Dominik Berner

C++ Coder, Agilist, Rock Climber


Project maintained by bernedom

initializers in selection statements - smarter coding in C++17

initializers in selection statements - smarter coding in C++17

C++ is hard! Luckily the new standards brought some features and tools to make writing C++ easier. Selection statements with initializers are one of these small-but-nice features that make our coding-life easier.

With the introduction of C++17 if and switch statements can be initialized inside the statement instead of as before only outside the statement. Variables that are only used in the condition or inside the body of the selection statement can be assigned a value similar to what we know from for loops. So in modern C++ we can write:

// i is assigned right in the if-clause
if (unsigned i = std::rand(); i % 2 == 0) {
	std::cout << "i is even" << std::endl;
} else {
	std::cout << "i is odd" << std::endl;
}

The most obvious benefit is that code becomes a tiny bit more compact than what we are using up to now.

Apart from a bit better readability, using the initializers helps with scoping temporary variables to the statement. Previously i would have been defined in the scope surrounding the if-else block. By using a initializer in the if-statement, i is de-scoped on the closing bracket of the if or else block. This is of course only useful if the conditional variable was a true temporary variable and not something to be reused a few times. Because of this scoping initializers in selection statements should usually only be used with functions returning data by value and not by raw pointer. It is possible to manually delete a variable that was created like this inside the selection statement, but this is not considered good coding form. Doing so would also mean that the cleanup-code has to be duplicated for each case of an if, else or each case in a switch-statement).

Initializers in selection statements become even more fun if combined with structured bindings another of my favorite features of C++17. This allows for instance for easy and readable consistency checks in fixed size containers.

if(const auto [x,y,z] = func_returning_a_tuple(); x + y < z)
{ 
  // Do something that relies on the check above 
}

A word of warning is that each if opens it’s own scope, so be careful when reinitializing variables in chained else if expressions. Also it is technically possible to use a function returning void as an initializer, but relying on “out”-parameters instead is hardly what was in the minds of the developers here.

All in all the initializers in selection statement are not the biggest change in C++, nor are they the feature which probably has the most impact on how code is written, but they are a very good example on the direction modern C++ is taking.

(This article was originally published at bbv.ch in german

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Written on February 5, 2018