Compile-time checks

Some mistakes can be caught before the program ever runs: a record whose layout changed, a build made without a setting it needs. The preprocessor can test these while it reads the source, and stop the build when the answer is wrong.

'' Questions answered while compiling, not while running.
type Header
    magic as uint32
    size as uint32
end type

'' #assert stops the build when its condition is false: if a field is
'' ever added to Header, this program no longer compiles.
#assert sizeof( Header ) = 8

dim as int32 count = 0
dim as int64 total = 0

'' #if typeof( ) compares two types exactly.
#if typeof( count ) = typeof( int32 )
    print "count is an int32"
#endif
#if typeof( total ) <> typeof( count )
    print "total is a different type"
#endif

'' #print reports a note while the program compiles.
#print the Header layout is checked

print "Header is"; sizeof( Header ); " bytes"

Output

count is an int32
total is a different type
Header is 8 bytes

While it compiles, the program also reports its #print line:

checks.tiko(23,1): note TKO0315: #print the Header layout is checked
     |
  23 | #print the Header layout is checked
     | ^

• #assert cond stops the build when cond is false. Change the 8 to 12 and the build fails with TKO0604, "#assert condition failed". A record that is written to files is a good place for one: the build stops before a file in the old layout is misread.
• #if typeof( a ) = typeof( b ) compares two types exactly: int32 is not int64.
• #print text reports its text as a note, TKO0315, and the build goes on.

#error stops the build with a message of your own:

'' This program needs a setting chosen when it is built.
#ifndef CONFIG
    #error "build with -d CONFIG=1 (development) or -d CONFIG=2 (release)"
#else
    print "configuration"; CONFIG
#endif

needconfig.tiko(3,5): error TKO0309: #error: "build with -d CONFIG=1 (development) or -d CONFIG=2 (release)"
     |
   3 |     #error "build with -d CONFIG=1 (development) or -d CONFIG=2 (release)"
     |     ^

Built with -d CONFIG=2, the same file prints configuration 2. Keep the code that needs the setting in the #else branch: the compiler reads on past an #error to report everything it can, so code after the #endif that used CONFIG would add a second error.

For checks on values while the program runs, see Error Handling.

Activities

1. Add a flags as uint16 field to Header and read the error.
2. Write an #if that stops the build with #error unless __FB_64BIT__ is defined.