"-opt1 val1 -opt2 val1 val2 -opt3 val3"

boost::split(target_vector, inputString, boost::is_any_of(" "), boost::token_compress_on);
["-opt1", "val1", "-opt2", "val1", "val2", "-opt3", "val3"]

["-opt1", "val1", "-opt2", "val1 val2", "-opt3", "val3"]

option : opt1 val1
  option name = opt1
  - value : val1
option : opt2 val1 val2
  option name = opt2
  - value : val1 val2
option : opt3 val3
  option name = opt3
  - value : val3

#include <iostream>
#include <limits>
#include <vector>
#include <string_view>
// a generic string splitting algorithm (tokenizer)
// string_views are used so there will be no string copies involved.
auto split(std::string_view string, std::string_view delimiters, std::size_t maximum_number_of_substrings = std::numeric_limits<std::size_t>::max())
{
    std::vector<std::string_view> substrings;
    if (string.size() == 0ul)
    {
        return substrings;
    }
    if (delimiters.size() == 0ul)
    {
        substrings.emplace_back(string);
        return substrings;
    }
    auto start_pos = string.find_first_not_of(delimiters);
    auto end_pos = start_pos;
    auto max_length = string.length();
    while (start_pos < max_length)
    {
        maximum_number_of_substrings--;
        if (maximum_number_of_substrings != 0ul)
        {
            end_pos = std::min(max_length, string.find_first_of(delimiters, start_pos));
        }
        else
        {
            end_pos = max_length;
        }
        if (end_pos != start_pos)
        {
            substrings.emplace_back(&string[start_pos], end_pos - start_pos);
            start_pos = string.find_first_not_of(delimiters, end_pos);
        }
    }
    return substrings;
}
int main()
{
    // demo command line
    // note I do NOT check for validity of the input string right now
    // this is a quick sketch of the approach.
    std::string command_line{ "-opt1 val1 -opt2 val1 val2 -opt3 val3" };
    // first create a vector of string_views based on splitting by "-"
    auto options = split(command_line, "-");
    // then each of those split substrings can be split by a space " "
    for (const auto& option : options)
    {
        std::cout << "option : " << option << "\n";
        auto values = split(option, " ", 2ul);
        // the first split value will be the option name
        std::cout << "  option name = " << values[0] << "\n";
        // the other substrings will be the values 
        for (std::size_t n{ 1ul }; n < values.size(); ++n)
        {
            std::cout << "  - value : " << values[n] << "\n";
        }
    }
    return 0;
}