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D Tip - Testing with TypeTuple

Posted: 2012-11-18 - Link

When testing a function, it’s usually a good idea to test it with a range of different inputs. To do this, you can easily use a for loop over an array of input values, but what if your input is a type, as it often is with template code?

The D Programming Language allows you to iterate over a TypeTuple, so all you need to do is declare a tuple of all the types you want to test, and iterate over them in the normal way:

import std.typetuple;
alias TypeTuple!(int, long, double) Types;
foreach (T; Types)
    test!T();

You might wonder what this compiles to. After all, the body of the loop varies with T, so the generated code must also vary on each iteration. How does the compiler handle this?

The answer is that the loop is completely unrolled. The code above is literally the same as:

test!int();
test!long();
test!double();

For this reason, you might want to keep an eye on the size of your TypeTuples, to avoid code bloat.

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