Technique: Immediately-Invoked Function Expression for Metaprogramming

Common C++ guidelines are to initialize variables on use and to make variables const whenever possible. But sometimes a variable is unchanged once initialized and the initialization is complex, like involving a loop. Then an IIFE – immediately-invoked function expression – can be used: the variable is initialized by a lambda that computes the value, which is then immediately invoked to produce the value. Then the variable is initialized on use and can also be made const.

I’ve been recently working on a meta-programming library where I found IIFEs useful in a slightly different context – computing type information.

TL;DR: decltype([] { ... } ())!

» read more »
Jonathan

Implementation Challenge: Replacing std::move and std::forward

When C++11 introduced move semantics, it also added two important helper functions: std::move and std::forward. They are essential when you want to manually indicate that you no longer care about an object or need to propagate the value category in generic code. As such, I’ve used them countless times in the past.

However, they are functions. Plain, old, standard library functions.

This is problematic for multiple reasons.

» read more »
Jonathan

Nifty Fold Expression Tricks

Suppose you need to have a variadic function and want to add all arguments together. Before C++17, you need two pseudo-recursive functions:

template <typename H, typename ... T>
auto add(H head, T... tail)
{
    return head + add(tail...);
}

template <typename H>
auto add(H head)
{
    return head;
}

However, C++17 added fold expressions, making it a one-liner:

template <typename H, typename ... T>
auto add(H head, T... tail)
{
    return (head + ... + tail);
    // expands to: head + tail[0] + tail[1] + ...
}

If we’re willing to abuse operator evaluation rules and fold expressions, we can do a lot more. This blog posts collects useful tricks.

» read more »
Jonathan

Tutorial: C++20’s Iterator Sentinels

You probably know that C++20 adds ranges. Finally we can write copy(container, dest) instead of copy(container.begin(), container.end(), dest)!

Ranges also do a lot more. Among other things, they add a new way of specifying an iterator to the end – sentinels.

» read more »
Jonathan