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


The latest version of this topic can be found at String and I-O Formatting (Modern C++).

C++ iostreams are capable of formatted string I/O. For example, the following code shows how to set cout to format an integer to output in hexadecimal, first saving off the current state and re-setting afterwards, because once state formatting is passed to cout, it stays that way until changed, not just for 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 can be entirely too cumbersome in many cases. 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++ iostreams, 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 are not 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  

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