std::make_unique和unique_ptr

Question

如果我没看错的话，cppreference说以下两个构造应该等同于C++14和更高版本中的非数组类型：

make_unique<T>(T());
unique_ptr<T>(new T());

很明显，我没有理解一个微妙的地方。下面是一个例子(包含头文件和脚手架的完整文件是here。在本例中，我希望cmp3和cmp4具有相同的效果，但cmp4无法编译。

template <typename T>
class DbCell {
   public:
    DbCell() {}
    DbCell(const T& v) : value_(v.value_) {}
    DbCell(T&& v) : value_(move(v.value_)) {}
    ~DbCell() {}
   private:
    T value_;
};

struct ChapterStats {
    ChapterStats() {}
};
class ChapterMap {
   public:
    ChapterMap() {}
    ChapterMap(const ChapterMap&) = delete;
    ChapterMap(ChapterMap&& cm) : the_map_(move(cm.the_map_)) {}

   private:
    map<string, unique_ptr<ChapterStats>> the_map_;
};
void foo() {
    DbCell<ChapterMap> cm;

    unique_ptr<DbCell<int>> cmp1 =
        make_unique<DbCell<int>>(DbCell<int>());

    unique_ptr<ChapterMap> cmp2 = make_unique<ChapterMap>(ChapterMap());

    unique_ptr<DbCell<ChapterMap>> cmp3 =
        unique_ptr<DbCell<ChapterMap>>(new DbCell<ChapterMap>());

    // This next fails, even though I think it should be equivalent to cmp3.
    cout << "> cmp4:" << endl;
    unique_ptr<DbCell<ChapterMap>> cmp4 =
        make_unique<DbCell<ChapterMap>>(new DbCell<ChapterMap>());
}

如果没有cmp4，输出是这样的(添加适当的打印语句)：

Constructed ChapterMap.
Constructed DbCell.
> cmp1:
Constructed DbCell.
> cmp2:
Constructed ChapterMap.
Moved ChapterMap.
> cmp3:
Constructed ChapterMap.
Constructed DbCell.

对于cmp4，编译器错误是这样的：

In file included from rgr.cc:6:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/memory:81:
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/unique_ptr.h:765:34: error: no matching constructor for initialization of 'DbCell<ChapterMap>'
    { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
                                 ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
rgr.cc:61:9: note: in instantiation of function template specialization 'std::make_unique<DbCell<ChapterMap>, DbCell<ChapterMap> *>' requested here
        make_unique<DbCell<ChapterMap>>(new DbCell<ChapterMap>());
        ^
rgr.cc:18:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'DbCell<ChapterMap> *' to 'const DbCell<ChapterMap>' for 1st argument; dereference the argument with *
class DbCell {
      ^
rgr.cc:21:5: note: candidate constructor not viable: no known conversion from 'DbCell<ChapterMap> *' to 'const ChapterMap' for 1st argument
    DbCell(const T& v) : value_(v.value_) {
    ^
rgr.cc:24:5: note: candidate constructor not viable: no known conversion from 'DbCell<ChapterMap> *' to 'ChapterMap' for 1st argument
    DbCell(T&& v) : value_(move(v.value_)) { cout << "Moved DbCell." << endl; }
    ^
rgr.cc:20:5: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
    DbCell() { cout << "Constructed DbCell." << endl; }
    ^
1 error generated.

令我困惑的一件事是编译器抱怨隐式删除的DbCell<ChapterMap>复制构造函数，但我已经定义了一个(我不需要它，只是为了示例)。

Answer 1

这两个并不完全相同：

   make_unique<T>(T());
   unique_ptr<T>(new T());

这两个是相同的：

   make_unique<T>();
   unique_ptr<T>(new T());

在第一个示例中，您有一个额外的移动/复制ctor，它涉及到make_unique<T>的调用。

Answer 2

对于cmp4，您应该使用

unique_ptr<DbCell<ChapterMap>> cmp4 =
        make_unique<DbCell<ChapterMap>>(/*empty*/);

std::make_unique调用其类型的构造函数，在本例中，该构造函数将是一个接受指向DbCell的指针的构造函数，而您没有该构造函数。编译器错误消息突出显示了这一点：

rgr.cc:18:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'DbCell<ChapterMap> *' to 'const DbCell<ChapterMap>' for 1st argument; dereference the argument with *