Scope guards

Syntax#

scope(exit) - statements are called no matter how the current block was exited
scope(success) - statements are called when the current block was exited normally
scope(failure) - statements are called when the current block was exited through exception throwing

Remarks#

Using scope guards makes code much cleaner and allows to place resource allocation and clean up code next to each other. These little helpers also improve safety because they make sure certain cleanup code is always called independent of which paths are actually taken at runtime.

The D scope feature effectively replaces the RAII idiom used in C++ which often leads to special scope guards objects for special resources.

Scope guards are called in the reverse order they are defined.

Play with scope guards or see an extensive tutorial.

Place allocation and cleanup code next to each other

Scope guards allow executing statements at certain conditions if the current block is left.

import core.stdc.stdlib;

void main() {
    int* p = cast(int*)malloc(int.sizeof);
    scope(exit) free(p);
}

Multiple, nested scopes

import std.stdio;

void main() {
    writeln("<html>");
    scope(exit) writeln("</html>");
    {
        writeln("\t<head>");
        scope(exit) writeln("\t</head>");
        "\t\t<title>%s</title>".writefln("Hello");
    } // the scope(exit) on the previous line is executed here

    writeln("\t<body>");
    scope(exit) writeln("\t</body>");

    writeln("\t\t<h1>Hello World!</h1>");
}

prints

<html>
    <head>
        <title>Hello</title>
    </head>
    <body>
        <h1>Hello World!</h1>
    </body>
</html>