Boost C++ Libraries

Values are functions! Examples:

val(3)
val("Hello, World")

The first evaluates to a nullary function (a function taking no arguments) that returns an int, 3. The second evaluates to a nullary function that returns a char const(&)[13], "Hello, World".

Lazy Evaluation

Confused? val is a unary function and val(3) invokes it, you say? Yes. However, read carefully: "evaluates to a nullary function". val(3) evaluates to (returns) a nullary function. Aha! val(3) returns a function! So, since val(3) returns a function, you can invoke it. Example:

std::cout << val(3)() << std::endl;

(See values.cpp)

The second function call (the one with no arguments) calls the nullary function which then returns 3. The need for a second function call is the reason why the function is said to be Lazily Evaluated. The first call doesn't do anything. You need a second call to finally evaluate the thing. The first call lazily evaluates the function; i.e. doesn't do anything and defers the evaluation for later.

Callbacks

It may not be immediately apparent how lazy evaluation can be useful by just looking at the example above. Putting the first and second function call in a single line is really not very useful. However, thinking of val(3) as a callback function (and in most cases they are actually used that way), will make it clear. Example:

template <typename F>
void print(F f)
{
    cout << f() << endl;
}

int
main()
{
    print(val(3));
    print(val("Hello World"));
    return 0;
}

(See callback.cpp)