Using array literal in macro

Question

I have a struct like this:

typedef struct {
  int num_files;
  audio_file files[];
} message_files;

and I want to create a macro so I don't have to specify the number of files of a statically known array:

#define FILES(arr) {sizeof((audio_file []) arr) / sizeof(audio_file), arr}

message_files files[3] = {
  FILES({{1,500}, {2,500}}),
  FILES({{3,1000}),
  FILES({{4,5000}, {5,10}})
};

But this doesn't work: I get error: macro "FILES" passed 4 arguments, but takes just 1. How can I make the macro accept the array literal as such?

{{1,500}, {2,500}} is not an array literal. What are you trying to do here? Assuming the error in question is fixed, macro expansion would lead to message_files files[1] = { {sizeof((audio_file []) {{1,500}, {2,500}}) / sizeof(audio_file), {{1,500}, {2,500}}} }; which doesn't make sense syntactically. — Yksisarvinen
– Yksisarvinen, Commented Oct 15, 2024 at 10:40
It doesn't make sense conceptually to form an array of message_files. This is a class with flexible array member (which btw. are not valid in standard C++), so it should be individually allocated for. (audio_file []) arr is also not valid C++ syntax. Compound literals exist in C, but not C++. — user17732522
– user17732522, Commented Oct 15, 2024 at 10:40
I'm programming for Arduino, this may explain the C/C++ mixture. — return true
– return true, Commented Oct 15, 2024 at 10:44
@returntrue It is not clear what you are trying to achieve, even if I take the non-standard extensions from C for granted. Flexible array members must be used differently with dynamic allocation. — user17732522
– user17732522, Commented Oct 15, 2024 at 10:45
Why bother with macros instead of templated reference to array and variadic templates if C++ is used anyway? What C++ version are you on? — alagner
– alagner, Commented Oct 15, 2024 at 11:05

Pepijn Kramer · Accepted Answer · 2024-10-15 14:14:18Z

3

Your code is decidedly a "C" style approach, in C++20 I would do this :

#include <array>
#include <string_view>
#include <concepts>

struct audio_file_t
{
    int id;
    std::string_view name;
};

int main()
{
    constexpr auto files = std::to_array<audio_file_t>({{ 1, "file1" }, { 2, "file2" }});
    static_assert(files.size() == 2);
}

Update to include a more nested structure (no longer constexpr)

#include <array>
#include <vector>
#include <string_view>
#include <iostream>
#include <format>

struct audio_file_t
{
    int id;
    std::string_view name;
};

struct message_files_t
{
    int id;
    std::vector<audio_file_t> files;
};

std::ostream& operator<<(std::ostream& os, const audio_file_t& file)
{
    std::cout << std::format("File id : {}, name : {}", file.id, file.name);
    return os;
}

int main()
{
    constexpr auto files = std::to_array<audio_file_t>({{ 1, "file1" }, { 2, "file2" }});
    static_assert(files.size() == 2);

    std::vector<audio_file_t> temp{{ 4, "file3.1" }, { 5, "file3.1" }, { 6, "file3.1" }};

    

    // This is the initialization you're looking for
    std::vector<message_files_t> multi_files{ 
        {11, {{ 1, "file1.1" }, { 2, "file2.1" }}},
        {22, {{ 3, "file2" }}},
        {33, {{ 4, "file3.1" }, { 5, "file3.2" }, { 6, "file3.3" }}}
    };
    
    for(const auto& message_files : multi_files)
    {
        std::cout << "Message Files, id = " << message_files.id << "\n";
        for( const auto& audio_file : message_files.files)
        {
            std::cout << audio_file << "\n";
        }
    }

}

edited Oct 15, 2024 at 14:14

answered Oct 15, 2024 at 11:10

Pepijn Kramer

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3 Comments

return true Over a year ago

How does this work when files consists of more than one message_files?

return true Over a year ago

I edited the question to clarify what I want to achieve.

Pepijn Kramer Over a year ago

I updated my answer whatever you do don't use macros

Toby Speight · Accepted Answer · 2024-10-16 07:06:06Z

0

If you need to pass an argument that contains , to a C or C++ macro, you generally need to ensure it's enclosed in at least one level of parentheses (braces don't work). For example:

message_files files[3] = {
    FILES(({{1,500}, {2,500}})),
    FILES(({{3,1000}})),
    FILES(({{4,5000}, {5,10}}))
};

That said, a macro is almost certainly the wrong approach here, and a template function would be much better.

answered Oct 16, 2024 at 7:06

Toby Speight

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Comments

return true · Accepted Answer · 2024-10-16 06:54:10Z

-2

I found a solution using va_args:

#define FILES(...) (message_files) { (sizeof((audio_file []) {__VA_ARGS__}) / sizeof(audio_file)), (audio_file[]) { __VA_ARGS__ }}

edited Oct 16, 2024 at 6:54

answered Oct 15, 2024 at 13:20

return true

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3 Comments

Pepijn Kramer Over a year ago

Proposing a "C" style macro with VA_ARGS no less is not really a C++ approach

user17732522 Over a year ago

It doesn't work though. audio_files doesn't appear in your question and as I said in my comments, you are fundamentally using flexible array members wrong. Any compiler that supports flexible array members that I know will complain about it. You can't initialize them with a braced-init-list. Instead they are supposed to cover memory after the structure in an overallocation for the structure via malloc.

return true Over a year ago

Oh it does work. I didn't ask for a C++-idiomatic solution, just something that works for my Arduino project.

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