What is the best way to initialize a vector member object in C++11? Will the compiler optimize away the copy in foo or will bar be more efficient?
#include <vector>
using namespace std;
// C++11 style
struct foo
{
vector<int> vec = vector<int>(256);
};
// traditional
struct bar
{
bar() : vec(256) {}
vector<int> vec;
};
In C++11 there probably isn't much difference between them. For example foo does not copy a large vector. The right hand side of the = is an rvalue and will be moved to the left hand side. The only difference is the creation (and quick removal) of the 0-element vector within foo. But that'll take up no time.
But, both C++11 and C++03 allow optimizations ('elision') to skip the assignment in foo. Therefore they can both be very efficient in both standards.
Related
I learned that from C++17, with the deduction guides, template arguments of std::vector can be deduced e.g. from the initialization:
std::vector vec = { function_that_calculate_and_return_a_specifically_templated_type() }
However I do not have the luxury of C++17 in the machine where I want to compile and run the code now.
Is there any possible workaround for C++11? If more solutions exist, the best would be the one that keep the readability of the code.
At the moment the only idea that I have is to track the various cases along the code (luckily they should not be too many) and make some explicit typedef/using.
Any suggestion is very welcome
The usual way to use type deduction for class template when CTAD is not available is providing a make_* function template, e.g. for your case (trailing return type is necessary for C++11):
#include <vector>
#include <type_traits>
#include <tuple>
template <class ...Args>
auto make_vec(Args&&... args) ->
std::vector<typename std::decay<typename std::tuple_element<0, std::tuple<Args...>>::type>::type>
{
using First = typename std::decay<typename std::tuple_element<0, std::tuple<Args...>>::type>::type;
return std::vector<First>{std::forward<Args>(args)...};
}
You can invoke the above with
const auto v = make_vec(1, 2, 3);
which gets at least kind of close to CTAD in the sense that you don't have to explicitly specify the vector instantiation.
While the answer by lubgr is a correct way, the following template is simpler and seems to work as well:
#include <vector>
#include <string>
template <typename T>
std::vector<T> make_vec(const std::initializer_list<T> &list)
{
return std::vector<T>(list);
}
int main()
{
auto v = make_vec({1,2,3});
auto v2 = make_vec({std::string("s")});
std::string s("t");
auto v3 = make_vec({s});
return v.size() + v2.size() + v3.size();
}
One advantage of using the initializer_list template directly are more clear error messages if you pass mixed types like in make_vec({1,2,"x"});, because the construction of the invalid initializer list now happens in non-templated code.
When initializing an atomic class member it requires a 'deleted' function, but adding it would make it no longer trivially copyable which is a requirement for an object/struct to be atomic. Am I just not understanding how to do this correctly, or is this a problem in the c++ standard?
Take the example below:
#include <atomic>
#include <cstdint>
template<typename T>
struct A
{
T * data;
std::atomic<uintptr_t> next;
};
template<typename T>
class B
{
std::atomic<A<T>> myA;
public:
B ( A<T> & a ) noexcept
{
myA.store(a, std::memory_order_relaxed );
}
};
int main ()
{
A<int> a;
B<int> b(a);
return 0;
}
Trying to compile this with g++ gives error: use of deleted function 'A<int>::A(const A<int>&)' myA.store(a, std::memory_order_relaxed);. My understanding of this error is that the atomic::store method is looking for that constructor in my struct A but not finding it.
Now here is what happens when I add that constructor:
#include <atomic>
#include <cstdint>
template<typename T>
struct A
{
T * data;
std::atomic<uintptr_t> next;
A(const A<T>& obj) { }
A( ) { }
};
template<typename T>
class B
{
std::atomic<A<T>> myA;
public:
B ( A<T> & a ) noexcept
{
myA.store(a, std::memory_order_relaxed );
}
};
int main ()
{
A<int> a;
B<int> b(a);
return 0;
}
I no longer receive the above compiler error but a new one coming from the requirements of the atomic class required from 'class B<int>' .... error: static assertion failed: std::atomic requires a trivially copyable type ... In other words by adding the used-defined constructors I have made my struct A a non-trivially copyable object which cannot be initialized in class B. However, without the user-defined constructors I cannot use the store method in myA.store(a, std::memory_order_relaxed).
This seems like a flaw in the design of the std::atomic class. Now maybe I am just doing something wrong because I don't have a lot of experience using C++11 and up (I'm old school). Since 11 there have been a lot of changes and the requirements seem to be a lot stricter. I'm hoping someone can tell me how to achieve what I want to achieve.
Also I cannot change std::atomic<A<T>> myA; to std::atomic<A<T>> * myA; (changed to pointer) or std::atomic<A<T>*> myA;. I realize this will compile but it will destroy the fundamental design of a class I am trying to build.
The problem here resides in the fact that std::atomic requires a trivially copiable type. This because trivially copyable types are the only sure types in C++ which can be directly copied by copying their memory contents directly (eg. through std::memcpy). Also non-formerly trivially copyable types could be safe to raw copy but no assumption can be made on this.
This is indeed important for std::atomic since copy on temporary values is made through std::memcpy, see some implementation details for Clang for example.
Now at the same time std::atomic is not copy constructible, and this is for reasonable reasons, check this answer for example, so it's implicitly not trivially copyable (nor any type which contains them).
If, absurdly, you would allow a std::atomic to contain another std::atomic, and the implementation of std::atomic contains a lock, how would you manage copying it atomically? How should it work?
I noticed that std::for_each requires it's iterators to meet the requirement InputIterator, which in turn requires Iterator and then Copy{Contructable,Assignable}.
That's not the only thing, std::for_each actually uses the copy constructor (cc) (not assignment as far as my configuration goes). That is, deleting the cc from the iterator will result in:
error: use of deleted function ‘some_iterator::some_iterator(const some_iterator&)’
Why does std::for_each need a cc? I found this particularly inconvenient, since I created an iterator which recursively iterates through files in a folder, keeping track of the files and folders on a queue. This means that the iterator has a queue data member, which would also have to be copied if the cc is used: that is unnecessarily inefficient.
The strange thing is that the cc is not called in this simple example:
#include <iostream>
#include <iterator>
#include <algorithm>
class infinite_5_iterator
:
public std::iterator<std::input_iterator_tag, int>
{
public:
infinite_5_iterator() = default;
infinite_5_iterator(infinite_5_iterator const &) {std::cout << "copy constr "; }
infinite_5_iterator &operator=(infinite_5_iterator const &) = delete;
int operator*() { return 5; }
infinite_5_iterator &operator++() { return *this; }
bool operator==(infinite_5_iterator const &) const { return false; }
bool operator!=(infinite_5_iterator const &) const { return true; }
};
int main() {
std::for_each(infinite_5_iterator(), infinite_5_iterator(),
[](int v) {
std::cout << v << ' ';
}
);
}
source: http://ideone.com/YVHph8
It however is needed compile time. Why does std::for_each need to copy construct the iterator, and when is this done? Isn't this extremely inefficient?
NOTE: I'm talking about the cc of the iterator, not of it's elements, as is done here: unexpected copies with foreach over a map
EDIT: Note that the standard does not state the copy-constructor is called at all, it just expresses the amount of times f is called. May I then assume that the cc is not called at all? Why is the use of operator++ and operator* and cc not specified, but the use of f is?
You have simply fallen victim to a specification that has evolved in bits and pieces over decades. The concept of InputIterator was invented a long time before the notion of move-only types, or movable types was conceived.
In hindsight I would love to declare that InputIterator need not be copyable. This would mesh perfectly with its single-pass behavior. But I also fear that such a change would have overwhelming backwards compatibility problems.
In addition to the flawed iterator concepts as specified in the standard, about a decade ago, in an attempt to be helpful, the gcc std::lib (libstdc++) started imposing "concepts" on things like InputIterator in the std-algorithms. I.e. because the standard says:
Requires: InputIterator shall satisfy the requirements of an input iterator (24.2.3).
then "concept checks" were inserted into the std-algorithms that require InputIterator to meet all of the requirements of input iterator whether or not the algorithm actually used all of those requirements. And in this case, it is the concept check, not the actual algorithm, that is requiring your iterator to be CopyConstructible.
<sigh>
If you write your own for_each algorithm, it is trivial to do so without requiring your iterators to be CopyConstructible or CopyAssignable (if supplied with rvalue iterator arguments):
template <class InputIterator, class Function>
inline
Function
for_each(InputIterator first, InputIterator last, Function f)
{
for (; first != last; ++first)
f(*first);
return f;
}
And for your use case I recommend either doing that, or simply writing your own loop.
I have the boost::variant over set of non-default constructible (and maybe even non-moveable/non-copyable and non-copy/move constructible) classes with essentialy different non-default constructor prototypes, as shown below:
#include <boost/variant.hpp>
#include <string>
#include <list>
struct A { A(int) { ; } };
struct B { B(std::string) { ; } };
struct C { C(int, std::string) { ; } };
using V = boost::variant< A const, B const, C const >;
using L = std::list< V >;
int main()
{
L l;
l.push_back(A(1)); // an extra copy/move operation
l.push_back(B("2")); // an extra copy/move operation
l.push_back(C(3, "3")); // an extra copy/move operation
l.emplace_back(4);
l.emplace_back(std::string("5"));
// l.emplace_back(3, std::string("3")); // error here
return 0;
}
I expect, that std::list::emplace_back allows me to construct-and-insert (in single operation) new objects (of all the A, B, C types) into list, even if they have T & operator = (T const &) = delete;/T & operator = (T &&) = delete; and T(T const &) = delete;/T(T &&) = delete;. But what should I do, if constructor is a non-conversion one? I.e. have more, than one parameter. Or what I should to do if two different variant's underlying types have ambiguous constructor prototypes? In my opinion, this is the defect of implementation of the boost::variant library in the light of the new features of C++11 standard, if any at all can be applyed to solve the problem.
I specifically asked about std::list and boost::variant in superposition, because they are both internally implement the pimpl idiom in some form, as far as I know (say, boost::variant currently designed by means of temporary heap backup approach).
emplace can only call the constructors of the type in question. And boost::variant's constructors only take single objects which are unambiguously convertible to one of the variant's types.
variant doesn't forward parameters arbitrarily to one of its bounded types. It just takes a value. A single value that it will try to convert to one of the bounded types.
So you're going to have to construct an object and then copy that into the variant.
Assuming you can modify your "C" class, you could give it an additional constructor that takes a single tuple argument.
I often need to deal with dynamically-allocated arrays in C++, and hence rely on Boost for scoped_array, shared_array, and the like. After reading through Stroustrup's C++11 FAQ and the C++11 Reference Wiki, I could not find a suitable replacement for these dynamic array wrappers that is provided by the C++11 standard. Is there something that I have overlooked, or do I have to continue relying on Boost?
There is a specialization of unique_ptr, like unique_ptr<T[]>.
#include <iostream>
#include <memory>
struct test
{
~test() { std::cout << "test::dtor" << std::endl; }
};
int main()
{
std::unique_ptr<test[]> array(new test[3]);
}
When you run it, you will get this messages.
test::dtor
test::dtor
test::dtor
If you want to use shared_ptr, you should use std::default_delete<T[]> for deleter since it doesn't have one like shared_ptr<t[]>.
std::shared_ptr<test> array(new test[3], std::default_delete<test[]>());
So far as vectors are intended as array wrappers, what if you use any suitable smart pointer with the vector as inner object?