STL Container for storing multiple types of values?

Question

I have a Message structure that I am using with a message bus, and I'd like to send data with messages. The problem is that the data will vary in type; maybe for one message I'll just want to send one int, but for another I'll want to send several ints, a string, maybe even a pointer to an object for example. I could do something like this:

struct Message {
    std::map<int, int> intPayload;
    std::map<int, std::string> strPayload;
    short id;
};

But not only is this ugly and unclean, and probably wastes space, but that doesn't account for if I want to pass a relatively exotic data type like a pointer to an instance of a class for example. What should I be using for this?

Answer 1

There are many ways to do this. Here's an example with C++17's std::variant:

std::vector<std::variant<int, std::string>> vec1;

vec1.emplace_back(1);
vec1.emplace_back("hello"s);

doSomethingWithInt( std::get<int>(vec1[0]) );
doSomethingWithString( std::get<std::string>(vec1[1]) );

vec1 is a list of element that are either int or std::string.

You can also use a static visitor:

std::vector<std::variant<int, std::string>> vec2;

// ...

for(auto&& variant : vec1) {
    variant.visit([](auto value){
        using t = decltype(value);

        if constexpr (std::is_same_v<t, int>) {
            std::cout << "value is a int!" << std::endl;
        } else if constexpr (std::is_same_v<t, std::string>) {
            std::cout << "value is a string!" << std::endl;
        }
    });
}