Question about the line_request structure of libgpiod

I am looking at the example libgpiod gpiogetcxx.cpp from github https://github.com/brgl/libgpiod/blob/master/bindings/cxx/examples/gpiogetcxx.cpp i sort of confuse on

The line_bulk::request(const line_request& config, const ::std::vector<int> default_vals) taking line_request structure as reference parameter as shown in below.

struct line_request
{
    /**
     * @brief Request types.
     */
    enum : int {
        DIRECTION_AS_IS = 1,
        /**< Request for values, don't change the direction. */
        DIRECTION_INPUT,
        /**< Request for reading line values. */
        DIRECTION_OUTPUT,
        /**< Request for driving the GPIO lines. */
        EVENT_FALLING_EDGE,
        /**< Listen for falling edge events. */
        EVENT_RISING_EDGE,
        /**< Listen for rising edge events. */
        EVENT_BOTH_EDGES,
        /**< Listen for all types of events. */
    };

    GPIOD_API static const ::std::bitset<32> FLAG_ACTIVE_LOW;
    /**< Set the active state to 'low' (high is the default). */
    GPIOD_API static const ::std::bitset<32> FLAG_OPEN_SOURCE;
    /**< The line is an open-source port. */
    GPIOD_API static const ::std::bitset<32> FLAG_OPEN_DRAIN;
    /**< The line is an open-drain port. */
    GPIOD_API static const ::std::bitset<32> FLAG_BIAS_DISABLE;
    /**< The line has neither pull-up nor pull-down resistor enabled. */
    GPIOD_API static const ::std::bitset<32> FLAG_BIAS_PULL_DOWN;
    /**< The line has a configurable pull-down resistor enabled. */
    GPIOD_API static const ::std::bitset<32> FLAG_BIAS_PULL_UP;
    /**< The line has a configurable pull-up resistor enabled. */

    ::std::string consumer;
    /**< Consumer name to pass to the request. */
    int request_type;
    /**< Type of the request. */
    ::std::bitset<32> flags;
    /**< Additional request flags. */
};

I dont understand the following code

lines.request({
    argv[0],
    ::gpiod::line_request::DIRECTION_INPUT,
    0
});

1. i observed that it seemed like the code populate the structure member in very random order,am i correct? i assumed argv[0] correspond to ::std::string consumer,DIRECTION_INPUT correspond to the enum, i not really get what the 0 try to do.

2. I don't see any declaration of structure variable such as line_request obj in the code using csope, i was wonder why this code will not encounter invalid initialization during compilation

3. what happens to other structure members that do not involve in initialization? will it become zero by default?