String and I/O Formatting (Modern C++)

C++ <iostream> classes, functions, and operators support formatted string I/O. For example, the following code shows how to set cout to format an integer to output in hexadecimal. First, it saves the current state to reset it afterwards, because once format state is passed to cout, it stays that way until changed. It doesn't just apply to the one line of code.

#include <iostream>
#include <iomanip>

using namespace std;

int main()
    ios state(nullptr);

    cout << "The answer in decimal is: " << 42 << endl;

    state.copyfmt(cout); // save current formatting
    cout << "In hex: 0x" // now load up a bunch of formatting modifiers
        << hex
        << uppercase
        << setw(8)
        << setfill('0')
        << 42            // the actual value we wanted to print out
        << endl;
    cout.copyfmt(state); // restore previous formatting

This approach is type-safe and extensible, but it's also complex and verbose.

Alternative format options

As an alternative, you can use Boost.Format from the Boost C++ libraries, even though it’s nonstandard. You can download any Boost library from the Boost website.

Some advantages of Boost.Format are:

  • Safe: Type-safe, and throws an exception for errors, for example, the specification of too few or too many items.

  • Extensible: Works for any type that can be streamed.

  • Convenient: Standard Posix and similar format strings.

Although Boost.Format is built on C++ <iostream> facilities, which are safe and extensible, they aren't performance-optimized. When you require performance optimization, consider C printf and sprintf, which are fast and easy to use. However, they aren't extensible or safe from vulnerabilities. (Safe versions exist, but they incur a slight performance penalty. For more information, see printf_s, _printf_s_l, wprintf_s, _wprintf_s_l and sprintf_s, _sprintf_s_l, swprintf_s, _swprintf_s_l).

The following code demonstrates some of the Boost formatting features.

    string s = str( format("%2% %2% %1%\n") % "world" % "hello" );
    // s contains "hello hello world"

    for( auto i = 0; i < names.size(); ++i )
        cout << format("%1% %2% %|40t|%3%\n") % first[i] % last[i] % tel[i];
    // Georges Benjamin Clemenceau             +33 (0) 123 456 789
    // Jean de Lattre de Tassigny              +33 (0) 987 654 321

See also

Welcome Back to C++ (Modern C++)
C++ Language Reference
C++ Standard Library