How to Reset `std::cout` to its Original State After Formatting Changes?

How to Restore the State of std::cout After Manipulating It

In C , manipulating the state of output streams such as std::cout allows for versatile formatting options. However, sometimes it becomes necessary to restore the original state after performing these alterations.

Consider the following code:

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..
}

Here, after calling printHex, the state of std::cout changes to hexadecimal output. As a result, subsequent print operations also print in hexadecimal, even though the programmer may not desire this behavior.

To address this issue, one can utilize the following technique:

#include <iostream>

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

// Your code here...

cout.flags( f );

By saving the current flags (formatting options) of std::cout in the variable f before performing any manipulations, one can restore the original state by setting the flags back to f after the desired changes.

Alternatively, for more convenient and object-oriented code, one can employ RAII (Resource Acquisition Is Initialization) as follows:

#include <iostream>

struct FlagsSaver {
    FlagsSaver() : flags( cout.flags() ) {}
    ~FlagsSaver() { cout.flags( flags ); }
private:
    std::ios_base::fmtflags flags;
};

void printHex(std::ostream& x) {
    FlagsSaver saver; // RAII to restore flags
    x << std::hex << 123;
}

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

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