A square with one argument evaluation
Consider `inline unsigned square_unsigned(unsigned x) { return x*x; }`. If `count()` increments a counter, `square_unsigned(count())` calls `count()` once. By contrast, `#define square_macro(X) (X*X)` expands `square_macro(count())` to `count()*count()`, calling it twice. When the argument count and type are known, prefer the function’s type-safe interface. section 16.0
An inline definition makes a function available for possible integration into its callers. Instead of performing a call, the compiler may use the function’s code at the call site, avoiding call overhead. Seeing that code in context may also expose dead branches or repeated computations. TAKEAWAY: `inline` makes an optimization possible; it does not promise that every call will be inlined. section 15.1
Keep visibility and linkage separate:
- A compiler traditionally needs the definition, not merely a declaration, to inline a function. A header can make that definition visible to each translation unit that includes it. section 15.1
- A header containing an inline definition can be paired with a non-inline declaration in exactly one translation unit to instantiate the function there. section 15.1
- Placing a `static` definition in a header is another way to expose code, but a function not actually inlined can then have a separate instance in every translation unit. section 15.1
1 // Inline definition in a header file.
2 // Function argument names and local variables are visible
3 // to the preprocessor and must be handled with care.
4 inline
5 toto* toto_init(toto* toto_x) {
6 if (toto_x) {
7 *toto_x = (toto){ 0 };
8 }
9 return toto_x;
10 }
!@%STYLE%@!
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!@%STYLE%@!
Inspect an inline definition intended for a header. Its parameter and local names are visible to the preprocessor, so name them with care. section 15.1
1 #include "toto.h"
2
3 // Instantiate in exactly one TU.
4 // The parameter name is omitted to avoid macro replacement.
5 toto* toto_init(toto*);
!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":0},{\"line\":0,\"ch\":1}],[{\"line\":0,\"ch\":4},{\"line\":0,\"ch\":12}],[{\"line\":1,\"ch\":0},{\"line\":1,\"ch\":1}],[{\"line\":2,\"ch\":0},{\"line\":2,\"ch\":1}],[{\"line\":3,\"ch\":0},{\"line\":3,\"ch\":1}],[{\"line\":4,\"ch\":0},{\"line\":4,\"ch\":1}]]"}
!@%STYLE%@!
Compare the separate declaration used to instantiate that function in exactly one translation unit. section 15.1
Inline functions can also supply the typed operations behind a type-generic macro: `_Generic` selects among separate floating-point minimum functions. Use such combinations when a macro provides something a fixed-argument function cannot. Do not trade correctness or safety for an assumed performance gain; inline is particularly suited to small, pure functions. section 16.6 section 15.4
