package main

import "core:mem"

main :: proc() {
	// In each scope, there is an implicit value named context. This
	// context variable is local to each scope and is implicitly passed
	// by pointer to any procedure call in that scope (if the procedure
	// has the Odin calling convention).

	// The main purpose of the implicit context system is for the ability
	// to intercept third-party code and libraries and modify their
	// functionality. One such case is modifying how a library allocates
	// something or logs something. In C, this was usually achieved with
	// the library defining macros which could be overridden so that the
	// user could define what they wanted. However, not many libraries
	// supported this in many languages by default which meant intercepting
	// third-party code to see what it does and to change how it does it is
	// not possible.

	c := context // copy the current scope's context

	context.user_index = 456
	{
		context.allocator = my_custom_allocator()
		context.user_index = 123
		what_a_fool_believes() // the `context` for this scope is implicitly passed to `what_a_fool_believes`
	}

	// `context` value is local to the scope it is in
	assert(context.user_index == 456)

	what_a_fool_believes :: proc() {
		c := context // this `context` is the same as the parent procedure that it was called from
		// From this example, context.user_index == 123
		// An context.allocator is assigned to the return value of `my_custom_allocator()`
		assert(context.user_index == 123)

		// The memory management procedure use the `context.allocator` by
		// default unless explicitly specified otherwise
		china_grove := new(int)
		free(china_grove)

		_ = c
	}

	my_custom_allocator :: mem.nil_allocator
	_ = c

	// By default, the context value has default values for its parameters which is
	// decided in the package runtime. What the defaults are are compiler specific.

	// To see what the implicit context value contains, please see the following
	// definition in package runtime.
}