Recursive merge - Githubissues

I want to implement a config system (using TOML) which supports inherits/includes type functionality, so I can have a base config and specializations, or include standard blocks in the final config (in order to avoid the usual DRY/copy-paste problems that configs inevitably end up with).

To be clear, I am not wanting this as some first-class feature in TOML, just something I can implement on top. I also realise this is not the same thing as appending or textually including .toml files.

It seems what is needed is a table merge. Something like C++17's std::map::merge(), or std::merge(). https://en.cppreference.com/w/cpp/container/map/merge

Although ideally I'd like to merge into a subtree (to include standard blocks in different sub-keys).

Either: (a) Some guidance on how to implement this using the existing public API. I had a go using iteration, visit and insert but couldn't get it working. and/or: (b) The functionality added to the API, e.g.

Could either do inplace merge where one table is also the output, or return a new table. Optionally add overloads where the whole table is passed to cut down on .begin() noise.

void toml::merge(iterator dest, iterator src, toml::replacement_policy = replace); // or src,dest depending on preferred style Where replacement_policy is e.g. keep, replace, throw. And/or: tbl toml::merge(input1, input2, replacement_policy);

Pseudo-code:

//////////////////////////////

auto base = load("base.toml");
auto overrides = load("overrides.toml");
auto config = toml::merge(base, overrides);

base.toml;
[section1]
key1="value1"
key2="value2"

overrides.toml:
[section1]
key2="value3"

Output:
[section1]
key1="value1"
key2="value3"

//////////////////////////////

auto table = load("my_server.toml"));
if (tbl.contains["inherits"])
    toml::merge(table, load(tbl["inherits"]), replacement_policy::keep);

server_base.toml:
[server]
ip=192.168.100.100
debug_mode=false

my_server.toml:
inherits="server_base.toml"
[server]
debug_mode=true

Result:
[server]
ip=192.168.100.100
debug_mode=true

//////////////////////////////

// Put a subtree into 2 different places in the top-level table.
toml::merge(find("client1"), load(tbl["default_client_config"]), replacement::policy::throw);
toml::merge(find("client2"), load(tbl["default_client_config"]), replacement::policy::throw);

default_client_config.toml:
connect=true
print_thing=false

config.toml
[server]
ip=192.168.100.100
debug_mode=false

[client1]
include="default_client_config.toml"

[client2]
include="default_client_config.toml"

Result:
[server]
ip=192.168.100.100
debug_mode=false

[client1]
connect=true
print_thing=false

[client2]
connect=true
print_thing=false

marzer / tomlplusplus

Recursive merge #155