That line is actually part of a string that contains javascript that is
embedded in html that is sent to the browser client when requested.

so the plus signs are actually in the javascript code.

server.sendContent(F("var arrpv=3D[];"
 ...
  "var pidupdate=3D'  " STRING_PREAMBLE "  ' +ID+ ' , ' +posVal;
console.log('pidupdate:'+pidupdate); "
 ...
"));

I guess it would have been clearer if I said I want to do something like
this

#define  STRING_PREAMBLE "@"

strHTML[] =3D "sometext "  STRING_PREAMBLE  "someothertext" ;

As best as I can tell the above expands to

strHTML[] =3D "sometext " "@" "someothertext"

and effectively produces

strHTML[] =3D "sometext @someothertext"

and works ok until I try

if (payload[0] =3D=3D STRING_PREAMBLE)

which expand to

if (payload[0] =3D=3D "@")

which the compiler does not like.

I am using the Arduino IDE to compile code for the ESP8266.  As best as i
can tell the IDE/compiler accepts both C and C++

justin

On Sun, Sep 29, 2019 at 8:35 PM Isaac M. Bavaresco <isaacbavaresco@gmail.co=
m>
wrote:

> Are you using C++?
>
> Because in
>
> pidupdate=3D'"STRING_PREAMBLE"'+ID+','+posVal;console.log('pid
> update:'+pidupdate)
>
> The plus signs are not valid in C to concatenate strings, also, if posVal
> is a variable then even without the plus signs it would not work.
>
> In C you could write:
>
> #define STRING_PREAMBLE @
> #define STRINGIFY_(c) #c
> #define STRINGIFY(c) STRINGIFY_(c)
>
> Then:
>
> char *pidupdate=3DSTRINGIFY(STRING_PREAMBLE) ID "," posVal;
>
> I just noticed you are using single quotes for some elements of the
> expression. I'm pretty sure you must be using C++.
>
> C++ is not C. They have important differences.
>
> Cheers,
> Isaac
>
>
>
> Em dom, 29 de set de 2019 05:03, Justin Richards <
> justin.richards@gmail.com>
> escreveu:
>
> > Using macros to make some code more readable and maintainable I always
> seem
> > to hit snags.
> >
> > Struggling to define the question, I have used examples below to try an=
d
> > illustrate my query.
> >
> > I wanted to convert this
> >
> > if (payload[0] =3D=3D '@')
> > {
> >  ...
> > }
> >
> > to
> > #define STRING_PREAMBLE '@'
> >
> > if (payload[0] =3D=3D STRING_PREAMBLE)
> > {
> >  ...
> > }
> >
> > The compiler complains with "expected ')' before '@'
> >
> > So i tried variations on the theme
> > #define STRING_PREAMBLE @
> > #define STRING_PREAMBLE "'@'"
> > #define STRING_PREAMBLE "@"
> >
> > The later two variations compiles but the following comparison  fails
> >    if (payload[0] =3D=3D 'STRING_PREAMBLE')
> >
> > The macro is used elsewhere in the code as part of a string of embedded
> > HTML that embeds javascript and behaves as required
> > "var
> >
> >
> pidupdate=3D'"STRING_PREAMBLE"'+ID+','+posVal;console.log('pidupdate:'+pi=
dupdate);"
> >
> > this was previously coded as
> >
> > "var pidupdate=3D'@'+ID+','+posVal;console.log('pidupdate:'+pidupdate);=
"
> >
> > Any hints or suggestions on how i should phrase my search.
> >
> > Justin
> > --
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