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Comment by smitherfield | Hacker News Reader

For Linux kernel purposes the following one-liner would suffice

    template<class T> constexpr T max(T l, T r) {return l < r ? r : l;}

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And this short snippet in D

    T max(T)(T a, T b) { return a < b ? b : a; }

without requiring explicit constexpr annotations and with less noisy template syntax ;).

Does it compile the Linux kernel? ;)

The C version works with distinct types for a and b. Does this?

No it doesn't, which is one of the major advantages it has over the C version; you have to do an explicit cast when the args are different types (almost always a potential source of subtle bugs).

Or what edflsafoiewq said: https://news.ycombinator.com/item?id=16721559

It is however trivial to write a version that works with distinct types, if that were really what you want.

  template<class L, class R>
  constexpr std::common_type_t<L, R> max(L l, R r) {return l < r ? r : l;}

Assuming the single template argument version. If T has an implicit constructor taking type R as argument, wouldn't the second argument into max of type R be implicitly converted to T before getting passed to the max function? So there is no need for a special version taking both L and R?

This is a property of c++ implicit constructor rules, it is not unique to this function. In most cases this is something you want to avoid but for integer promotion it can some times be useful.

Nope, template type deduction never implicitly converts: https://godbolt.org/g/stZBK1

Oh, "conflicting types for parameter 'T' ('T2' vs. 'T1')". I'm surprised but it's a nice surprise. Finally something in c++ where safety is valued more than implicit dangerous magic.

C++'s type system has always been stricter than C's. Even the following, totally valid C program, is ill-formed C++

    int* x = malloc(5 * sizeof (int));

because in C++ you cannot implicitly cast a void pointer.

You'll have to give the template parameter explicitly, like

    max<int>(-1, 2u) // => 2

A difference is that the linux macro works for non-constant expressions as well.

All the crazyness is in order to support both constant expressions (and keep them constant expressions) and non-constant expressions with the same max() macro.

That one-liner I posted works with both constant and non-constant expressions (and in fact is more likely to be evaluated at compile-time than the macro), is type-safe, and has no side effects assuming both arguments are arithmetic types or pointers (like all uses of `max` in the Linux kernel / C-language programs).

That version does this. From cppreference (emphasis mine):

>The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time. Such variables and functions can then be used where only compile time constant expressions are allowed (provided that appropriate function arguments are given).

In C++, all the craziness and brittleness you find in the "C version" is actually encompassed entirely within the `constexpr` keyword. The compiler gets to deal with a lot of complexity to make that possible, but for users it is genuinely "that easy".

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