Re: c++ build question

James Kanze <>
Mon, 2 Nov 2009 02:08:52 -0800 (PST)
On Nov 1, 11:02 am, Angus <> wrote:

I hope this is an ok topic for this forum.

When I compile a C++ file I can specify on the command line a

Formally, the C++ standard doesn't say anything about this:-).
But in practice, yes; every compiler has some means of
"predefining" preprocessor variables (and every compiler also
has a few that are automatically predefined as well). For that
matter, every compiler I've seen uses the D option (typically
-D, but /D is generally accepted under Windows as well), with
some variation as to whether there can be a space between the D
and the preprocessor symbol. (In other words, "-D_DEBUG" can be
used more or less portably. But since most of the rest of the
command line is totally unportable, it really doesn't matter.)

In my compiler it can be eg /D "_DEBUG". But on my command
line I can also specify /Od - which disables optimisations.
Are they the same thing?

No. The "/O" part and the "/D" part are the same thing:
compiler options (or directives). The first directs the
compiler to pre-define the following symbol; the second directs
it configure optimization according to the following flags.

I can also use #define _DEBUG in my source file. Does that
have the same effect as the command line /D "DEBUG"?

As /D _DEBUG? More or less, provided the #define _DEBUG is the
first line of your program (e.g. before any includes).

Note that _DEBUG is a special symbol, whose effects are defined
by the standard, and that it is not designed to be defined in
the command line or at the top of the code, but rather somewhere
within the code, before including <assert.h> (for a second
time), so that you only turn off assertions around the critical
block, and not everywhere.

James Kanze

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