Pragmas in C++

The Big Difference vs C

In C, pragmas specifically name the entity they operate on, and thus can appear some distance away from where that entity is defined. For example ...

<syntaxhighlight lang="cpp">

1. pragma CODE_SECTION(fxn, "mysect");

// many lines later ...

void fxn(int arg) { ... // fxn is finally defined </syntaxhighlight>

In C++, the pragmas are positional. They do not name the entity they operate on. They always operate on the next entity defined after the pragma. The above example, in C++, must be written ...

<syntaxhighlight lang="cpp">

1. pragma CODE_SECTION("mysect") // NO semi-colon!

// The affected function must be the very next entity void fxn(int arg) { ... </syntaxhighlight>

Hide It in a Macro

If you need your code to work whether it is compiled as either C or C++ code, a macro like this is useful.

<syntaxhighlight lang="cpp">

1. define PRAGMA(x) _Pragma(#x)
2. ifdef __cplusplus
3. define CSPRAGMA(f,s) PRAGMA(CODE_SECTION(s))
4. else
5. define CSPRAGMA(f,s) PRAGMA(CODE_SECTION(f, s))
6. endif

CSPRAGMA(fxn, "mysect") // NO semi-colon void fxn(int arg) { ... </syntaxhighlight>

This macro is, of course, positional when used in C++. So, for the code to be portable, the macro must be viewed as positional even when used in C code.

Pragmas You Can Understand is a good article on wrapping pragmas in a macro, via _Pragma, for better portability.