最烦人的解析会阻止类中初始化std :: vector< int>。 [英] most vexing parse prevents in-class initializing a std::vector<int>

问题描述

C ++ 11允许类内初始化：

C++11 allows in-class initialization:

struct Foo{
    std::vector<std::string> v{3}; // vector of 3 empty strings
};

如果我们想在类中初始化 ints 的向量，我们会得到其他结果：

If we wanted to initialize in-class a vector of ints, we would get something else:

struct Foo{
    std::vector<int> v{3}; // vector of one element with value 3
};

此问题似乎是语言的局限性，。但是，如果这不是类内初始化，则可以使用括号代替大括号，并获得所需的结果：

This issue seems to be a limitation of the language, as discussed in previous questions. However, if this were not an in-class initialization, we would be able to use parentheses instead of braces, and get the desired result:

std::vector<int> v(3); // vector of three zeros

但是，由于最令人讨厌的解析：

struct Foo{
    std::vector<int> v(3); // most vexing parse; doesn't compile
};

当然，上面的代码是否是好的设计实践尚有争议，因为我们可以轻松地移动我们正在尝试做一个构造函数。但是暂时将其搁置一旁，是否有一种方法可以执行所需的初始化，尽可能接近第一个 std :: string 示例，该示例可以正常工作？

Of course, it's debatable whether the code above is good design practice, since we can easily just move what we're trying to do into a constructor. But temporarily putting that aside, is there a way to perform the desired initialization, as closely as possible to the first std::string example, which works with no problem?

推荐答案

默认成员初始化程序也可以与 = 一起使用。因此

Default member initializers work with = as well. So

struct Foo{
    std::vector<int> v = std::vector<int>(3);
};

会做到这一点。尽管很明显，一个主要警告是我们在这里重复了类型名称。

Will do it. Though obviously, a major caveat is the fact that we are repeating the type name here.

我们可以使用 decltype ：

struct Foo{
    std::vector<int> v = decltype(v)(3);
};

但这仍然让我们命名两次。

But that still has us naming things twice.

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