How Can I Achieve Golang-Style Defer in C ?

Understanding Golang-Style Defer in C

Introduction:

In C , the concept of deferred execution, similar to Golang's defer, provides a convenient way to execute code after a specific function returns or exits.

STL and Library Implementations:

As of C 14 and later, the Standard Template Library (STL) or other libraries like Boost do not provide a direct implementation of the deferred execution pattern. However, there are several third-party libraries and techniques available to achieve this functionality.

Custom Implementation:

The provided code snippet offers a zero-overhead, easy-to-use implementation of defer:

<code class="cpp">#ifndef defer
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{} *[&]()
#endif // defer</code>

Usage:

To use this implementation, simply place defer before the scope of code you want to execute after the function exits:

<code class="cpp">defer {
   // Code to be executed after the function returns
};</code>

Example:

The following example demonstrates how to use this defer implementation to open and read a file, automatically closing the file when the scope exits:

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

bool read_entire_file(char *filename, std::uint8_t *&data_out,
                      std::size_t *size_out = nullptr) {
    ...

    defer { std::fclose(file); }; // Automatically close the file

    ...
}</code>

Advantages:

This implementation provides several advantages over other approaches:

• Zero-overhead, without any additional runtime overhead
• Syntactically simpler and easier to use
• Zero dependencies, reducing compile times
• Can be easily pasted anywhere in the codebase

Conclusion:

This custom defer implementation offers a convenient and efficient way to achieve deferred execution in C , providing a similar functionality to Golang's defer keyword without the need for custom classes or additional dependencies.

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