c++ - 为什么下面的代码不能用 gcc 编译但用 clang 编译得很好

Question

下面的代码可以用 clang 编译，但不能用 gcc 编译，请问这是 gcc 中的错误吗？

它只是一个包含 unique_ptr vector 和 std::function 作为成员的类，当我创建此类的 vector 时，我不能对此说保留或调整大小。 push_back 与 std::move 一起工作正常，而这只发生在 gcc 而不是 clang 上。

#include <algorithm>                                                                                                                                               
#include <memory>                                                                                                                                                          
#include <utility>                                                                                                                                                         
#include <iostream>  
#include <vector>
#include <functional>

using namespace std;                                                                                                                                                       

class ABC                                                                                                                                                                  
{                                                                                                                                                                          
public:                                                                                                                                                                    
    ABC()                                                                                                                                                                  
          {}                                                                                                                                                               
  private:                                                                                                                                                                 
    std::vector<std::unique_ptr<int>> up;                                                                                                                                  
    std::function<void (int*)> func;                                                                                                                                       
};                                                                                                                                                                         

int main()                                                                                                                                                                 
{                                                                                                                                                                          
    ABC a;                                                                                                                                                                 
    std::vector<ABC> vec;                                                                                                                                                  
    vec.reserve(1);                                                                                                                                                        
}    

gcc 的错误信息如下所示

In file included from /opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_tempbuf.h:60:0,
                 from /opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_algo.h:62,
                 from /opt/wandbox/gcc-7.1.0/include/c++/7.1.0/algorithm:62,
                 from prog.cc:1:
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<int>; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}]':
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:83:18:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; bool _TrivialValueTypes = false]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:134:15:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:289:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; _Tp = std::unique_ptr<int>]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_vector.h:331:31:   required from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<int>; _Alloc = std::allocator<std::unique_ptr<int> >]'
prog.cc:10:7:   required from 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = ABC; _Args = {const ABC&}]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:83:18:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const ABC*; _ForwardIterator = ABC*; bool _TrivialValueTypes = false]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:134:15:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const ABC*; _ForwardIterator = ABC*]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_uninitialized.h:289:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const ABC*; _ForwardIterator = ABC*; _Tp = ABC]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_vector.h:1263:35:   required from 'std::vector<_Tp, _Alloc>::pointer std::vector<_Tp, _Alloc>::_M_allocate_and_copy(std::vector<_Tp, _Alloc>::size_type, _ForwardIterator, _ForwardIterator) [with _ForwardIterator = const ABC*; _Tp = ABC; _Alloc = std::allocator<ABC>; std::vector<_Tp, _Alloc>::pointer = ABC*; std::vector<_Tp, _Alloc>::size_type = long unsigned int]'
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/vector.tcc:73:40:   required from 'void std::vector<_Tp, _Alloc>::reserve(std::vector<_Tp, _Alloc>::size_type) [with _Tp = ABC; _Alloc = std::allocator<ABC>; std::vector<_Tp, _Alloc>::size_type = long unsigned int]'
prog.cc:24:18:   required from here
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/stl_construct.h:75:7: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]'
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/wandbox/gcc-7.1.0/include/c++/7.1.0/memory:80:0,
                 from prog.cc:2:
/opt/wandbox/gcc-7.1.0/include/c++/7.1.0/bits/unique_ptr.h:388:7: note: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^~~~~~~~~~

Answer 1

发生这种情况是因为 std::function 的 move ctor 不是 noexcept，但是 std::vector 只能使用 move ctor 如果它是 noexcept(强异常保证)。

问题是 std::unique_ptr 是(显然)不可复制的，所以这使得 ABC 作为一个整体不可复制。要使 ABC noexcept 隐式可移动，它需要它的每个成员也是 noexcept - 可移动的。

如果删除 std::function，就会发生这种情况:.resize() 不需要复制 A.up(std::vector 的移动运算符是 noexcept)，所以 std::unique_ptr(在 up) 可以移动，一切正常。

参见 this coliru : 如果您评论 noexcept，vec.reserve() 将需要复制所有内容，问题又回来了。

添加 ABC(ABC&&) = default 解决了这个问题，因为用户声明的(虽然 defaulted)移动构造函数禁止复制构造函数的生成(参见 here ) .

我们也可以反其道而行之，在coliru中手动删除A的复制构造函数(并保留move ctor not noexcept ): here .

---

要解决这个问题，您可以将 std::function 存储在 std::unique_ptr 中，它有一个 nothrow 移动构造函数。 See here