error: invalid application of 'sizeof' to incomplete type

[ricomoss]

I'm running into an error when trying to update legacy code.

Small Portion of Code:

    if (WhichLayout == STD_LAYOUT)
    {
        int num_stand_fields = sizeof(StandardLayout)/sizeof(FirmFieldStruct);
        for (int i = 0; i < num_stand_fields; ++i)
        {
            if (StandardLayout[i].ReportCounts)
            {
                ReportOnField[StandardLayout[i].Id] = 1;
            }
        }
    }
    else if (WhichLayout == MATCH_LAYOUT)
    {
226     int num_match_fields = sizeof(MatchLayout)/sizeof(FirmFieldStruct);
        for (int i = 0; i < num_match_fields; ++i)
        {
            if (MatchLayout[i].ReportCounts)
            {
                ReportOnField[MatchLayout[i].Id] = 1;
            }
        }
    }

Error:

226: error: invalid application of 'sizeof' to incomplete type 'FirmFieldStruct []'

I'm not sure what is being done wrong here. This is not code I've changed.

Also, why would int num_stand_fields = sizeof(StandardLayout)/sizeof(FirmFieldStruct); be ok but int num_match_fields = sizeof(MatchLayout)/sizeof(FirmFieldStruct); cause an error?

[Bazzy]

Probably MatchLayout was only forward declared

[ricomoss]

Yes, this is true. Why would this cause a problem? What can be done here to fix it?

[filipe]

The compiler can't tell the size of an object just from a forward declaration (for obvious reasons: there's nothing there but a name). You'll need to #include the proper header to use that approach.

[ricomoss]

I understand what went wrong. Thank you.