achieve the effect of the following:
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
c++ c++11 templates sfinae enable-if
edited Jan 1 at 16:16
max66
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
add a comment |
2
How do I create a function that adds the contents of one collection to another, using std::back_inserter() if possible for efficiency? I don't see an obvious trait for push_back() and I'm not an expert with std::enable_if, but I'm hoping some combination will achieve the effect of the following:
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
c++ c++11 templates sfinae enable-if
edited Jan 1 at 16:16
max66
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
add a comment |
2
2
2
How do I create a function that adds the contents of one collection to another, using std::back_inserter() if possible for efficiency? I don't see an obvious trait for push_back() and I'm not an expert with std::enable_if, but I'm hoping some combination will achieve the effect of the following:
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
c++ c++11 templates sfinae enable-if
edited Jan 1 at 16:16
max66
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
How do I create a function that adds the contents of one collection to another, using std::back_inserter() if possible for efficiency? I don't see an obvious trait for push_back() and I'm not an expert with std::enable_if, but I'm hoping some combination will achieve the effect of the following:
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
c++ c++11 templates sfinae enable-if
c++ c++11 templates sfinae enable-if
edited Jan 1 at 16:16
max66
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
edited Jan 1 at 16:16
max66
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
edited Jan 1 at 16:16
max66
37.7k74370
edited Jan 1 at 16:16
max66
37.7k74370
edited Jan 1 at 16:16
max66
37.7k74370
37.7k74370
asked Jan 1 at 15:54
xanxan
5,81822439
asked Jan 1 at 15:54
xanxan
5,81822439
asked Jan 1 at 15:54
xanxan
5,81822439
5,81822439
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
3
How do I create a function that adds the contents of one collection to another, using back_inserter if possible for efficiency?
I suppose you can declare a template function that return std::true_type when there is push_back()
template <typename T>
constexpr auto hasPushBack (int)
-> decltype( std::declval<T>().push_back(*(std::declval<T>().begin())),
std::true_type() );
and the fail-back function that return std::false_type
template <typename>
constexpr std::false_type hasPushBack (long);
so you can modify your functions as follows
template<typename CIn, typename COut>
typename std::enable_if<true == decltype(hasPushBack<COut>(0))::value>::type
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
typename std::enable_if<false == decltype(hasPushBack<COut>(0))::value>::type
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
If you can use C++14 or newer, you can also define a template variable with the value
template <typename T>
constexpr bool hasPushBack_v = decltype(hasPushBack<T>(0))::value;
and you can simplify the functions as follows
template<typename CIn, typename COut>
std::enable_if_t<true == hasPushBack_v<COut>>
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
std::enable_if_t<false == hasPushBack_v<COut>>
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
answered Jan 1 at 16:09
max66max66
37.7k74370
add a comment |
2
Just for fun, from C++14 onward you can also use variable template
template <class...> using void_t = void; // (compensate C++14 lack)
template <class T, class = void>
constexpr bool HasPushBack{false};
template <class T>
constexpr bool HasPushBack<T, void_t<
decltype(std::declval<T>().push_back(std::declval<typename std::decay_t<T>::value_type>()))>
>{true};
template<typename CIn, typename COut, std::enable_if_t< HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
template<typename CIn, typename COut, std::enable_if_t<!HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
answered Jan 1 at 16:38
JansJans
9,06422635
Nice and funny. Unfortunatelystd::void_tis C++17 (but it's easy substitute it in C++14)
– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
add a comment |
1
You can apply SFINAE with the help of std::enable_if and std::void_t.
template <typename T, typename = void>
struct has_push_back : std::false_type {};
template <typename T>
struct has_push_back<T, std::void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>
: std::true_type {};
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<!has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
LIVE
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
3
How do I create a function that adds the contents of one collection to another, using back_inserter if possible for efficiency?
I suppose you can declare a template function that return std::true_type when there is push_back()
template <typename T>
constexpr auto hasPushBack (int)
-> decltype( std::declval<T>().push_back(*(std::declval<T>().begin())),
std::true_type() );
and the fail-back function that return std::false_type
template <typename>
constexpr std::false_type hasPushBack (long);
so you can modify your functions as follows
template<typename CIn, typename COut>
typename std::enable_if<true == decltype(hasPushBack<COut>(0))::value>::type
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
typename std::enable_if<false == decltype(hasPushBack<COut>(0))::value>::type
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
If you can use C++14 or newer, you can also define a template variable with the value
template <typename T>
constexpr bool hasPushBack_v = decltype(hasPushBack<T>(0))::value;
and you can simplify the functions as follows
template<typename CIn, typename COut>
std::enable_if_t<true == hasPushBack_v<COut>>
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
std::enable_if_t<false == hasPushBack_v<COut>>
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
answered Jan 1 at 16:09
max66max66
37.7k74370
add a comment |
3
How do I create a function that adds the contents of one collection to another, using back_inserter if possible for efficiency?
I suppose you can declare a template function that return std::true_type when there is push_back()
template <typename T>
constexpr auto hasPushBack (int)
-> decltype( std::declval<T>().push_back(*(std::declval<T>().begin())),
std::true_type() );
and the fail-back function that return std::false_type
template <typename>
constexpr std::false_type hasPushBack (long);
so you can modify your functions as follows
template<typename CIn, typename COut>
typename std::enable_if<true == decltype(hasPushBack<COut>(0))::value>::type
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
typename std::enable_if<false == decltype(hasPushBack<COut>(0))::value>::type
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
If you can use C++14 or newer, you can also define a template variable with the value
template <typename T>
constexpr bool hasPushBack_v = decltype(hasPushBack<T>(0))::value;
and you can simplify the functions as follows
template<typename CIn, typename COut>
std::enable_if_t<true == hasPushBack_v<COut>>
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
std::enable_if_t<false == hasPushBack_v<COut>>
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
answered Jan 1 at 16:09
max66max66
37.7k74370
add a comment |
3
3
3
How do I create a function that adds the contents of one collection to another, using back_inserter if possible for efficiency?
I suppose you can declare a template function that return std::true_type when there is push_back()
template <typename T>
constexpr auto hasPushBack (int)
-> decltype( std::declval<T>().push_back(*(std::declval<T>().begin())),
std::true_type() );
and the fail-back function that return std::false_type
template <typename>
constexpr std::false_type hasPushBack (long);
so you can modify your functions as follows
template<typename CIn, typename COut>
typename std::enable_if<true == decltype(hasPushBack<COut>(0))::value>::type
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
typename std::enable_if<false == decltype(hasPushBack<COut>(0))::value>::type
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
If you can use C++14 or newer, you can also define a template variable with the value
template <typename T>
constexpr bool hasPushBack_v = decltype(hasPushBack<T>(0))::value;
and you can simplify the functions as follows
template<typename CIn, typename COut>
std::enable_if_t<true == hasPushBack_v<COut>>
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
std::enable_if_t<false == hasPushBack_v<COut>>
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
answered Jan 1 at 16:09
max66max66
37.7k74370
How do I create a function that adds the contents of one collection to another, using back_inserter if possible for efficiency?
I suppose you can declare a template function that return std::true_type when there is push_back()
template <typename T>
constexpr auto hasPushBack (int)
-> decltype( std::declval<T>().push_back(*(std::declval<T>().begin())),
std::true_type() );
and the fail-back function that return std::false_type
template <typename>
constexpr std::false_type hasPushBack (long);
so you can modify your functions as follows
template<typename CIn, typename COut>
typename std::enable_if<true == decltype(hasPushBack<COut>(0))::value>::type
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
typename std::enable_if<false == decltype(hasPushBack<COut>(0))::value>::type
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
If you can use C++14 or newer, you can also define a template variable with the value
template <typename T>
constexpr bool hasPushBack_v = decltype(hasPushBack<T>(0))::value;
and you can simplify the functions as follows
template<typename CIn, typename COut>
std::enable_if_t<true == hasPushBack_v<COut>>
addAll (CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::back_inserter(to)); }
template<typename CIn, typename COut>
std::enable_if_t<false == hasPushBack_v<COut>>
addAll(CIn && from, COut && to)
{ std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin())); }
answered Jan 1 at 16:09
max66max66
37.7k74370
edited Jan 1 at 16:23
answered Jan 1 at 16:09
max66max66
37.7k74370
answered Jan 1 at 16:09
max66max66
37.7k74370
answered Jan 1 at 16:09
max66max66
37.7k74370
37.7k74370
add a comment |
add a comment |
2
Just for fun, from C++14 onward you can also use variable template
template <class...> using void_t = void; // (compensate C++14 lack)
template <class T, class = void>
constexpr bool HasPushBack{false};
template <class T>
constexpr bool HasPushBack<T, void_t<
decltype(std::declval<T>().push_back(std::declval<typename std::decay_t<T>::value_type>()))>
>{true};
template<typename CIn, typename COut, std::enable_if_t< HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
template<typename CIn, typename COut, std::enable_if_t<!HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
answered Jan 1 at 16:38
JansJans
9,06422635
Nice and funny. Unfortunatelystd::void_tis C++17 (but it's easy substitute it in C++14)
– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
add a comment |
2
Just for fun, from C++14 onward you can also use variable template
template <class...> using void_t = void; // (compensate C++14 lack)
template <class T, class = void>
constexpr bool HasPushBack{false};
template <class T>
constexpr bool HasPushBack<T, void_t<
decltype(std::declval<T>().push_back(std::declval<typename std::decay_t<T>::value_type>()))>
>{true};
template<typename CIn, typename COut, std::enable_if_t< HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
template<typename CIn, typename COut, std::enable_if_t<!HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
answered Jan 1 at 16:38
JansJans
9,06422635
Nice and funny. Unfortunatelystd::void_tis C++17 (but it's easy substitute it in C++14)
– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
add a comment |
2
2
2
Just for fun, from C++14 onward you can also use variable template
template <class...> using void_t = void; // (compensate C++14 lack)
template <class T, class = void>
constexpr bool HasPushBack{false};
template <class T>
constexpr bool HasPushBack<T, void_t<
decltype(std::declval<T>().push_back(std::declval<typename std::decay_t<T>::value_type>()))>
>{true};
template<typename CIn, typename COut, std::enable_if_t< HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
template<typename CIn, typename COut, std::enable_if_t<!HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
answered Jan 1 at 16:38
JansJans
9,06422635
Just for fun, from C++14 onward you can also use variable template
template <class...> using void_t = void; // (compensate C++14 lack)
template <class T, class = void>
constexpr bool HasPushBack{false};
template <class T>
constexpr bool HasPushBack<T, void_t<
decltype(std::declval<T>().push_back(std::declval<typename std::decay_t<T>::value_type>()))>
>{true};
template<typename CIn, typename COut, std::enable_if_t< HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
template<typename CIn, typename COut, std::enable_if_t<!HasPushBack<COut>,bool> = true>
void addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
answered Jan 1 at 16:38
JansJans
9,06422635
edited Jan 1 at 17:26
answered Jan 1 at 16:38
JansJans
9,06422635
answered Jan 1 at 16:38
JansJans
9,06422635
answered Jan 1 at 16:38
JansJans
9,06422635
9,06422635
Nice and funny. Unfortunatelystd::void_tis C++17 (but it's easy substitute it in C++14)
– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
add a comment |
Nice and funny. Unfortunatelystd::void_tis C++17 (but it's easy substitute it in C++14)
– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
Nice and funny. Unfortunately
std::void_t is C++17 (but it's easy substitute it in C++14)– max66
Jan 1 at 17:03
Nice and funny. Unfortunately
std::void_t is C++17 (but it's easy substitute it in C++14)– max66
Jan 1 at 17:03
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
@max66 - you're right. I added it for completeness.
– Jans
Jan 1 at 17:10
add a comment |
1
You can apply SFINAE with the help of std::enable_if and std::void_t.
template <typename T, typename = void>
struct has_push_back : std::false_type {};
template <typename T>
struct has_push_back<T, std::void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>
: std::true_type {};
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<!has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
LIVE
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
add a comment |
1
You can apply SFINAE with the help of std::enable_if and std::void_t.
template <typename T, typename = void>
struct has_push_back : std::false_type {};
template <typename T>
struct has_push_back<T, std::void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>
: std::true_type {};
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<!has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
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answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
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1
You can apply SFINAE with the help of std::enable_if and std::void_t.
template <typename T, typename = void>
struct has_push_back : std::false_type {};
template <typename T>
struct has_push_back<T, std::void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>
: std::true_type {};
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<!has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
LIVE
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
You can apply SFINAE with the help of std::enable_if and std::void_t.
template <typename T, typename = void>
struct has_push_back : std::false_type {};
template <typename T>
struct has_push_back<T, std::void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>
: std::true_type {};
// IF HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::back_inserter(to));
}
// IF ! HAS_PUSH_BACK:
template<typename CIn, typename COut>
std::enable_if_t<!has_push_back<std::remove_reference_t<COut>>::value> addAll(CIn && from, COut && to) {
std::copy(std::begin(from), std::end(from), std::inserter(to, to.begin()));
}
LIVE
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
answered Jan 1 at 16:22
songyuanyaosongyuanyao
92.6k11178243
92.6k11178243
add a comment |
add a comment |
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