initializer_list immutable nature leads to excessi

问题:

Why does the access to std::initializer_list not allow us to change its content? It's a big disadvantage of std::initializer_list when using it for its main purpose (to initialize a container), since it's use leads to excessive copy-construction/copy-assignment, instead of move-construction/move-assignment.

#include <initializer_list>
#include <iostream>
#include <vector>

#include <cstdlib>

struct A
{

    A() = default;
    A(A const &) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
    A(A &&) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
    A & operator = (A const &) { std::cout << __PRETTY_FUNCTION__ << std::endl; return *this; }
    A & operator = (A &&) { std::cout << __PRETTY_FUNCTION__ << std::endl; return *this; }

};

int
main()
{
    std::vector< A >{A{}, A{}, A{}};
    return EXIT_SUCCESS;
}

Output (as expected):

A::A(const A &)
A::A(const A &)
A::A(const A &)

Why is its design so constrained?

回答1:

There is a recent proposal for movable initializer lists, where, in particular, the authors say:

std::initializer_list was designed around 2005 (N1890) to 2007 (N2215), before move semantics matured, around 2009. At the time, it was not anticipated that copy semantics would be insufficient or even suboptimal for common value-like classes. There was a 2008 proposal N2801 Initializer lists and move semantics but C++0x was already felt to be slipping at that time, and by 2011 the case had gone cold.

回答2:

good (if unfortunate) answer by Anton.

Here's the source code of the implementation in libc++:

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
vector<_Tp, _Allocator>::vector(initializer_list<value_type> __il)
{
#if _LIBCPP_DEBUG_LEVEL >= 2
    __get_db()->__insert_c(this);
#endif
    if (__il.size() > 0)
    {
        allocate(__il.size());
        __construct_at_end(__il.begin(), __il.end());
    }
}

no move iterators in sight, hence copy construction.

in case it's useful, here's a workaround using a variadic argument list:

#include <initializer_list>
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <utility>

#include <cstdlib>

struct A
{

    A() noexcept{ std::cout << __PRETTY_FUNCTION__ << std::endl; }
    A(A const &) noexcept { std::cout << __PRETTY_FUNCTION__ << std::endl; }
    A & operator  = (A const &) noexcept { std::cout << __PRETTY_FUNCTION__ << std::endl; return *this; }
    A(A &&) noexcept { std::cout << __PRETTY_FUNCTION__ << std::endl; }
    A & operator = (A &&) noexcept { std::cout << __PRETTY_FUNCTION__ << std::endl; return *this; }

};

template<class T, class...Args>
void append_it(std::vector<T>& v)
{
}

template<class T, class...Args>
void append_it(std::vector<T>& v, T&& t1, Args&&...args)
{
    v.push_back(std::move(t1));
    append_it(v, std::forward<Args&&>(args)...);
}

template<class T, class...Args>
std::vector<T> make_vector(T&& t1, Args&&...args)
{
    std::vector<T> result;
    result.reserve(1 + sizeof...(args));
    result.push_back(std::move(t1));
    append_it(result, std::forward<Args&&>(args)...);
    return result;
}

int
main()
{
    auto v2 = make_vector( A{}, A{}, A{} );

    return EXIT_SUCCESS;
}