How Can I Restore the Original State of `std::cout` After a Function Modifies Its Formatting Flags?

Regaining Control: Restoring the Integrity of std::cout

Question:

In a scenario where a function, let's call it printHex, modifies the state of std::cout (e.g., base, field width, etc.), how can we restore the original state after the function invocation? This issue arises when subsequent output using std::cout becomes corrupted by the state changes made within printHex.

Answer:

To regain control over std::cout and revert it to its original settings, we can utilize the capabilities of the header (or header). This restoration process involves two key steps:

1. Capture the Current State: Using std::ios_base::fmtflags f(cout.flags()), retrieve the current formatting flags and store them in a variable named f. Doing so captures the state of std::cout before it can be altered by printHex.
2. Restore the Original State: Once printHex has completed its task, we can restore the original settings of std::cout by invoking cout.flags(f). This action resets std::cout to the state it had prior to the function call, ensuring that subsequent output operations function as expected.

Alternatively, we can implement the restoration process using the Resource Acquisition Is Initialization (RAII) idiom to manage the flags automatically. Here's an example:

class RestoreCoutFlags {
public:
    RestoreCoutFlags(std::ostream& stream)
        : _savedFlags(stream.flags()) {}

    ~RestoreCoutFlags() {
        stream.flags(_savedFlags);
    }

    operator std::streambuf*() const {
        return stream.rdbuf();
    }

private:
    std::ios_base::fmtflags _savedFlags;
};

With this class, the restoration can be achieved as follows:

{
    RestoreCoutFlags r(std::cout);

    // Code that modifies std::cout flags

} // When the block exits, r is automatically destroyed, restoring the flags.

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