My code:

module http.HttpRequest;

import std.container;
import std.array : Appender;

struct HttpRequest
{
    struct Header()
    {
        Appender!string name;
        Appender!string value;
    }

    string method;
    string uri;
    int versionMajor = 0;
    int versionMinor = 0;
    Array!Header headers;
    ubyte[] content;
    bool keepAlive = false;
}

Error code:

source/http/HttpRequest.d(18,5): Error: struct `std.container.array.Array` does not match any template declaration

On Tuesday, 14 December 2021 at 08:12:04 UTC, zoujiaqing wrote:

module http.HttpRequest;

import std.container;
import std.array : Appender;

struct HttpRequest
{
    struct Header()
    {
        Appender!string name;
        Appender!string value;
    }

    string method;
    string uri;
    int versionMajor = 0;
    int versionMinor = 0;
    Array!Header headers;
    ubyte[] content;
    bool keepAlive = false;
}

source/http/HttpRequest.d(18,5): Error: struct `std.container.array.Array` does not match any template declaration

the problem is that your header is a template, so you need to instantiate it:

Array!(Header!()) headers;

the error message is kinda poor here.

Alternatively, remove the template () from your struct Header

On Tuesday, 14 December 2021 at 08:28:01 UTC, WebFreak001 wrote:
[...]

What is the semantic sense of a template having no parameters?

Although the documentation declares such a template to be syntactically correct, not a single example suggests a meaning.

On Wednesday, 15 December 2021 at 11:36:41 UTC, Manfred Nowak wrote:

For functions there is a use-case: they will not be compiled/included in the executable until they are actually instantiated once.

Additionally for functions that means for libraries they aren't usually compiled into the resulting library file, unless they are used in the library themself. (only really meaningful when you don't distribute the D source or when interoping with other languages)

For types similarly, if you have any template instantiations inside your templated type, they will not be instantiated or compiled, until that template is used at least once in code anywhere.

So:

struct Header()
{
    Appender!string name;
}

unless Appender!string is used anywhere else in the code, that Appender struct will never be initiated, thus no member functions will be emitted or compile time will be used, until you instantiate the Header struct once.

On Wednesday, 15 December 2021 at 11:36:41 UTC, Manfred Nowak wrote:

In this case, nothing.

In other cases to only compile said "object" in specific conditions.

On 12/15/21 6:36 AM, Manfred Nowak wrote:

It's just a template with no parameters, like a function with no parameters. It provides a separate namespace for items.

A struct with empty template parameters is equivalent to:

template T()
{
   struct T {}
}

-Steve

On Wednesday, 15 December 2021 at 11:36:41 UTC, Manfred Nowak wrote:

To add to other responses, certain features are only available for templates, such as constraints and auto ref parameters.

On 12/15/21 9:41 AM, Stanislav Blinov wrote:

Oh yeah, that reminds too -- a templated struct will have all member function attributes inferred.

-Steve