Defer Implementation in C

The concept of Go-style defer, which allows for clean and concise resource cleanup, has gained popularity in C . However, finding a standard or well-supported library implementation for this feature can be challenging.

Despite the absence of built-in support for defer in the Standard Template Library (STL) or Boost, there are external implementations available. One such implementation is a lightweight, zero-overhead solution:

#ifndef defer
struct defer_dummy {};
template  struct deferrer { F f; ~deferrer() { f(); } };
template  deferrer operator*(defer_dummy, F f) { return {f}; }
#define DEFER_(LINE) zz_defer##LINE
#define DEFER(LINE) DEFER_(LINE)
#define defer auto DEFER(__LINE__) = defer_dummy{} *[&]()
#endif // defer

This implementation requires minimal setup and can be easily integrated into your codebase. The syntax is straightforward:

defer { statements; };

For example, in the following code snippet, the fclose operation is automatically executed when the scope of the read_entire_file function is exited:

auto file = std::fopen(filename, "rb");
if (!file)
    return false;

defer { std::fclose(file); }; // don't need to write an RAII file wrapper.

// ...

This zero-overhead implementation offers a convenient and efficient way to manage resource cleanup in C , providing a Go-like defer feature without the need for complex RAII classes or custom memory management techniques.