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Legacy Code

Contents

body keyword
alias target name; syntax
Aliasing an instance member

To maintain compatibility with older D code, many legacy features remain supported. This page describes each legacy feature that is supported, with a suggestion of how to modernize the code.

Legacy Features
Feature	Summary	Edition check
body keyword	body after a contract statement - use do instead	2024
alias target name; syntax	use alias name = target; instead	2024
Aliasing an instance member	Use typeof(instance).member instead	2024
Escaping scope data	scope is enforced in @safe code	2024
Struct/union postblit	use a copy constructor instead.

body keyword

body was a keyword used to specify a function/method's body after a contract statement:

class Foo
{
    void bar(int i)
    in { assert(i >= 42); }
    body { /* Do something interesting */ }

    string method(string s)
    out(v) { assert(v.length == s.length); }
    body { /* Do something even more interesting */ }

    void noBody() { /* No contracts, no body */ }
}

Corrective Action

Use the do keyword instead (introduced in v2.075.0):

void bar(int i)
in { assert(i >= 42); }
do { /* Look ma, no body! */ }

Rationale: The body keyword was only used for this single purpose. Since D grammar aims to be context free, this common word was reserved, which led to frequent trouble for people interfacing with other languages (e.g. javascript) or auto-generating code.

alias target name; syntax

From the 2024 edition, AliasDeclaration only supports alias name = target; syntax. This is easier to find the symbol identifier, particularly when target is a complex symbol. The target first order came from C's typedef syntax.

Aliasing an instance member

E.g. alias a = instance.field;. Such an alias actually aliases a member of the instance's type, not the instance member itself. That could be confusing. Instead, alias a member of the type.

Note: Aliasing this.member and super.member are valid inside methods.

struct Bar
{
    Foo f;
    alias v = f.v; // Error, use `typeof(f).v`
}

struct Foo
{
    int v;
    void test(Foo that) const
    {
        alias a = this.v; // OK
        alias b = that.v; // Error, use `typeof(that).v` instead
        assert(&a is &b); // passes
        assert(&b !is &that.v);
    }
}

Glossary

Editions