How Can I Restore the Original State of std::cout After Modifying its Formatting Flags?

Restoring the State of std::cout after Modification

In C , manipulating input/output streams can temporarily alter their state. For example, the code snippet below modifies the base of std::cout to hexadecimal:

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..
}

After returning from printHex, subsequent output to std::cout will continue to use the hexadecimal base, potentially leading to unexpected results. To address this, we need a way to restore the original state of std::cout.

Solution

The solution involves using the std::ios_base::fmtflags class to capture and restore the stream's formatting flags. Here's how to do it:

1. Include or .

2. At the beginning of the function that modifies std::cout:

   std::ios_base::fmtflags f( cout.flags() );

   This stores the current formatting flags in the f variable.

3. Perform the necessary modifications to std::cout.

4. At the end of the function, after returning the stream to its original state:

   cout.flags( f );

   This restores the formatting flags that were captured at the beginning of the function.

Example

In the code snippet below, the restoreCoutState function captures and restores the state of std::cout:

void restoreCoutState(std::ostream& os){
    std::ios_base::fmtflags f( os.flags() );
    os << std::hex << 123;
    os.flags( f );
}

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

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