In D, a Voldemort type is a type that cannot be directly named outside of the scope it's declared in, but code outside the scope can still use this type by taking advantage of D's static type inference.

Usage

For example:

// Note: the return type is auto, because we cannot actually name it outside the function! auto createVoldemortType ( int value ) { struct TheUnnameable { int getValue () { return value ; } } return TheUnnameable (); }

Here is how the Voldemort type can be used:

auto voldemort = createVoldemortType ( 123 ); writeln ( voldemort . getValue ()); // prints 123

The type of the variable voldemort cannot be explicitly named here, because that name is only accessible inside the function createVoldemortType. Furthermore, instances of TheUnnameable cannot be created outside of this function, because its definition relies on a local variable in the function. However, we can still declare variables of that type by using D's static type inference, to store the function's return value.

Purpose

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Voldemort types are widely used in Phobos's range-based functions. They are used as internal types that implement that particular range function. Voldemort types were chosen for this in order to improve encapsulation: users of the standard library don't need to, and shouldn't need to, know how a particular range function was implemented and instead the simple knowledge of the primitives implemented by the return type is sufficient.

In fact, some of the range-based functions will return different types based on what kind of range was passed in. This happens when the function that contains a Voldemort type is actually a templatized function.

Rather than forcing the user to remember which type to use for which occasion, the use of Voldemort types forces the user to depend on static type inference, and thus automatically get the right type every time.

This also allows the Phobos developers to change the underlying implementation without requiring all users to update their code. Otherwise, every time the implementation changes you will have to update every instance of the type to refer to the correct type name. Using Voldemort types alleviates this problem by letting the compiler fill in the correct type for you automatically.

Side effects

Since Voldemort types are often implemented in templatized functions that are used in a CTFE fashion, their usage tends to create huge monolithic symbols that are not useful when debugging and that increase the binary size. In extreme cases the mangle of a call chain could occupy up to 1 MB!

Solutions exist. One of them would be to change the way that DMD generates the mangles, using a specialized compressor. Another solution, proposed by Steve Schveighoffer, is to move the Voldemort out of the function body.