C++ int、int*和int[]与std:：unique_ptr之间的差异

由于您可以执行

std:：unique\u ptr upi{new int}

，我立即感到困惑，为什么

std:：unique\u ptr upi{new int[42]{}

不起作用。有人能解释一下以下片段之间的区别吗

{
    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];
}

{
std:：unique_ptr upi{new int[42]{}；
upi.get（）[0]；
}
//不起作用
{
std:：unique_ptr upi{new int[42]{}；
upi[0]；
}
{
std:：unique_ptr upi{new int[42]{}；
upi.get（）[0]；
}    
{
std:：unique_ptr upi{new int[42]{}；
upi[0]；
}     
//不起作用
{
std:：unique_ptr upi{new int[42]{}；
upi.get（）[0]；
}      
//不起作用
{
std:：unique_ptr 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];
            ^

prog.cpp:在函数“int main（）”中：
程序cpp:8:20:警告：未使用计算的值[-Wunused value]
upi.get（）[0]；
^
程序cpp:14:12:错误：“运算符[]”不匹配（操作数类型为“std:：unique_ptr”和“int”）
upi[0]；
^
程序cpp:19:20:警告：未使用计算的值[-Wunused value]
upi.get（）[0]；
^
prog.cpp:29:48:错误：调用“std:：unique\u ptr:：unique\u ptr（）”时没有匹配的函数
std:：unique_ptr upi{new int[42]{}；
^
项目cpp:29:48：注：候选人为：
在/usr/include/c++/4.8/memory:81:0中包含的文件中，
来自项目cpp:2：
/usr/include/c++/4.8/bits/unique\u ptr.h:176:2：注：模板std:：unique\u ptr:：unique\u ptr（std:：auto\u ptr&）
唯一（自动）无例外；
^
/usr/include/c++/4.8/bits/unique_ptr.h:176:2:注意：模板参数推导/替换失败：
程序cpp:29:48:注意：类型“std:：auto_ptr”和“int*”不匹配
std:：unique_ptr upi{new int[42]{}；
^
在/usr/include/c++/4.8/memory:81:0中包含的文件中，
来自项目cpp:2：
/usr/include/c++/4.8/bits/unique_ptr.h:169:2：注：模板std:：unique_ptr:：unique_ptr（std:：unique_ptr&）
唯一的（唯一的）无例外
^
/usr/include/c++/4.8/bits/unique_ptr.h:169:2:注意：模板参数推导/替换失败：
程序cpp:29:48:注意：类型“std:：unique_ptr”和“int*”不匹配
std:：unique_ptr upi{new int[42]{}；
^
在/usr/include/c++/4.8/memory:81:0中包含的文件中，
来自项目cpp:2：
/usr/include/c++/4.8/bits/unique_ptr.h:160:7：注：std:：unique_ptr:：unique_ptr（std:：unique_ptr&）[with _Tp=int*；_Dp=std:：default_delete]
唯一的（唯一的）无例外
^
/usr/include/c++/4.8/bits/unique\u ptr.h:160:7：注意：参数1从“int*”到“std:：unique\u ptr&”的转换未知
/usr/include/c++/4.8/bits/unique_ptr.h:157:17：注：constepr std:：unique_ptr:：unique_ptr（std:：nullptr_t）[带_Tp=int*；_Dp=std:：default_delete；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：注意：参数1从“int*”到“std:：nullptr_t”的转换未知
/usr/include/c++/4.8/bits/unique\u ptr.h:151:7：注：std:：unique\u ptr:：unique\u ptr:：pointer，typename std:：remove\u reference:：type&）[with _Tp=int*；_Dp=std:：default\u delete；std:：unique\u ptr:：pointer=int**；typename std:：remove\u reference:：type=std:：default\u delete]
唯一指针，
^
/usr/include/c++/4.8/bits/unique\u ptr.h:151:7：注意：候选者需要2个参数，提供1个
/usr/include/c++/4.8/bits/unique\u ptr.h:146:7：注：std：：unique\u ptr：：unique\u ptr：：unique\u ptr：：pointer，typename std：：conditional：：type）[withTp=int*；Dp=std：：default\u delete；std：：unique\u ptr：：pointer=int**；typename std：：const：：std：：default delete&]
唯一指针，
^
/usr/include/c++/4.8/bits/unique\u ptr.h:146:7：注意：候选者需要2个参数，提供1个
/usr/include/c++/4.8/bits/unique\u ptr.h:141:7：注：std:：unique\u ptr:：unique\u ptr:：pointer[带_Tp=int*；_Dp=std:：default\u delete；std:：unique\u ptr:：pointer=int**]
唯一\u ptr（指针\u p）无异常
^
/usr/include/c++/4.8/bits/unique\u ptr.h:141:7:注意：参数1从'int*'到'std:：unique\u ptr:：pointer{aka int**}的转换未知
/usr/include/c++/4.8/bits/unique_ptr.h:135:17：注：constepr std:：unique_ptr:：unique_ptr（）[带_Tp=int*；_Dp=std:：default_delete]
constexpr unique_ptr（）noexcept
^
/usr/include/c++/4.8/bits/unique\u ptr.h:135:17：注意：候选者需要0个参数，提供1个参数
程序cpp:30:20:警告：未使用计算的值[-Wunused value]
upi.get（）[0]；
^
prog.cpp:35:48:错误：调用“std:：unique\u ptr:：unique\u ptr（）”时没有匹配的函数
std:：unique_ptr upi{new int[42]{}；
^
项目cpp:35:48：注：候选人为：
在/usr/include/c++/4.8/memory:81:0中包含的文件中，
来自项目cpp:2：
/usr/include/c++/4.8/bits/unique\u ptr.h:176:2：注：模板std:：unique\u ptr:：unique\u ptr（std:：auto\u ptr&）
唯一（自动）无例外；
^
/usr/include/c++/4.8/bits/unique_ptr.h:176:2:注意：模板参数推导/替换失败：
程序cpp:35:48:注意：类型“std:：auto_ptr”和“int*”不匹配
std:：unique_ptr upi{new int[42]{}；
^
在/usr/include/c++/4.8/memory:81:0中包含的文件中，
来自项目cpp:2：
/usr/include/c++/4.8/bits/unique_ptr.h:169:2：注：模板std:：unique_ptr:：unique_ptr（std:：unique_ptr&）
唯一的（唯一的）无例外
^
/usr/include/c++/4.8/bits/unique_ptr.h:169:2:注意：模板参数推导/替换失败：
程序cpp:35:48:注：类型不匹配
 std::unique_ptr<int> upi{new int[42]{}};

upi[0];

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

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