In this part we will complete the implementation of the if expression to replace the ternary operator. Furthermore, we will look into limitations and extensions of the implementation.

In the previous part we have looked at implementing an if...else... expression which effectively replaces the ternary operator. To make it really useful we have to implement else if... branches as well and we will start with that.

Additional Branches with ElseIf

Following the logic of the previous article, we create new methods and types for the else...if branch to implement that part of the logic. We obviously need an ElseIf method on the If_Then type. This method returns another type (call it If_Then_Else_If) containing all the necessary information of the chain up to that point. It is composed of a bool condition value, a value of type T in case the bool is true, and another bool value for the ElseIf condition. However, this is where the necessity of creating new expression helper types stops. Because as soon as we call the Then method of this type, we only need to remember the first value whose condition is true (if any). We then package the result as a call to an If(...).Then(...). Let’s first look at the If_Then_Else_If type:

template<typename T>
class If_Then_Else_If
{
public:
    constexpr If_Then<T> Then(T t_now) const &&  {
        return (previous_condition)? If(true).Then(previous_t) : If(condition_now).Then(t_now);
    }
private:
    constexpr If_Then_Else_If(bool _previous_condition, T  _previous_t, bool _condition_now)
    : previous_condition(_previous_condition), previous_t(_previous_t),
    condition_now(_condition_now)
    {}

    bool previous_condition;
    T previous_t;
    bool condition_now;
    friend class If_Then<T>;
};

Now we can add an ElseIf method to our If_Then type:

template<typename T>
class If_Then
{
public:
    //-- see previous implementation --

    //this is new:
    constexpr If_Then_Else_If<T> ElseIf(bool condition_elseif) const && {
        return If_Then_Else_If<T>(condition,t,condition_elseif);
    }
private:
    // -- see previos implementation --
};

And that is how we can stick an ElseIf in there. Once the Else method on a chain of conditions is called, the first value whose condition evaluates to true is returned. If no such value exists, the value given to Else is returned.

constexpr All The Things

You might have already noticed that I snuck a constexpr in every constructor and method. So if all conditions and values are constexpr, the whole expression can be evaluated at compile time. We can use the result in all places that require a compile time constant expression. In this case, we don’t pay at runtime and we have an expressive syntax for evaluating compile time constants.

//planar_coordinates is a compile time bool
//maybe it came from a class template argument list
using coordinates = std::array<double,If(planar_coordinates).Then(2).Else(3)>;

In the example above example, we use the if expression to fix the size of an array at compile time.

Problems, Limitations and Improvements

Finally, I want to go over some limitations and improvements of my implementation. This list is not complete, and there are probably many more things that could be done to improve the implementation even more.

Fixing the Desired Return Type for Different Arguments

Say we wanted to initialize a string depending on some condition like so:

//this will produce a compiler error:
std::string str = If(is_french).Then("Bon Jour").Else("Hello");

This will give us a compiler error¹. The way to make this compile is to be explicit about the type in the first Then method. We do this by writing

std::string str2 = If(is_french).Then(std::string("Bon Jour")).Else("Hello");
//or equivalent:
std::string str  = If(is_french).Then<std::string>("Bon Jour").Else("Hello");

We just have to be explicit in the first Then method. This is because that method is templated and it passes the deduced type to all other expression helper types down the line. Those can take advantage of converting constructors.

Eager Evaluation in Then Clauses

A problem of my implementation is that every temporary passed to a Then method is constructed eagerly. That means it is always constructed and not only when its corresponding condition is actually true. I have experimented with implementing a Then_Lazy method taking a type std::function<T(void)>, which produces the type only when it is necessary. Thinking about lazy evaluation too much made me lazy myself and I ended up not implementing it. After all, I didn’t feel it would be a giant improvement for common use cases. However, it is something to keep in mind for types that drain a lot of resources upon construction.

Taking Advantage of Move Semantics

In the implementations given in these posts, I have taken all arguments of type T by value. I did not perform any calls to std::move for clarity. In my repository I made the effort to overload all methods and constructors for const lvalue references as well as rvalue references to T. Furthermore I have made use of std::move and std::forward to avoid copies wherever possible. This leads to more efficient code for movable types².

I have chosen move semantics over making the class members themselves references, because that would send me to the C++ lifetime purgatory. I experimented with using reference members and it seemed to work (when it probably shouldn’t have), but also I feel as though it is very thin ice. So I kept away from that for now.

Code and Conclusion

This was my shot at mimicking Rust-style if expressions in C++. These expressions are way more powerful in Rust because expressions are a more natural part of the language than in C++. This is why I only targeted the niche case of conditional initialization. I wonder how useful my implementation is at the end of the day, but the journey was fun for sure. The code is available as part of my func++ repository on Github.

Endnotes

c++11 c++17 constexpr if-expression rust

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