int、int* 和 int[] 与 std::unique_ptr 之间的区别 [关闭]答案

【问题标题】：Differences between int, int* and int[] with std::unique_ptr [closed]int、int* 和 int[] 与 std::unique_ptr 之间的区别 [关闭]
【发布时间】：2014-10-29 15:35:17
【问题描述】：

既然你可以做到std::unique_ptr<int> upi{new int}，我立刻就对为什么std::unique_ptr<int*> upi{new int[42]{}} 不起作用感到困惑。有人能解释一下以下sn-ps之间的区别吗？

{
    std::unique_ptr<int> upi{new int[42]{}};
    upi.get()[0];
}

// Doesn't work
{
    std::unique_ptr<int> upi{new int[42]{}};
    upi[0];
}

{
    std::unique_ptr<int[]> upi{new int[42]{}};
    upi.get()[0];
}    

{
    std::unique_ptr<int[]> upi{new int[42]{}};
    upi[0];
}     

// Doesn't work
{
    std::unique_ptr<int*> upi{new int[42]{}};
    upi.get()[0];
}      

// Doesn't work    
{
    std::unique_ptr<int*> upi{new int[42]{}};
    upi[0];
}

编译器错误：

prog.cpp: In function ‘int main()’:
prog.cpp:8:20: warning: value computed is not used [-Wunused-value]
         upi.get()[0];
                    ^
prog.cpp:14:12: error: no match for ‘operator[]’ (operand types are ‘std::unique_ptr<int>’ and ‘int’)
         upi[0];
            ^
prog.cpp:19:20: warning: value computed is not used [-Wunused-value]
         upi.get()[0];
                    ^
prog.cpp:29:48: error: no matching function for call to ‘std::unique_ptr<int*>::unique_ptr(<brace-enclosed initializer list>)’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
prog.cpp:29:48: note: candidates are:
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:176:2: note: template<class _Up, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::auto_ptr<_Up>&&)
  unique_ptr(auto_ptr<_Up>&& __u) noexcept;
  ^
/usr/include/c++/4.8/bits/unique_ptr.h:176:2: note:   template argument deduction/substitution failed:
prog.cpp:29:48: note:   mismatched types ‘std::auto_ptr<_Up>’ and ‘int*’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:169:2: note: template<class _Up, class _Ep, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Up, _Ep>&&)
  unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept
  ^
/usr/include/c++/4.8/bits/unique_ptr.h:169:2: note:   template argument deduction/substitution failed:
prog.cpp:29:48: note:   mismatched types ‘std::unique_ptr<_Tp, _Dp>’ and ‘int*’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:160:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>&&) [with _Tp = int*; _Dp = std::default_delete<int*>]
       unique_ptr(unique_ptr&& __u) noexcept
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:160:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::unique_ptr<int*>&&’
/usr/include/c++/4.8/bits/unique_ptr.h:157:17: note: constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr(std::nullptr_t) [with _Tp = int*; _Dp = std::default_delete<int*>; std::nullptr_t = std::nullptr_t]
       constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
                 ^
/usr/include/c++/4.8/bits/unique_ptr.h:157:17: note:   no known conversion for argument 1 from ‘int*’ to ‘std::nullptr_t’
/usr/include/c++/4.8/bits/unique_ptr.h:151:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer, typename std::remove_reference<_To>::type&&) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**; typename std::remove_reference<_To>::type = std::default_delete<int*>]
       unique_ptr(pointer __p,
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:151:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.8/bits/unique_ptr.h:146:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer, typename std::conditional<std::is_reference<_Dp>::value, _Dp, const _Dp&>::type) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**; typename std::conditional<std::is_reference<_Dp>::value, _Dp, const _Dp&>::type = const std::default_delete<int*>&]
       unique_ptr(pointer __p,
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:146:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.8/bits/unique_ptr.h:141:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**]
       unique_ptr(pointer __p) noexcept
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:141:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::unique_ptr<int*>::pointer {aka int**}’
/usr/include/c++/4.8/bits/unique_ptr.h:135:17: note: constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr() [with _Tp = int*; _Dp = std::default_delete<int*>]
       constexpr unique_ptr() noexcept
                 ^
/usr/include/c++/4.8/bits/unique_ptr.h:135:17: note:   candidate expects 0 arguments, 1 provided
prog.cpp:30:20: warning: value computed is not used [-Wunused-value]
         upi.get()[0];
                    ^
prog.cpp:35:48: error: no matching function for call to ‘std::unique_ptr<int*>::unique_ptr(<brace-enclosed initializer list>)’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
prog.cpp:35:48: note: candidates are:
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:176:2: note: template<class _Up, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::auto_ptr<_Up>&&)
  unique_ptr(auto_ptr<_Up>&& __u) noexcept;
  ^
/usr/include/c++/4.8/bits/unique_ptr.h:176:2: note:   template argument deduction/substitution failed:
prog.cpp:35:48: note:   mismatched types ‘std::auto_ptr<_Up>’ and ‘int*’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:169:2: note: template<class _Up, class _Ep, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Up, _Ep>&&)
  unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept
  ^
/usr/include/c++/4.8/bits/unique_ptr.h:169:2: note:   template argument deduction/substitution failed:
prog.cpp:35:48: note:   mismatched types ‘std::unique_ptr<_Tp, _Dp>’ and ‘int*’
         std::unique_ptr<int*> upi{new int[42]{}};
                                                ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:160:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>&&) [with _Tp = int*; _Dp = std::default_delete<int*>]
       unique_ptr(unique_ptr&& __u) noexcept
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:160:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::unique_ptr<int*>&&’
/usr/include/c++/4.8/bits/unique_ptr.h:157:17: note: constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr(std::nullptr_t) [with _Tp = int*; _Dp = std::default_delete<int*>; std::nullptr_t = std::nullptr_t]
       constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
                 ^
/usr/include/c++/4.8/bits/unique_ptr.h:157:17: note:   no known conversion for argument 1 from ‘int*’ to ‘std::nullptr_t’
/usr/include/c++/4.8/bits/unique_ptr.h:151:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer, typename std::remove_reference<_To>::type&&) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**; typename std::remove_reference<_To>::type = std::default_delete<int*>]
       unique_ptr(pointer __p,
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:151:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.8/bits/unique_ptr.h:146:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer, typename std::conditional<std::is_reference<_Dp>::value, _Dp, const _Dp&>::type) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**; typename std::conditional<std::is_reference<_Dp>::value, _Dp, const _Dp&>::type = const std::default_delete<int*>&]
       unique_ptr(pointer __p,
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:146:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.8/bits/unique_ptr.h:141:7: note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer) [with _Tp = int*; _Dp = std::default_delete<int*>; std::unique_ptr<_Tp, _Dp>::pointer = int**]
       unique_ptr(pointer __p) noexcept
       ^
/usr/include/c++/4.8/bits/unique_ptr.h:141:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::unique_ptr<int*>::pointer {aka int**}’
/usr/include/c++/4.8/bits/unique_ptr.h:135:17: note: constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr() [with _Tp = int*; _Dp = std::default_delete<int*>]
       constexpr unique_ptr() noexcept
                 ^
/usr/include/c++/4.8/bits/unique_ptr.h:135:17: note:   candidate expects 0 arguments, 1 provided
prog.cpp:36:12: error: no match for ‘operator[]’ (operand types are ‘std::unique_ptr<int*>’ and ‘int’)
         upi[0];
            ^

【问题讨论】：

你能不能再说“不工作”？
int 不是int*。你所拥有的不是编译，更不用说“工作”了。
这意味着我得到了编译器错误。s
总是发布您遇到的实际错误，并显示哪些行产生了错误。
+1 用于发布错误。为了进一步改进，请稍微整理一下错误。比如第一个警告：你明白了吗？也许通过将错误放在代码旁边来将错误与代码联系起来。

标签： c++ pointers c++11 unique-ptr

【解决方案1】：

 std::unique_ptr<int> upi{new int[42]{}};

具有未定义的行为。您正在使用new[] 分配内存，而unique_ptr 将在其析构函数中调用delete，而不是delete[]。

upi[0];

无法编译，因为 unique_ptr::operator[] 重载只存在于 unique_ptr<T[]> 部分特化。

std::unique_ptr<int*> upi{new int[42]{}};

无法编译，因为unique_ptr 构造函数需要int**，但您传递给它的是int*。如果它确实编译了，由于与前一种情况相同的原因，它会有未定义的行为。

你应该使用

std::unique_ptr<int[]> upi{new int[42]{}};
upi.get()[0] = 0;  // this works
upi[0] = 0;        // and so does this

【讨论】：

【解决方案2】：

为了使用unique_ptr的数组版本，您必须在第一个模板参数中指定一个数组类型（即int[]而不是int），否则您将使用指针版本。

int[] 可以降级为int*。您可以将new int[] 传递给unique_ptr<int> 的构造函数，因为它需要int* 作为输入，但是您不能访问upi[] 运算符，因为它在unique_ptr 的那个版本中不存在，只有在使用@ 时改为 987654331@。

【讨论】：

如何为指针版本编写特化？
如果你想使用std::unique_ptr<int*>，你必须给构造函数提供一个int**，比如std::unique_ptr<int*> upi{new int*[42]{}};，但是这当然是错误的，因为delete[]不会被使用。您必须改用std::unique_ptr<int*[]> upi{new int[42]{}};。
让upi[0]为std::unique_ptr<int>工作
upi[] 不适用于std::unique_ptr<int>，因为operator[] 在该版本中根本不存在。甚至不要尝试使用它。无论如何，将int[] 放入unique_ptr<int> 是错误的。您必须改用std::unique_ptr<int[]>。这就是它的用途。不要规避这一点。