特征::矩阵<double,1,3>结构类型函数中的返回类型函数

Question

I want to write a function which is of struct type, but it returns eigen::matrix type vector (so to say...)

For example:

struct foo (){ double a,b,c; };  
foo FOO(){  
    typedef eigen::Matrix<double,1,3> foofoo;
    foo f;
    // .....                 // some expressions that generate some numerical values
    f.a; f.b;f.c;         // numerical values are put in here
    foofoo<<f.a, f.b,f.c; // assigned to the eigen::matrix
    return foofoo;        // attempt to return eigen::matrix type vector
}

I am unsure about where to declare the eigen::matrix type vector. Should it be inside the function or in the struct or should it be a separate struct of eigen::matrix type or any other way is preferred.

Answer 1

There is no such thing as "a function of struct type", and your struct declaration syntax is really weird. You seem to be confusing types with objects.

Here's what I think you need, simply a function that returns an instance of your eigen::Matrix specialisation (which you've named foofoo via a type alias):

struct foo
{
   double a, b, c;
};

using foofoo = eigen::Matrix<double, 1, 3>;

foofoo FOO()
{
   foofoo result;

   foo f;
   // ... populate members of f ...
   result << f.a, f.b, f.c;

   return result;
}