C++ code:

#include <cstdint>
#include <string>
#include <sstream>
#include <vector>
#include <iterator>

int main() {
    std::string my_string{ "123 456 789" };

    std::istringstream stream{ my_string };

    // You can move in C++20 to avoid a copy.
    // std::istringstream stream{ std::move(my_string) };

    // Construct numbers using an istream_iterator.
    // Need an empty one for the end because the constructor can't handle a default_sentinel.
    std::vector<int64_t> numbers{ std::istream_iterator<int64_t>{stream}, std::istream_iterator<int64_t>{} };

    return 0;
}