GenConfig

Aims to provide less-painful C/C++ app configuration support.

Motivation

Created because all of the config systems I have used (or created) in the past had too many pain points, like:

• Requiring things to be defined in multiple places
• Not using standard, human-friendly configuration formats
• Lacking validation of basic types and structure
• Not supporting hierarchical data structures, arrays, strings very well
• Requiring tedious structure maintenance
• Requiring accessor calls to get at simple data fields
• Not supporting configuration deltas

This system attempts to alleviate those pain points and make configuration suck less:

• Automatically generates structure definition code from an input specification, which is easy to write
• Loads application configuration from TOML format, which is standardized and human-friendly
• Validates user configuration matches expected structure and fields are of expected type
• Support hierarchical structures of basic types: bool, int, float, enum, strings (UTF-8), and arrays
• Minimizes development effort: just define the setting once in a spec file and the rest is handled
• Usable in C/C++, without having to do any lookups or extra validation: just read from a struct
• Supports saving configuration deltas

How it Works

• You define the options you want for your app in a spec file
• You run gen_config.py to generate a header file with struct config definition
• User runs your app, providing their settings in a .toml file matching your specification
  • Your code calls a function to load, parse, and store the config file to the config structure
  • Your code accesses the config by just reading from and writing to the config struct
  • Your code changes config structure in response to some user action
  • Your code calls a function to save the config delta to the user's .toml config file
• You retain a bit more hair that you might have ripped out using/creating another configuration system
• Your app makes lots of money and you send some my way because you support open-source development. Thanks, that's nice of you.

Requirements

• toml++, which is used to parse config file
• Compiler that supports C++17

Example

This is the specification file that defines your config options:

company:
  name: string
  headquarters:
    state: string
    city: string

  products:
    type: array
    items:
      name:
        type: string
        default: New Product
      price: float
      inventory: int
      international_shipping:
        type: bool
        default: true
      suppliers:
        type: array
        items: string
      category:
        type: enum
        values: ['fruit', 'vegetable', 'beverage', 'explosive']

After running gen_config.py, this C code is generated automatically.

enum CONFIG_COMPANY_PRODUCTS_CATEGORY {
	CONFIG_COMPANY_PRODUCTS_CATEGORY_FRUIT,
	CONFIG_COMPANY_PRODUCTS_CATEGORY_VEGETABLE,
	CONFIG_COMPANY_PRODUCTS_CATEGORY_BEVERAGE,
	CONFIG_COMPANY_PRODUCTS_CATEGORY_EXPLOSIVE,
	CONFIG_COMPANY_PRODUCTS_CATEGORY__COUNT
};

struct config {
  struct company {
    const char *name;
    struct headquarters {
      const char *state;
      const char *city;
    } headquarters;
    struct products {
      const char *name;
      float price;
      int inventory;
      bool international_shipping;
      const char **suppliers;
      unsigned int suppliers_count;
      enum CONFIG_COMPANY_PRODUCTS_CATEGORY category;
    } *products;
    unsigned int products_count;
  } company;
};

Additionally, a corresponding `CNode` tree is created, that's used to support everything:

CNode config_tree =
 ctab("config", {
  ctab("company", {
   cstring(
    offsetof(struct config, company.name),
    "name", ""),
   ctab("headquarters", {
    cstring(
     offsetof(struct config, company.headquarters.state),
     "state", ""),
    cstring(
     offsetof(struct config, company.headquarters.city),
     "city", "")
   }),
   carray(
    offsetof(struct config, company.products),
    offsetof(struct config, company.products_count),
    sizeof(struct config::company::products),
    "products",
    ctab("", {
     cstring(
      offsetof(struct config::company::products, name),
      "name", "New Product"),
     cnumber(
      offsetof(struct config::company::products, price),
      "price", 0.0),
     cinteger(
      offsetof(struct config::company::products, inventory),
      "inventory", 0),
     cbool(
      offsetof(struct config::company::products, international_shipping),
      "international_shipping", true),
     carray(
      offsetof(struct config::company::products, suppliers),
      offsetof(struct config::company::products, suppliers_count),
      sizeof(((struct config::company::products *){0})->suppliers[0]),
      "suppliers",
      cstring(
       0, "", "")
     ),
     cenum(
      offsetof(struct config::company::products, category),
      "category", {"fruit", "vegetable", "beverage", "explosive"}, "fruit")
    })
   )
  })
 });

Config file to be loaded at runtime, in TOML format:

[company]
	name = 'Acme Corp'
	products = [
		{ name = 'Apple', price = 1.2, inventory = 100, suppliers = ['Midwest Orchard', 'Tasty Apples Inc.'], category = 'fruit' },
		{ name = 'TNT', price = 50, inventory = 1000, category = 'explosive', international_shipping = false },
		]

[company.headquarters]
	city = 'Phoenix'
	state = 'Arizona'

Loading the config basically looks like:

#include <toml.hpp> // Required for basic parsing
#include <cnode.h> // Required data structure for config mgmt

#define DEFINE_CONFIG_TREE
#include "config.h"

// Load config from user file
struct config s;
auto toml_table = toml::parse_file("config.toml");
config_tree.update_from_table(toml_table);
config_tree.store_to_struct(&s);

Then when you are ready to save the config again:

// Update config tree from structure if you have modified it
config_tree.update_from_struct(&s);

// Save config
FILE *f = fopen("config.toml", "wb");
fprintf(f, "%s", config_tree.generate_delta_toml().c_str());