使用 GCC 4.7 从初始化列表初始化 unique_ptrs 的容器失败

2021-12-31 00:00:00 initialization compiler-errors c++ c++11

我正在尝试以与 Bjarne Stroustrup 的 C++11 常见问题:

using namespace std;
vector<unique_ptr<string>> vs { new string{"Doug"}, new string{"Adams"} }; // fails
unique_ptr<string> ps { new string{"42"} }; // OK

我看不出为什么这个语法会失败.这种初始化容器的方式有什么问题吗?
编译器错误信息很大；我找到的相关部分如下:

I can see no reason why this syntax should fail. Is there something wrong with this way of initializing the container?
The compiler error message is huge; the relevant segment I find is below:

/usr/lib/gcc-snapshot/lib/gcc/i686-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/stl_construct.h:77:7: 错误:没有匹配的函数调用'std::unique_ptr>::unique_ptr(std::basic_string<char>&)'

/usr/lib/gcc-snapshot/lib/gcc/i686-linux-gnu/4.7.0/../../../../include/c++/4.7.0 /bits/stl_construct.h:77:7: error: no matching function for call to 'std::unique_ptr<std::basic_string<char> >::unique_ptr(std::basic_string<char>&)'

有什么办法可以解决这个错误?

What is the way to fix this error ?

推荐答案

unique_ptr 的构造函数是 explicit.所以你不能用 from new string{"foo"} 隐式地创建一个.它需要类似于 unique_ptr{ new string{"foo"} }.

unique_ptr's constructor is explicit. So you can't create one implicitly with from new string{"foo"}. It needs to be something like unique_ptr<string>{ new string{"foo"} }.

导致我们这样做

// not good
vector<unique_ptr<string>> vs {
    unique_ptr<string>{ new string{"Doug"} },
    unique_ptr<string>{ new string{"Adams"} }
};

但是，如果其中一个构造函数失败，它可能会泄漏.使用 make_unique 更安全:

However it may leak if one of the constructors fails. It's safer to use make_unique:

// does not work
vector<unique_ptr<string>> vs {
     make_unique<string>("Doug"),
     make_unique<string>("Adams")
};

但是... initializer_lists 总是执行复制，而 unique_ptrs 是不可复制的.这对于初始化列表来说真的很烦人.您可以破解它，或者通过调用emplace_back回退到初始化.

But... initializer_lists always perform copies, and unique_ptrs are not copyable. This is something really annoying about initializer lists. You can hack around it, or fallback to initialization with calls to emplace_back.

如果您实际上使用智能指针管理 string 并且不仅仅是示例，那么您可以做得更好:只需制作一个 vector.std::string 已经处理了它使用的资源.

If you're actually managing strings with smart pointers and it's not just for the example, then you can do even better: just make a vector<string>. The std::string already handles the resources it uses.

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