scl-utility

SCL_FORWARD

Identity variadic macro that forwards tokens unchanged and can be used to encourage expansion in macro-chaining scenarios.

• Header: #include <scl/utility/preprocessor/forward.h>
• Declaration: #define SCL_FORWARD(...) __VA_ARGS__

Purpose

• Forward comma-separated tokens into another macro, function call, or template parameter list without altering tokenization.
• Encourage expansion when a macro produces comma-separated tokens that need to be consumed by another macro.

This macro introduces no runtime overhead; it operates purely at preprocessing time.

Usage

#include <scl/utility/preprocessor/forward.h>

// Forwarding function arguments
#define CALL(fn, ...) fn(__VA_ARGS__)
int add(int a, int b) { return a + b; }
int result = CALL(add, SCL_FORWARD(1, 2)); // -> add(1, 2)

// Forcing expansion of a macro that yields a type list
#define BASIC_TYPES int, long, double
template <typename... Ts>
struct type_list {};
using TL = type_list<SCL_FORWARD(BASIC_TYPES)>; // -> type_list<int, long, double>

// Chaining macros that produce comma-separated tokens
#define MAKE_PAIR(a, b) a, b
using P = std::pair<SCL_FORWARD(MAKE_PAIR(int, double))>; // -> std::pair<int, double>

// Forwarding arguments through an intermediate macro
#define FORWARD_TO_EMPLACE(obj, ...) obj.emplace_back(__VA_ARGS__)
std::vector<std::pair<int, std::string>> v;
FORWARD_TO_EMPLACE(v, SCL_FORWARD(42, "answer")); // -> emplace_back(42, "answer")

Notes and pitfalls

• SCL_FORWARD does not add parentheses or change token grouping. If grouping is required, parenthesize at call sites.
• It does not turn multiple arguments into “one argument”; the preprocessor still splits by commas at the outer macro level.
• Useful in macro chains where an intermediate expansion is needed before the next macro consumes the tokens.

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