using __base_type  = __vector_layout<_Tp, _Allocator>;

  pointer __begin_ = nullptr;#ifdef _LIBCPP_ABI_VECTOR_LAYOUT_SIZE_BASED  size_type __size_     = 0;  size_type __capacity_ = 0;  _LIBCPP_NO_UNIQUE_ADDRESS allocator_type __alloc_;#else  pointer __end_ = nullptr;  _LIBCPP_COMPRESSED_PAIR(pointer, __capacity_ = nullptr, allocator_type, __alloc_);#endif

std::vector<int> a(10);

template <class _Tp, class _Alloc = allocator<_Tp> >class vector;

  explicit vector(size_type __n) {    auto __guard = std::__make_exception_guard(__destroy_vector(*this));    if (__n > 0) {      __vallocate(__n);      __construct_at_end(__n);    }    __guard.__complete();  }

      __vallocate(__n);      __construct_at_end(__n);

void __vallocate(size_type __n) {    // .... 这里省略一些前置检查语句    if (__n > max_size())      this->__throw_length_error();    auto __allocation = std::__allocate_at_least(this->__layout_.__alloc(), __n);    __layout_.__set_layout(__allocation.ptr, 0, __allocation.count);    __annotate_new(0);  }

std::__allocate_at_least

    __layout_.__set_layout(__allocation.ptr, 0, __allocation.count);

template <class _Tp, class _Alloc>void __vector_layout<_Tp, _Alloc>::__set_layout(    pointer __new_begin, size_type __new_size, size_type __new_capacity) _NOEXCEPT {  __begin_    = __new_begin;  __end_      = __new_begin + __new_size;  __capacity_ = __new_begin + __new_capacity;}

//  Default constructs __n objects starting at __layout_.__end_ptr()//  throws if construction throws//  Precondition:  __n > 0//  Precondition:  size() + __n <= capacity()//  Postcondition:  size() == size() + __ntemplate <class _Tp, class _Allocator>void vector<_Tp, _Allocator>::__construct_at_end(size_type __n) {  _ConstructTransaction __tx(*this, __n);  const_pointer __new_end = __tx.__new_end_;  for (pointer __pos = __tx.__pos_; __pos != __new_end; __tx.__pos_ = ++__pos) {    __alloc_traits::construct(this->__layout_.__alloc(), std::__to_address(__pos));  }}

  struct _ConstructTransaction { explicit _ConstructTransaction(vector& __v, size_type __n)        : __v_(__v), __pos_(__v.__layout_.__end_ptr()), __new_end_(__pos_ + __n) {      __v_.__annotate_increase(__n);    }  ~_ConstructTransaction() {      __v_.__layout_.__set_bound_using_pointer(__pos_);      if (__pos_ != __new_end_) {        __v_.__annotate_shrink(__new_end_ - __v_.__layout_.__begin_ptr());      }    }    vector& __v_;    pointer __pos_;    const_pointer const __new_end_;    _ConstructTransaction(_ConstructTransaction const&)            = delete;    _ConstructTransaction& operator=(_ConstructTransaction const&) = delete;  };

    __alloc_traits::construct(this->__layout_.__alloc(), std::__to_address(__pos));

  template <class _Up, class... _Args>  voidconstruct(_Up* __p, _Args&&... __args){    ::new ((void*)__p) _Up(std::forward<_Args>(__args)...);  }

classA；std::vector<A> a(10);

#include<cstdio>#include<vector>#include<iostream>class A {  A(int a) {    std::cout << "A(int)\n";  }};intmain(){    std::vector<A> a(5);    return 0;}

In file included from /home/insights/insights.cpp:2:In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/vector:66:/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_construct.h:133:38: error: no matching constructor for initialization of 'A'  133 |       ::new(static_cast<void*>(__p)) _Tp(std::forward<_Args>(__args)...);      |                                      ^/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_uninitialized.h:899:11: note: in instantiation of function template specialization 'std::_Construct<A>' requested here  899 |             std::_Construct(std::addressof(*__first));      |                  ^/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_uninitialized.h:970:2: note: in instantiation of function template specialization 'std::__uninitialized_default_n_1<false>::__uninit_default_n<A *, unsigned long>' requested here  970 |         __uninit_default_n(__first, __n);      |         ^/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_uninitialized.h:1025:19: note: in instantiation of function template specialization 'std::__uninitialized_default_n<A *, unsigned long>' requested here 1025 |     { return std::__uninitialized_default_n(__first, __n); }      |                   ^/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_vector.h:2001:9: note: in instantiation of function template specialization 'std::__uninitialized_default_n_a<A *, unsigned long, A>' requested here 2001 |           std::__uninitialized_default_n_a(this->_M_impl._M_start, __n,      |                ^/usr/bin/../lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_vector.h:578:9: note: in instantiation of member function 'std::vector<A>::_M_default_initialize' requested here  578 |       { _M_default_initialize(__n); }      |         ^/home/insights/insights.cpp:13:20: note: in instantiation of member function 'std::vector<A>::vector' requested here   13 |     std::vector<A> a(5);      |                    ^/home/insights/insights.cpp:6:3: note: candidate constructor not viable: requires single argument 'a', but no arguments were provided    6 |   A(int a) {      |   ^ ~~~~~/home/insights/insights.cpp:5:7: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were provided    5 | class A {      |       ^/home/insights/insights.cpp:5:7: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 0 were provided    5 | class A {      |       ^1 error generated.Error while processing /home/insights/insights.cpp.

std::vector<int> a{10};