Benefits of ternary operator vs. if statement

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".

Answer 2

The only potential benefit to ternary operators over plain if statements in my view is their ability to be used for initializations, which is particularly useful for const:

E.g.

const int foo = (a > b ? b : a - 10);

Doing this with an if/else block is impossible without using a function cal as well. If you happen to have lots of cases of const things like this you might find there's a small gain from initializing a const properly over assignment with if/else. Measure it! Probably won't even be measurable though. The reason I tend to do this is because by marking it const the compiler knows when I do something later that could/would accidentally change something I thought was fixed.

Effectively what I'm saying is that ternary operator is important for const-correctness, and const correctness is a great habit to be in:

1. This saves a lot of your time by letting the compiler help you spot mistakes you make
2. This can potentially let the compiler apply other optimizations

Answer 3

Well...

I did a few tests with GCC and this function call:

add(argc, (argc > 1)?(argv[1][0] > 5)?50:10:1, (argc > 2)?(argv[2][0] > 5)?50:10:1, (argc > 3)?(argv[3][0] > 5)?50:10:1);

The resulting assembler code with gcc -O3 had 35 instructions.

The equivalent code with if/else + intermediate variables had 36. With nested if/else using the fact that 3 > 2 > 1, I got 44. I did not even try to expand this into separate function calls.

Now I did not do any performance analysis, nor did I do a quality check of the resulting assembler code, but at something simple like this with no loops e.t.c. I believe shorter is better.

It appears that there is some value to ternary operators after all :-)

That is only if code speed is absolutely crucial, of course. If/else statements are much easier to read when nested than something like (c1)?(c2)?(c3)?(c4)?:1:2:3:4. And having huge expressions as function arguments is not fun.

Also keep in mind that nested ternary expressions make refactoring the code - or debugging by placing a bunch of handy printfs() at a condition - a lot harder.

Answer 4

If you're worried about it from a performance perspective then I'd be very surprised if there was any different between the two.

From a look 'n feel perspective it's mainly down to personal preference. If the condition is short and the true/false parts are short then a ternary operator is fine, but anything longer tends to be better in an if/else statement (in my opinion).

Answer 5

You assume that there must be a distinction between the two when, in fact, there are a number of languages which forgo the "if-else" statement in favor of an "if-else" expression (in this case, they may not even have the ternary operator, which is no longer needed)

Imagine:

x = if (t) a else b

Anyway, the ternary operator is an expression in some languages (C,C#,C++,Java,etc) which do not have "if-else" expressions and thus it serves a distinct role there.