toml::format incorrectly formats an empty array of tables which fails to be parsed later on

[ifexception]

I have a scenario where I am using Array of Tables and it can be empty in certain cases. When this occurs, toml11 will correctly parse the file and my program will initialize as the array of tables is formatted according to the toml specification.

However, during the operation of the program, the array of tables can be reset. When this occurs toml11 does not format an empty array of tables as I would expect and then throws a bad_format error when I later try to parse the toml file.

Expected Behavior

Given the following toml structure in code:

toml::value output(
	toml::table{
		{
			"array-of-tables",
			toml::array{}
		},
		{
			"subtable",
			toml::table{}
		},
	}
);

toml::format should return:

[[array-of-tables]]

[subtable]

Which should align to the spec

Current Behavior

Given the following toml structure in code:

toml::value output(
	toml::table{
		{
			"array-of-tables",
			toml::array{}
		},
		{
			"subtable",
			toml::table{}
		},
	}
);

toml::format returns:

array-of-tables = 

[subtable]

Which throws a bad format error when parsing:

[error] bad format: unknown value appeared
--> test.toml
|
1 | array-of-tables =0x0d(CARRIAGE RETURN)
| ^-- here

Steps to Reproduce

test.toml:

array-of-tables =

[subtable]

C++ MRE:

#include <iostream>
#include <string>

#include <toml.hpp>

int main()
{
    toml::value output(
        toml::table{
            {
                "array-of-tables",
                toml::array{}
            },
            {
                "subtable",
                toml::table{}
            },
        }
    );

    auto& aot = output.at("array-of-tables");
    aot.as_array_fmt().fmt = toml::array_format::array_of_tables;
    aot.as_array_fmt().body_indent = 4;
    aot.as_array_fmt().closing_indent = 2;

    std::string asString = toml::format(output);

    std::cout << asString << std::endl;

    try
    {
        auto p = toml::parse("test.toml");
    }
    catch (const std::exception& e)
    {
        std::cerr << e.what() << "\n";
    }

    std::cin.get();
    return 0;
}

Context (Environment)

Visual Studio 2022 CMake project
toml11 installed via vcpkg
toml11 version 4.0.0
64 bits

[ToruNiina]

thank you for the detailed explanation of the problem you met.

But actually, your example code written here contains a mistake.

In TOML, the following defines an empty table named table under the root table.

[table]

In JSON, it corresponds to

{ "table": {} }

And, similarly, the following TOML:

[[tables]]

defines an empty table that is the first element of an array named tables.

So it corresponds to:

{ "tables": [ {} ] }

That means that, an array named tables has one element and is not empty.

The TOML file you intended:

[[array-of-tables]]

[subtable]

corresponds the following in JSON:

{
    "array-of-tables": [{}]
    "subtable":{}
}

And that is NOT equivalent to:

{
    "array-of-tables": []
    "subtable":{}
}

array-of-tables actually contains one empty table, so is not an empty array.

Based on these, you need to add one empty table to your c++ code.

    toml::value output(
        toml::table{
            {
                "array-of-tables",
                toml::array{ toml::table{} } // here
            },
            {
                "subtable",
                toml::table{}
            },
        }
    );

This returns the following:

[subtable]
[[array-of-tables]]

toml::parse also parses the file to this. You can see that the array-of-tables has one element.

https://wandbox.org/permlink/EndLu9HddNBw1A3K

---

That said, it is still a problem that toml::format outputs an invalid toml string in this case. The correct output should be the following if the array is empty.

array-of-tables = []
[subtable]

I have written a patch for this change. Later I will merge and then close this.

[ifexception]

Hi there,

Many thanks for your detailed response and highlighting my erroneous assumption with regards to array of tables.

I added a toml::table{} to the empty toml::array and now formats it correctly and it addresses my original issue :)