most vexing parse prevents in-class initializing a std::vector<int>

问题

C++11 allows in-class initialization:

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

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
};

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

However, we cannot do this in a class because of most vexing parse:

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

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?

回答1:

Default member initializers work with = as well. So

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

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

We can alleviate it somewhat with decltype:

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

But that still has us naming things twice.

来源：https://stackoverflow.com/questions/48723015/most-vexing-parse-prevents-in-class-initializing-a-stdvectorint