A control-flow construct in C for scoped variable declarations that satisfy a follow-up conditional expression.

let_if.h

//
//  let_if.h
//
//  A control-flow construct in C for scoped variable declarations that satisfy
//  a follow-up conditional expression.
//
//      let_if (<declaration>; <condition>) {
//          <code-block>
//      }
//
//  This can be useful as a form of error handling when reading encoded data.
//
//      let_if (fpos_t position; fgetpos(file, &position) == 0) {
//          let_if (size_t offset; fscanf(file, "%8zx", &offset) == 1)
//          let_if (int next = fgetc(file); next != EOF && ispunct(next))
//          let_if (size_t length;  fscanf(file, "%8zx", &length) == 1) {
//              if (fseek(file, offset, SEEK_CUR) == 0) {
//                  return length;
//              }
//          }
//          fsetpos(file, &position);
//      }
//

#if defined(__clang__)
#   define let_if(...) for (__VA_ARGS__; __BREAK_EXPR)
#   define let_in(...) for (__VA_ARGS__; 1; __BREAK_EXPR)
#   define __BREAK_EXPR __extension__({ \
        _Pragma("clang diagnostic push") \
        _Pragma("clang diagnostic ignored \"-Wgcc-compat\"") \
        break; 0; \
        _Pragma("clang diagnostic pop") \
    })
#elif defined(__GNUC__) || (defined(_MSC_VER) && _MSC_VER >= 1938)
#   define let_if(...) __LET_IF(__LET_IF_ID(__COUNTER__), __VA_ARGS__)
#   define let_in(...) __LET_IN(__LET_IN_ID(__COUNTER__), __VA_ARGS__)
#   define __LET_IF(ID, ...) __LET_ONCE(ID) for (__VA_ARGS__; __BREAK_EXPR)
#   define __LET_IN(ID, ...) __LET_ONCE(ID) for (__VA_ARGS__; ID; __BREAK_EXPR)
#   define __LET_IF_ID(COUNTER) __CONCAT(__let_if_,COUNTER)
#   define __LET_IN_ID(COUNTER) __CONCAT(__let_in_,COUNTER)
#   define __LET_ONCE(ID) for (int ID = 1; ID; ID = 0)
#   define __BREAK_EXPR __extension__({ break; 0; })
#   ifndef __CONCAT
#       define __CONCAT(x, y) x ## y
#   endif
#else
#   error let_if relies on language extensions not available for this compiler.
#endif