G++ paste operator - expands as desired but compile fails

[Chris]

Hello all,

Using g++ 3.3.1 on Linux ("Linux From Scratch") I have a data
structure SCSIParams_t that I wish to print out, field-by-field.
Rather than code a long line of the form

std::cout << "fieldname = " << basepointer->fieldname <<
std::endl;

for every field, I'd like to do it with a short macro, e.g.

EMITSCSI(SCSIDriverType);

I've coded a macro definition

#define EMITSCSI(fieldname) \
std::cout << # field << " = " << basepointer->##fieldname <<
std::endl

and coded, as a test, a single line identical to the "short macro,
e.g..." example shown above.

That line generates the compile error,

pasting "->" and "fieldname" does not give a valid
preprocessing token

However, using the -E switch on the compile line, I find that the
macro does, in fact, expand to a line identical to the "long line..."
shown above. In fact, if I cut-and-paste the expansion text from the
preprocessor output into my source file, and comment out the line that
uses the macro, my code compiles cleanly. That is to say, "expanding
the macro" MANUALLY, i.e. "by hand," leads to successful compilation,
but letting the preprocessor do it leads to failiure-to-compile.

Can someone suggest a way around this? I also have g++ 3.4.3
available, although that's not what we use for standard development.

Thanks,

Chris

PS - the e-mail address appearing on this message is no longer valid,
so don't e-mail. My new address is similar but different:
(e-mail address removed) . I'll create a new Google Groups login soon...

 

[Ondra Holub]

Hello all,

Using g++ 3.3.1 on Linux ("Linux From Scratch") I have a data
structure SCSIParams_t that I wish to print out, field-by-field.
Rather than code a long line of the form

std::cout << "fieldname = " << basepointer->fieldname <<
std::endl;

for every field, I'd like to do it with a short macro, e.g.

EMITSCSI(SCSIDriverType);

I've coded a macro definition

#define EMITSCSI(fieldname) \
std::cout << # field << " = " << basepointer->##fieldname <<
std::endl

and coded, as a test, a single line identical to the "short macro,
e.g..." example shown above.

That line generates the compile error,

pasting "->" and "fieldname" does not give a valid
preprocessing token

However, using the -E switch on the compile line, I find that the
macro does, in fact, expand to a line identical to the "long line..."
shown above. In fact, if I cut-and-paste the expansion text from the
preprocessor output into my source file, and comment out the line that
uses the macro, my code compiles cleanly. That is to say, "expanding
the macro" MANUALLY, i.e. "by hand," leads to successful compilation,
but letting the preprocessor do it leads to failiure-to-compile.

Can someone suggest a way around this? I also have g++ 3.4.3
available, although that's not what we use for standard development.

Thanks,

Chris

PS - the e-mail address appearing on this message is no longer valid,
so don't e-mail. My new address is similar but different:
(e-mail address removed) . I'll create a new Google Groups login soon...

1. Delete extra white-space between # and field
2. Delete ## - it is not necessary. -> and member name are different
preprocessor tokens, so you do not need to concat them with ##
#define EMITSCSI(fieldname) \
std::cout << #field << " = " << basepointer->fieldname <<
std::endl

 

[red floyd]

Hello all,

Using g++ 3.3.1 on Linux ("Linux From Scratch") I have a data
structure SCSIParams_t that I wish to print out, field-by-field.
Rather than code a long line of the form

std::cout << "fieldname = " << basepointer->fieldname <<
std::endl;

for every field, I'd like to do it with a short macro, e.g.

EMITSCSI(SCSIDriverType);

I've coded a macro definition

#define EMITSCSI(fieldname) \
std::cout << # field << " = " << basepointer->##fieldname <<
std::endl

Don't bother with ## here. It's only needed when you want to combine
two preprocessor tokens into one.

I.e. you want to create a new identifier. Since -> and fieldname are
two separate tokens anyways, just go with

std::cout << #fieldname " = " << basepointer->fieldname <<
std::endl;

Note that I corrected your typo, and that I elided the (unneeded) <<
between #field and " = ". Since they're both string literals, the
compiler will concatenate them.

You might want to look into overloading operator<< for SCSIParams_t,
i.e.

std:stream& operator<<(std:stream&, const SCSIParams_t&);

 

[David Harmon]

On Wed, 15 Aug 2007 10:35:30 -0700 in comp.lang.c++, Chris

#define EMITSCSI(fieldname) \
std::cout << # field << " = " << basepointer->##fieldname <<
std::endl

and coded, as a test, a single line identical to the "short macro,
e.g..." example shown above.

That line generates the compile error,

pasting "->" and "fieldname" does not give a valid
preprocessing token

Read the message. It contains the explanation of what the problem is.
In fact, pasting "->" can never begin a larger preprocessing token.
The pasting is extraneous, you want simply instead
<< basepointer -> fieldname

By the way, you are committing endl abuse. Use << '\n'

 

[Chris]

Thanks to all who replied. To paraphrase and respond a little,

#define EMITSCSI(fieldname) \
std::cout << # field << " = " << basepointer->##fieldname <<
std::endl
[...]

pasting "->" and "fieldname" does not give a valid
preprocessing token

Read the message. It contains the explanation of what the problem is.

With all due respect, if I understood what the error message was
talking about, I would have fixed the problem instead of posting.

In fact, pasting "->" can never begin a larger preprocessing token.

I don't understand what you mean by that. I expected the preprocessor
to paste together whatever I told it to paste together, and pass the
result along to the compiler. You're saying that's not the way it
works?

The pasting is extraneous, you want simply instead
<< basepointer -> fieldname

I'm amazed that it's as simple as that. Clearly the scope-and-
behavior of tokens within a macro definition is a completely "other
animal" than in ordinary code. This is why I've programmed for 18+
years WITHOUT USING macros. Can never get 'em to do what I want, and
spend an inordinate amount of time fighting with the compiler over the
matter.

By the way, you are committing endl abuse. Use << '\n'

Another shock. Every C++ book I've ever read instructs the reader to
use 'endl' (or, worse, 'std::endl') to generate end-of-line. Did this
change? When?

Chris