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add docs for usingnamespace 

closes #1589
master
Andrew Kelley 2019-09-24 21:23:12 -04:00
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commit 993d5bc9c9
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@ -5187,6 +5187,41 @@ test "@intToPtr for pointer to zero bit type" {
</p>
{#header_close#}
{#header_open|usingnamespace#}
<p>
{#syntax#}usingnamespace{#endsyntax#} is a top level declaration that imports all the declarations of
the operand, which must be a {#link|struct#}, {#link|union#}, or {#link|enum#}, into the current scope:
</p>
{#code_begin|test|usingnamespace#}
usingnamespace @import("std");
test "using std namespace" {
debug.assert(true);
}
{#code_end#}
<p>
Instead of the above pattern, it is generally recommended to explicitly alias individual declarations.
However, {#syntax#}usingnamespace{#endsyntax#} has an important use case when organizing the public
API of a file or package. For example, one might have <code>c.zig</code> with all of the
{#link|C imports|Import from C Header File#}:
</p>
<pre>{#syntax#}
pub usingnamespace @cImport({
@cInclude("epoxy/gl.h");
@cInclude("GLFW/glfw3.h");
@cDefine("STBI_ONLY_PNG", "");
@cDefine("STBI_NO_STDIO", "");
@cInclude("stb_image.h");
});
{#endsyntax#}</pre>
<p>
The above example demonstrates using {#syntax#}pub{#endsyntax#} to qualify the
{#syntax#}usingnamespace{#endsyntax#} additionally makes the imported declarations
{#syntax#}pub{#endsyntax#}. This can be used to forward declarations, giving precise control
over what declarations a given file exposes.
</p>
{#header_close#}
{#header_open|comptime#}
<p>
Zig places importance on the concept of whether an expression is known at compile-time.