Thread: Assert expression

Hi everyone,

I am looking a code snippet for ring buffering nRF5 SDK v15.2.0: Ring buffer library

Code:

void foo(void)
{
    ret_code_t err_code;
    uint8_t data_in[] = {1,2,3,4};
    uint8_t data_out[5];
    size_t  len_in = sizeof(data_in);
    size_t  len_out = sizeof(data_out);

    err_code = nrf_ringbuf_cpy_put(&m_ringbuf, data_in, &len_in);

    err_code = nrf_ringbuf_cpy_get(&m_ringbuf, data_out, &len_out);
    ASSERT(len_out == len_in);
    ASSERT(memcmp(data_in, data_out, sizeof(data_in)) == 0);
} 

I do not understand the last line. I have two questions:
1. What does it means sizeof(data_in)) == 0?
2. When the expression will assert?