The toml table order is changed

[NPYK]

Hi,
Thank you for your developing of this library.

When I use this library to save some data to a toml file, I found if the toml file have many tables like below,
the order of the table will be changed and has no rule for every saving.
Is there a way to save the data with the original order of these tables?
I use the following code to serialize the data to the file:
toml::value tomlData = toml::parse<toml::preserve_comments, std::map>(file);
std::ofstream ofs;
ofs.open(file, std::ios::out | std::ios::binary);
ofs << tomlData << std::endl;

[Title]
Title = "TOML Configration File"
[Owner]
Author = "XXX"
[Table1]
Key1 = "234"
[Table2]
Key2 = 233

[ToruNiina]

Hi,

It seems that the problem is that you explicitly specified the type of tomlData as toml::value at the first line. Since toml::value is an alias of basic_value with default parameters, it uses unordered_map internally. You need to change it to auto (or toml::basic_value<toml::preserve_comments, std::map>, but it might be too long to write) to take the result value as a customized type.

auto tomlData = toml::parse<toml::preserve_comments, std::map>(file);
// ^
std::ofstream ofs;
ofs.open(file, std::ios::out | std::ios::binary);
ofs << tomlData << std::endl;

If you want to preserve the insertion order, you can use a std::map compatible container, like https://github.com/Tessil/ordered-map, instead of std::map. (related issue: #111)

[NPYK]

Thank you, but I tried both the way by declaring the "tomlData" with "auto" or "toml::basic_value<toml::preserve_comments, std::map>", the file still cannot keep the original order, it seems ordered by the ascending order of charactor. (ABCDE...)

[ToruNiina]

Sorry for the confusing wording.

The problem of using toml::value is that, by default, it uses unordered_map that randomizes the order. If we use toml::value, the toml::parse result will be converted into toml::value and the order will be randomized. So we need to use full type (toml::basic_value<...>) or auto. Here we are changing the internal container, so we need to take care about that.

The reason why it is sorted is that std::map is a kind of binary tree and internally sorts the elements. std::map does not preserve the insertion order. The reason why I used map in the last comment is that I wanted to minimize the difference from your example. To keep the original order, we need to use another map-like container that keeps the insertion order.

And there are several implementations of a map that keeps the insertion order in github, like https://github.com/Tessil/ordered-map . To keep the original order, you can use those libraries instead of std::map. (related issue: #111)

So, I mean, you can try the following.

#include "ordered_map.hpp"
#include "toml.hpp"
#include <iostream>

int main()
{
    auto tomlData = toml::parse<toml::preserve_comments, tsl::ordered_map>("sample.toml");
    std::cout << tomlData << std::endl;
}

[NPYK]

Thank you very much!

The order now is saved as expected.
The solution is the same as the related issue: #111.
Only change the parse parameter still cannot save the original order, it's nessasary to change the toml::value default parameter.

[ToruNiina]

I'm glad the problem is solved. But I'm not sure why you need to change the definition of toml::value. Could you give me some more information about

• the version (commit id) of this library you are using
• the TOML file you tried (if it is not a problem)
• the cpp code you wrote (if it is not a problem)

Actually I have tried the following code without any modification.

#include "ordered-map/include/tsl/ordered_map.h"
#include "toml.hpp"
#include <iostream>

int main()
{
    auto tomlData = toml::parse<
        toml::preserve_comments, tsl::ordered_map>("issue133.toml");

    std::cout << tomlData << std::endl;
    return 0;
}

[table3]
[table3.3]
key2 = "foo"
key1 = "bar"
[table3.2]
[table3.1]

[table2]
[table2.2]
[table2.3]
[table2.1]

[table1]
[table1.3]
[table1.1]
[table1.2]

And the order seems to be preserved. I want to make it easier (and because of some curiosity), so if it is not too much trouble, I would like to see the code that does not work.

[NPYK]

Sorry for late reply, as the project is too large, so I'll try to reproduce it with a new simple project and later share with you.

[NPYK]

Ther verison of toml11 I used is the latest code I guess it's v3.6.0