问题
Consider the following snippet:
requires
designide,
rtl,
vcl,
{$IF RTLVersion < 19.0} // E2026 Constant expression expected
//{$IF CompilerVersion = 22.0} // same as above
vcljpg;
{$ELSE}
vclimg;
{$IFEND}
It seems to be absolutely syntactically correct. However, the compiler chokes on it and reports Constant expression expected
. What really happens here?
Technical: currently tested on XE (15.0.3953.35171) only.
Of course, workaround suggestions are welcome too.
回答1:
I found the same issue in the past even with delphi 2007. As workaround, I use a inc file with the conditional defines and then use {$IFDEF}
instead of {$IF}
something like so
{$I MyDefines.INC}
requires
designide,
rtl,
vcl,
{$IFDEF DELPHI_XE_UP} //the DELPHI_XE_UP is defineed inside of MyDefines.INC
uNewlib;
{$ELSE}
uOldLib;
{$ENDIF}
回答2:
package
modules are different from program
and library
modules. They do not contain executable code and you cannot use
units. Therefore, symbols like RTLVersion
are simply not visible from a package file. Your only option is to use $IFDEF
.
回答3:
I'm convinced what i just found the cause. Consider the following:
{$IF not Declared(RTLVersion)}
{$MESSAGE WARN 'There is no RTL'}
{$IFEND}
{$IF not Declared(CompilerVersion)}
{$MESSAGE WARN 'nor are compiler intrinsics at all'}
{$IFEND}
{$IF not Declared(System)}
{$MESSAGE ERROR 'Because package not uses System implicitly'}
{$IFEND}
So, it appears to be what compiler behaves correctly, but issues a rather misleading (if not erroneous) message about symbol not being a constant expression, while symbol in question actually is undeclared!
来源:https://stackoverflow.com/questions/8305657/are-conditional-expressions-broken-within-packages