C ++如何在编译时检查变量映射的类型

Question

I would like to have a map of key, value pairs; the keys can be implemented as string or enum class; but the values need to support multiple types, eg int or bool

Now, I would want to have a function with the following signature

template<typename T>
void getOption(KeyClass key_, T& value_)

Given that the map will be constant, is it possible to make the function checking the type at compile time?

For example, if the map is something like {"start_time": 2, "can_abort": false}

There should be a compilation error on

bool can_abort;
getOption("start_time", can_abort);

Answer 1

If you can use enum values for your keys, and the types and values are known at compile-time, why not just have a plain struct?

struct options_t {
    int start_time;
    bool can_abort;
};

If you really must iterate over elements of different types, you can go full voodoo and use Louis Dionne's Boost.Hana library. It has a multi-type map class . Everything will be checked at compile time.

Alternatively, if you want more run-time-oriented behavior, and you want flexibility wrt the types you stick in your map, you could use type erasure and make it, say, an std::unordered_map<std::string, std::any> . The std::any type holds, well, a value of any type; but it is up to you to know which type to try and extract from it (if you're wrong you get an exception) - ie you won't have compile time checking.

As for using a variant - you can use those, but if your keys are std::string s, then nothing is going to be checked at compile time, as std::string s involve heap allocation.