example-dao.cpp

/**
 * ***README***
 * This example shows how attributes can be used to build powerful
 * abstractions. Here we utilize two user-defined attributes to
 * generate SQL queries which insert elements into a database.
 * The SQL queries are generated all at compile-time using const_strings.
 */
#include <cstdint>
#include <sstream>
#include <iostream>

#include "refl.hpp"

/** An attribute for specifying a DB table's properties */
struct Table : refl::attr::usage::type
{
    const char* name;

    constexpr Table(const char* name) noexcept
        : name(name)
    {
    }
};

enum class DataType
{
    ID,
    TEXT,
};

/** An attribute for specifying a DB column's properties */
struct Column : refl::attr::usage::field
{
    const char* name;
    const DataType data_type;

    constexpr Column(const char* name, DataType dataType) noexcept
        : name(name), data_type(dataType)
    { }
};

/** User will serve as the "model" for the "Users table" */
struct User
{
    std::uint32_t id;
    std::string email;
};

/** Define the reflection metadata for the model */
REFL_AUTO(
    type(User, Table{"Users"}),
    field(id, Column{"ID", DataType::ID}),
    field(email, Column{"Email", DataType::TEXT})
);

template <typename Member>
constexpr auto make_sql_field_spec(Member)
{
    using namespace refl;

    constexpr auto col = descriptor::get_attribute<Column>(Member{});

    if constexpr (DataType::ID == col.data_type) {
        // convert the const char* data member to a refl::const_string<N>
        // (necessary to be able to do compile-time string concat)
        return REFL_MAKE_CONST_STRING(col.name) + " int PRIMARY KEY";
    }
    else if constexpr (DataType::TEXT == col.data_type) {
        return REFL_MAKE_CONST_STRING(col.name) + " TEXT";
    }
}

/**
 * Creates a CREATE TABLE SQL query.
 * Returns the query as a refl::const_string<N>.
 */
template <typename T>
constexpr auto make_sql_create_table()
{
    using namespace refl;
    using Td = type_descriptor<T>;

    // Verify that the type has the Table attribute.
    static_assert(descriptor::has_attribute<Table>(Td{}));

    // Concatenate all the members' column definitions together
    constexpr auto fields = accumulate(Td::members,
        [](auto acc, auto member) {
            return acc + ",\n\t" + make_sql_field_spec(member);
        }, make_const_string())
        .template substr<2>(); // remove the initial ",\n"

    constexpr auto tbl = descriptor::get_attribute<Table>(Td{});
    // convert the const char* data member to a refl::const_string<N>
    // (necessary to be able to do compile-time string concat)
    constexpr auto tbl_name = REFL_MAKE_CONST_STRING(tbl.name);

    // return the resulting compile-time string
    return "CREATE TABLE " + tbl_name + " (\n" + fields + "\n);";
}

int main()
{
    // sql is of type const_string<N> where N is the length of the
    // compile-time string.
    constexpr auto sql = make_sql_create_table<User>();
    std::cout << sql << "\n";
    std::cout << "Number of characters: " << sizeof(sql) - 1 << "\n";
}