C++14 Variable Templates: what is their purpose? Any usage example?

Question

C++14 will allow the creation of variables that are templated. The usual example is a variable \'pi\' that can be read to get the value of the mathematical constant π for va

Answer 1

I wonder whether something along these lines would be possible: (assuming availability of template lambdas)

void some_func() {
    template
    std::map storage;

    auto store = [](int key, const T& value) { storage[key] = value; };

    store(0, 2);
    store(1, "Hello"s);
    store(2, 0.7);

    // All three values are stored in a different map, according to their type. 
}

Now, is this useful?

As a simpler use, notice that the initialization of pi uses explicit conversion (explicit call of a unary constructor) and not uniform initialization. Which means that, given a type radians with a constructor radians(double), you can write pi.