What are futures?

Question

What are futures? It\'s something to do with lazy evaluation.

Answer 1

There is a Wikipedia article about futures. In short, it's a way to use a value that is not yet known. The value can then be calculated on demand (lazy evaluation) and, optionally, concurrently with the main calculation.

C++ example follows.

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Say you want to calculate the sum of two numbers. You can either have the typical eager implementation:

int add(int i, int j) { return i + j; }
// first calculate both Nth_prime results then pass them to add
int sum = add(Nth_prime(4), Nth_prime(2)); 

or you can use the futures way using C++11's std::async, which returns an std::future. In this case, the add function will only block if it tries to use a value that hasn't yet been computed (one can also create a purely lazy alternative).

int add(future i, future j) { return i.get() + j.get(); }
int sum = add(async(launch::async, [](){ return Nth_prime(4); }),
              async(launch::async, [](){ return Nth_prime(2); }));