macroscope

A step-by-step C / C++ macro expansion visualizer. Paste in any #defines and an invocation, and watch each substitution, stringization, paste, and rescan happen one token at a time.

Live at https://macroscope.orlac.io

What it does

Macroscope implements Dave Prosser's macro-replacement algorithm — iterative splicing with per-token hide sets — so it shows expansion exactly the way a conforming C23 preprocessor performs it (§6.10.5 + §6.10.3.5), including the trickier cases like __VA_OPT__, paint-blocking, and deferred expansion.

Examples

Function-like macros and argument prescan

#define SQUARE(x) ((x) * (x))

SQUARE(3 + 4)
// → ((3 + 4) * (3 + 4))

Stringization (#)

#define STR(x) #x

STR(hello world)
// → "hello world"

Token pasting (##)

#define CONCAT(a, b) a##b

CONCAT(foo, bar)
// → foobar

Variadic macros and __VA_OPT__

#define LOG(fmt, ...) printf(fmt __VA_OPT__(,) __VA_ARGS__)

LOG("hi\n")
// → printf("hi\n")
LOG("%d\n", 42)
// → printf("%d\n", 42)

Deferred expansion (rescan + hide sets)

#define EMPTY
#define DEFER(M) M EMPTY()
#define A() 123

DEFER(A)()
// → A ()      after first scan (A is hide-painted)
// → 123       after rescan

This is where most hand-rolled expanders go wrong — macroscope walks the hide-set bookkeeping token by token so you can see exactly why the second scan succeeds.

Scope

This is a macro-replacement simulator, not a full preprocessor. It deliberately doesn't handle:

• conditional compilation (#if, #ifdef, #else, #elif, #endif)
• #include, #pragma, #embed, #error, #warning, #line
• context-dependent predefined macros (__FILE__, __LINE__, …)

All such directives are flagged in the live error strip as "not supported".