How to Implement Defer Functionality in C ?

What is Standard Defer/Finalizer Implementation in C ?

The concept of "defer" functions, as inspired by Go, allows for deferred execution of specified tasks after the current scope exits. This allows for resource cleanup, logging, or other actions that should be triggered when a specific point in the code is reached.

STL and Boost Implementations

The C Standard Library (STL) and Boost library do not provide a built-in implementation of a defer class. However, there are third-party libraries that address this functionality.

A Custom Defer Implementation

If external libraries are not an option, the following custom implementation can be used to emulate the defer functionality:

<code class="cpp">struct defer_dummy {};
template <class F> struct deferrer { F f; ~deferrer() { f(); } };
template <class F> deferrer<F> 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{} *[&amp;]()</code>

Usage

To use this implementation, simply write:

<code class="cpp">defer { statements; };</code>

Example

Consider the following code:

<code class="cpp">#include <cstdio>
#include <cstdlib>

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

// File reading code...

return 0;</code>

In this example, the defer block ensures that the file is closed, regardless of whether the function returns successfully or otherwise.

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