Given a vector of strings which is known at compile-time (and say some numbers they should map to), I wish to create such a map (f.e. unordered_map) at compile-time. The goal is to start up quickly and perform just look-ups at runtime. Take this example:
enum category {fruit, vegetable};
const std::vector<std::string> fruits = {"apple", "pear", "orange"};
const std::vector<std::string> vegetables = {"cucumber", "zucchini", "tomato"};
const std::unordered_map<std::string, category> lookup_category = // ?
However, constexpr forbids the use of non-literals. Templating is a solution, however it is a massive headache to implement and maintain.
Does C++(17?) have something in the STL which can aid me in the construction of such map at compile time?
There are no vectors, strings, or maps at compile time. In fact, an object of class type with a non-trivial destructor can't be used in a constexpr context. So this simply can't be done.
From c++20, you can have non-trivial destructors, so this is possible in principle. As far as I know, only vector and string are available in constexpr contexts though, and even then, all the storage that they allocate, must be deallocated before run-time. So you can't even have a vector or string at compile time that you can also use at run-time.
From c++20, you can guarantee that the vector or string, or map is initialized at program-load time though, using the keyword constinit.
In case you just want to do a complex initialization of a const map, you can do that with an IIILE (immediately invoked initializing lambda expression) even before c++20. e.g.
const map<string, int> m = [] {
map<string,int> x; // construct a map ...
x["hi"] = 42; // populate it
return x;// return it
}(); // and call the expression straight away.