In order to be able to pass a const reference of the SBE Message and access the underlying buffer
consider adding method:

char const* buffer() const SBE_NOEXCEPT;
char const* buffer() const SBE_NOEXCEPT
{
    return m_buffer;
} 

The generator only generates a non-const version:

char* buffer() SBE_NOEXCEPT
{
     return m_buffer;
}