nocast — Frictionless C-to-C++ Bridge Cast

nocast solves two specific incompatibilities between C and C++ that arise constantly when you try to compile C code as C++:

  1. void* assignment. C allows void* → any pointer implicitly. C++ does not.
  2. Enum zero. C allows SomeEnum e = 0. C++ does not.

Without nocast, solving these requires either ugly double-casts or extra wrapper macros. With it:

SomeType* ptr = nocast malloc(sizeof(SomeType));   // C: no-op; C++: proxy cast
SomeEnum  e   = nocast 0;                          // C: no-op; C++: constexpr zero

In C, nocast expands to nothing. In C++ it expands to a proxy object and an operator+ overload that performs the right conversion at the call site.

needful_nocast_0 — Generic Zero

needful_nocast_0 is a constexpr zero that converts to any type. It is used internally by Option(T) and Result(T) as a sentinel:

Option(int*)   p = needful_nocast_0;   // same as none / nullptr
Result(int)    r = needful_nocast_0;   // used internally by fail(...)

Note: Unlike nocast, there is no unprefixed shorthand for nocast_0. Use needful_nocast_0 or use none (which is aliased to it for option/result contexts). This asymmetry is intentional: needful_nocast_0 is plumbing, not a typical user-facing API.

downcast and raw_downcast

For C codebases that model inheritance, downcast casts a base pointer to a derived pointer. In C it expands via void*; in C++ it uses nocast to accomplish the same without warnings:

Derived* d = downcast(base_ptr);   // base -> derived (trust the programmer)

raw_downcast is the unhooked version.

Compile-Time Tests

nocast bridges void* and enum zero 

#define NEEDFUL_CPP_ENHANCED  1
#include <cassert>
#include "needful.h"

typedef enum { RED = 0, GREEN = 1, BLUE = 2 } Color;

int main() {
    // void* -> T*: C allows implicitly; C++ requires cast
    void* raw = nullptr;
    int* p = nocast raw;
    assert(p == nullptr);

    // enum from zero: C allows; C++ does not
    Color c = needful_nocast_0;
    assert(c == RED);

    // int* from zero: portable across C and C++
    int* np = needful_nocast_0;
    assert(np == nullptr);

    return 0;
}