mattkretz/defer

defer a callable until the end of the current scope

Defer a function/lambda until the end of the scope

Inspired by the defer keyword in Go, this simple defer function in C++ provides similar functionality. In contrast to Go, the deferred function is called at the end of the current scope. In Go the deferred function is only called at the end of the current function scope.

The main use for this C++ defer function is to simplify resource cleanup safety in simple situations where the definition of a complete new RAII class would be overkill.

Example:

void f(const std::string &filename) {
  auto fp = std::fopen(filename.c_str(), "r");
  if (!fp) {
    std::perror("File opening failed");
    return;
  }
  DEFER({ std::fclose(fp); });
  // do stuff that can throw or return early. The file stream will be closed in
  // any case
}

There are two possible interfaces to using the defer function in defer.h:

1. Either use the preprocessor macro (yuck) as shown above (this avoids some boilerplate code/syntax). The macro constructs a lambda and expects the lambda body as its argument.
2. Or declare a local object, which you capture from the return value of ext::defer. Note that ext::defer works with any callable, so you could also use a std::function or standard function pointer here.

const auto &defer_fclose = ext::defer([&fp] { std::fclose(fp); });