varquirements

Varquirements is a library that let you define and enforce arbitrary constraints between the types of a variadic pack. The types are treated in sequence and each one can define new properties, modify existing properties or require that certain preconditions are met.

Prerequisites

A C++14 compiler

Installation

Add the varquirements directory in your includes. The library is header only!
Tests use the google test framework. Examples are in the src directory and don't use any extra dependency.

How to use

For example, if we have some sort of pipeline defined by types, we can add simple constraints in this way:

struct resource_created {};

struct Create_resource : varquirements::have<resource_created> {};
struct Use_resource : varquirements::require<resource_created> {};

const auto value = varquirements::check_requirements<Create_resource, Use_resource>::value;

varquirements::have let us add new properties while varquirements::require let us state the properties that a given type requires.

There can be multiple requirements and properties:

struct resource_created {};
struct initialize {};

struct Create_resource : varquirements::have<resource_created> {};
struct Init : varquirements::require<resource_created>, varquirements::have<initialize> {};
struct Use_resource : varquirements::require<resource_created, initialize> {};

const auto value = varquirements::check_requirements<Create_resource, Init, Use_resource>::value;

Properties customization

The interaction between various properties can be customized like this:

struct resource_created {};
struct resource_deleted {};
struct initialize {};

namespace varquirements
{
    template <>
    struct apply_property<resource_created, resource_deleted>
    {
        using type = void;
        static constexpr bool append = true;
    };
}

struct Create_resource : varquirements::have<resource_created> {};
struct Init : varquirements::require<resource_created>, varquirements::have<initialize> {};
struct Use_resource : varquirements::require<resource_created, initialize> {};
struct Delete_resource : varquirements::require<resource_created>, varquirements::have<resource_deleted> {};
struct Close : varquirements::require<resource_deleted> {};

const auto value = varquirements::check_requirements<Create_resource, Init, Use_resource, Delete_resource, Close>::value;

Disable static assertions

By default, varquirements generates a static assertion when a requirements is violated. If you want to disable this behaviour you must define VARQUIREMENTS_NO_STATIC_ASSERT and then to manually make a check just query varquirements::check_requirements<>::value.

Trisfald / varquirements