How to Ensure Immediate Output in C Using `std::flush`?

Enforcing Immediate Output in C Using std::flush

In C programs, it is common to encounter a scenario where text printed to the screen using std::cout does not appear instantly. This can be problematic, especially when the text is meant to provide real-time feedback to the user while long-running computations are being performed.

To address this issue, consider the following code:

<code class="cpp">std::cout << "Beginning computations...";  // Output 1
computations();
std::cout << " done!\n";                   // Output 2</code>

Output #1 informs the user that computations are underway, while Output #2 signals their completion. However, Output #1 might not appear on the screen until after computations() returns.

Fortunately, there is a simple solution: introducing std::flush. Inserting std::flush into the code forces the std::cout buffer to be immediately printed to the screen:

<code class="cpp">std::cout << "Beginning computations..." << std::flush;</code>

This ensures that Output #1 is displayed before the computations() call, giving the user immediate visual feedback.

Alternatively, instead of std::cout, one could use std::cerr, which writes to the standard error stream. By default, std::cerr flushes its buffer immediately, resolving the problem without the need for std::flush.

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