为什么我不能将 unique_ptr 推回到向量中?

问题描述

这个程序有什么问题?

#include <memory>
#include <vector>

int main()
{
    std::vector<std::unique_ptr<int>> vec;

    int x(1);
    std::unique_ptr<int> ptr2x(&x);
    vec.push_back(ptr2x); //This tiny command has a vicious error.

    return 0;
}

错误:

In file included from c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/mingw32/bits/c++allocator.h:34:0,
                 from c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/allocator.h:48,
                 from c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/memory:64,
                 from main.cpp:6:
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/unique_ptr.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, const _Tp&) [with _Tp = std::unique_ptr<int>, _Tp* = std::unique_ptr<int>*]':
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/stl_vector.h:745:6:   instantiated from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::unique_ptr<int>, _Alloc = std::allocator<std::unique_ptr<int> >, value_type = std::unique_ptr<int>]'
main.cpp:16:21:   instantiated from here
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/unique_ptr.h:207:7: error: deleted function 'std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = int, _Tp_Deleter = std::default_delete<int>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<int>]'
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/ext/new_allocator.h:105:9: error: used here
In file included from c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/vector:69:0,
                 from main.cpp:7:
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/unique_ptr.h: In member function 'void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::unique_ptr<int>&}, _Tp = std::unique_ptr<int>, _Alloc = std::allocator<std::unique_ptr<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >, typename std::vector<_Tp, _Alloc>::_Base::_Tp_alloc_type::pointer = std::unique_ptr<int>*]':
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/stl_vector.h:749:4:   instantiated from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::unique_ptr<int>, _Alloc = std::allocator<std::unique_ptr<int> >, value_type = std::unique_ptr<int>]'
main.cpp:16:21:   instantiated from here
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/unique_ptr.h:207:7: error: deleted function 'std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = int, _Tp_Deleter = std::default_delete<int>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<int>]'
c:mingwin../lib/gcc/mingw32/4.5.0/include/c++/bits/vector.tcc:314:4: error: used here

推荐答案

你需要移动 unique_ptr:

You need to move the unique_ptr:

vec.push_back(std::move(ptr2x));

unique_ptr 保证单个 unique_ptr 容器拥有持有的指针的所有权.这意味着您不能复制 unique_ptr(因为两个 unique_ptr 将拥有所有权)，因此您只能移动它.

unique_ptr guarantees that a single unique_ptr container has ownership of the held pointer. This means that you can't make copies of a unique_ptr (because then two unique_ptrs would have ownership), so you can only move it.

但是请注意，您当前使用的 unique_ptr 是不正确的.您不能使用它来管理指向局部变量的指针.局部变量的生命周期是自动管理的:局部变量在块结束时被销毁(例如，在这种情况下，当函数返回时).您需要动态分配对象:

Note, however, that your current use of unique_ptr is incorrect. You cannot use it to manage a pointer to a local variable. The lifetime of a local variable is managed automatically: local variables are destroyed when the block ends (e.g., when the function returns, in this case). You need to dynamically allocate the object:

std::unique_ptr<int> ptr(new int(1));

在 C++14 中，我们有一个更好的方法:

In C++14 we have an even better way to do so:

make_unique<int>(5);

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