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Mixins (not to be confused with template mixins) enable string constants to be compiled as regular D code and inserted into the program. Combining this with compile time manipulation of strings enables the creation of domain-specific languages.

For example, here we can create a template that generates a struct with the named members:

template GenStruct(string Name, string M1)
    const char[] GenStruct = "struct " ~ Name ~ "{ int " ~ M1 ~ "; }";

mixin(GenStruct!("Foo", "bar"));

which generates:

struct Foo { int bar; }

Superficially, since D mixins can manipulate text and compile the result, it has some similar properties to the C preprocessor. But there are major, fundamental differences: