Menu

- Learn
- Documentation
  - Language Reference
  - Library Reference
  - Command-line Reference
  - Feature Overview
  - Articles
- Downloads
- Packages
- Community
  - Blog
  - Orgs using D
  - Twitter
  - Calendar
  - Forums
  - IRC
  - Community Discord
  - Wiki
  - GitHub
  - Issues
  - Get involved
  - Contributors
  - Foundation
  - Security Team
  - Donate
  - Sponsors
- Resources
  - Tour
  - Books
  - Tutorials
  - Tools
  - Editors
  - IDEs
  - run.dlang.io
  - Visual D
  - Acknowledgments
  - D Style
  - Glossary
  - Sitemap

Search
Report a bug
If you spot a problem with this page, click here to create a Bugzilla issue.
Improve this page
Quickly fork, edit online, and submit a pull request for this page. Requires a signed-in GitHub account. This works well for small changes. If you'd like to make larger changes you may want to consider using a local clone.

# Mixins

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:

- The C preprocessing step occurs before lexical analysis. This makes it impossible to lex or parse C without access to all of the context, including all #include'd files, paths and all relevant compiler switches. Mixins occur during semantic analysis, and do not affect the lexing or parsing process. Lexing and parsing can still occur without semantic analysis.
- The C preprocessor can be used to create what appears to be different syntax:

      #define BEGIN {
      #define END   }

      BEGIN
        int x = 3;
        foo(x);
      END

  This monkey business is impossible with mixins. Mixed in text must form complete declarations, statements, or expressions.
- C macros will affect everything following that has the same name, even if they are in nested scopes. C macros cut across all scopes. This problem is called being not "coding hygenic". Mixins follow the usual scoping rules, and is hygenic.
- C preprocessing expressions follow a different syntax and have different semantic rules than the C language. The C preprocessor is technically a different language. Mixins are in the same language.
- C const declarations and C++ templates are invisible to C preprocessing. Mixins can be manipulated using templates and const declarations.

Copyright © 1999-2020 by the D Language Foundation | Page generated by Ddoc on Sun Feb 9 20:05:23 2020