So yesterday I decided to exercise my template chops. What I wanted to create was a lookup table that would allow you to insert and retrieve values based on any of several keys. It’s easiest to explain with some sample usage:
struct Thing {
Thing(int id, int group, std::string name) : id(id), group(group), name(name) {}
int id;
int group;
std::string name;
void print() const {
printf("id - %d, group - %d, name - \"%s\"\n", id, group, name.c_str());
}
};
int main(int argc, char* argv[]) {
LookupTable<Thing, LookupKey<'__id', int>, LookupKey<'_grp', int>, LookupKey<'name', std::string>> table;
Thing a(1, 2, "a");
Thing b(2, 5, "b");
Thing c(3, 3, "c");
Thing d(4, 5, "d");
Thing e(5, 8, "e");
table.insert(a, a.id, a.group, a.name);
table.insert(b, b.id, b.group, b.name);
table.insert(c, c.id, c.group, c.name);
table.insert(d, d.id, d.group, d.name);
table.insert(e, e.id, e.group, e.name);
printf("searching for id == 3\n");
for (auto thing : table.lookup<'__id'>(3)) {
thing.print();
}
printf("searching for group == 5\n");
for (auto thing : table.lookup<'_grp'>(5)) {
thing.print();
}
printf("searching for name == \"b\"\n");
for (auto thing : table.lookup<'name'>("b")) {
thing.print();
}
return 0;
}
Kinda cool, right? One import design goal was to make this as fool-proof as possible. Everything is checked at compile time so if you slip up and mistype a key or use a wrong type as an argument, you’ll get a static assertion. For example, trying to call table.lookup<'namw'>("b") in the above example would give you a “key not found” compile message.
Here’s the source:
https://s3.amazonaws.com/daxnitro/misc/LookupTable.h
There’s a decent chance that this code is unlike anything you’ve ever seen or written before. It demonstrates several new C++11 features and many older, yet rarely seen ones. The key parts are the ways it utilizes variadic templates and static assertions, while leveraging SFINAE and partial ordering. If you’re into C++ and any of these buzz terms are unfamiliar to you, I recommend reading up on them. Templates can be magical (and really disorienting).
It should be noted that the above code isn’t meant to be used with objects that aren’t trivially copyable and might be more useful with the addition of a mechanism to remove items, but the primary purpose here is to demonstrate concepts.
