vector of vector with different types

Question

I would like to construct a vector of vectors. This question has already been posted many time on SO but I did not find a satisfying answer. That because:

• each vector can have a different type
• I don't know the types at compilation time
• I don't know how many vectors it will contain

Basically I would like to be able to do something like that in pseudo code later

types = ["char", "int", "double", "int"]
container<vector> x

foreach (type in types)
{
  if (type == "char")
    x.push_back(vector<char>)
  else if (type == "int")
    x.push_back(vector<int>)
  else
    x.push_back(vector<double>)
}

and then I would like to be able to do for example

x[0].push_back("a")
x[1].push_back(1)
x[2].push_back(3.1416)

I think boost::any may help me but I'm not familiar with boost yet.

The point is, even if it sound weird, it is really what I want to do. I don't want a vector of structures, I really want a container (no matter which one) containing std::vector of different types. The reason is because I'm reading binary files. The header of the file states the number of data and their types but it can change from a file to another. Thus it cannot be known at compilation time.

Answer 1

Perhaps you could use a vector of variants?

using ints = std::vector<int>;
using chars = std::vector<char>;
using doubles = std::vector<double>;
using mixed_data_t = std::vector<std::variant<chars, ints, doubles>>;

If each file has the same type of data you can find out what it is at run time and push back into the appropriate variant-vector.

If the files have mixed data then you could use

std::vector<std::vector<std::variant<char, int, double>>>;

but you will have to check on each insertion.

I'm fairly confident that boost::any is not the solution you are looking for as type information will be lost after each insertion.