Question

I’m browsing through some code and I found a few ternary operators in it. This code is a library that we use, and it’s supposed to be quite fast.

I’m thinking if we’re saving anything except for space there.

What’s your experience?

Answer 1

Performance

The ternary operator shouldn’t differ in performance from a well-written equivalent if/else statement… they may well resolve to the same representation in the Abstract Syntax Tree, undergo the same optimisations etc..

Things you can only do with ? :

If you’re initialising a constant or reference, or working out which value to use inside a member initialisation list, then if/else statements can’t be used but ? : can be:

const int x = f() ? 10 : 2;

X::X() : n_(n > 0 ? 2 * n : 0) { }

Factoring for concise code

Keys reasons to use ? : include localisation, and avoiding redundantly repeating other parts of the same statements/function-calls, for example:

if (condition)
    return x;
else
    return y;

…is only preferable to…

return condition ? x : y;

…on readability grounds if dealing with very inexperienced programmers, or some of the terms are complicated enough that the ? : structure gets lost in the noise. In more complex cases like:

fn(condition1 ? t1 : f1, condition2 ? t2 : f2, condition3 ? t3 : f3);

An equivalent if/else:

if (condition1)
    if (condition2)
        if (condition3)
            fn(t1, t2, t3);
        else
            fn(t1, t2, f3);
    else if (condition3)
            fn(t1, f2, t3);
        else
            fn(t1, f2, f3);
else
    if (condition2)
       ...etc...

That’s a lot of extra function calls that the compiler may or may not optimise away.

Further, ? allows you to select an object, then use a member thereof:

(f() ? a : b).fn(g() ? c : d).field_name);

The equivalent if/else would be:

if (f())
    if (g())
        x.fn(c.field_name);
    else
        x.fn(d.field_name);
else
    if (g())
        y.fn(c.field_name);
    else
        y.fn(d.field_name);

Can’t named temporaries improve the if/else monstrosity above?

If the expressions t1, f1, t2 etc. are too verbose to type repeatedly, creating named temporaries may help, but then:

• To get performance matching ? : you may need to use std::move, except when the same temporary is passed to two && parameters in the function called: then you must avoid it. That’s more complex and error-prone.

• c ? x : y evaluates c then either but not both of x and y, which makes it safe to say test a pointer isn’t nullptr before using it, while providing some fallback value/behaviour. The code only gets the side effects of whichever of x and y is actually selected. With named temporaries, you may need if / else around or ? : inside their initialisation to prevent unwanted code executing, or code executing more often than desired.

Functional difference: unifying result type

Consider:

void is(int) { std::cout << "int\n"; }
void is(double) { std::cout << "double\n"; }

void f(bool expr)
{
    is(expr ? 1 : 2.0);

    if (expr)
        is(1);
    else
        is(2.0);
}

In the conditional operator version above, 1 undergoes a Standard Conversion to double so that the type matched 2.0, meaning the is(double) overload is called even for the true/1 situation. The if/else statement doesn’t trigger this conversion: the true/1 branch calls is(int).

You can’t use expressions with an overall type of void in a conditional operator either, whereas they’re valid in statements under an if/else.

Emphasis: value-selection before/after action needing values

There’s a different emphasis:

An if/else statement emphasises the branching first and what’s to be done is secondary, while a ternary operator emphasises what’s to be done over the selection of the values to do it with.

In different situations, either may better reflect the programmer’s “natural” perspective on the code and make it easier to understand, verify and maintain. You may find yourself selecting one over the other based on the order in which you consider these factors when writing the code – if you’ve launched into “doing something” then find you might use one of a couple (or few) values to do it with, ? : is the least disruptive way to express that and continue your coding “flow”.