How Can I Restore the Original State of std::cout After Manipulating Its Formatting Flags?

Restoring std::cout State After Manipulation

In C , modifying the state of I/O streams, such as std::cout, can have unintended consequences in subsequent operations. Consider the following code snippet:

void printHex(std::ostream& x){
    x << std::hex << 123;
}

int main(){
    std::cout << 100; // prints 100 base 10
    printHex(std::cout); //prints 123 in hex
    std::cout << 73; //problem! prints 73 in hex..
}

In this example, std::cout is modified by the printHex function to print in hexadecimal. However, this modification persists when returning to main, causing subsequent output using std::cout to be printed in hexadecimal instead of decimal.

To address this issue, we need to restore the original state of std::cout after the printHex function finishes. This can be achieved using the flags member function of std::ios_base.

#include <ios>
#include <iostream>

int main(){
    std::ios_base::fmtflags f( std::cout.flags() );

    std::cout << 100; // prints 100 base 10
    printHex(std::cout); //prints 123 in hex

    std::cout.flags( f );

    std::cout << 73; //prints 73 base 10
}

By capturing the current flags using std::cout.flags() before modifying it, we can restore them using std::cout.flags(f) to reset the stream's state to its original format.

This technique allows for temporary modifications to I/O stream properties, ensuring that subsequent I/O operations are performed as expected.

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