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small, fast, printf-like formatting for C++ streams

In a nutshell, io::fmt:

  • provides a printf-like syntax to control formatting on C++'s I/O streams (std::iostream, std::stringstream, etc.);
  • provides a shorter syntax than the standard C++ manipulators (std::fixed, std::setprecision, etc.).
  • runs faster than boost::format (but does not enforce typing).

How to use

  1. copy iofmt.h in your project,
  2. include iofmt.h in your C++ code,
  3. use io::fmt("...") on your C++ streams.

For example:

#include "iofmt.h"
#include <iostream>

using namespace std;
int main() {
   int v;
   cout << "Enter an octal value: " << flush;
   cin  >> io::fmt("o") >> v;
   cout << "In hex: " << io::fmt("#x") << v << endl;
   return 0;

will print "0x7b" before terminating.

This example shows:

  • io::fmt("o") which is like std::oct;
  • io::fmt("#x") which is like but shorter than std::hex << std::showbase;
  • that io::fmt works for both input and output streams.

Format strings

The argument to io::fmt is a C string containing zero or more of the following sequences in any order:

Sequence printf equivalent Description C++ equivalent
s s String (default)
d d Decimal integer std::dec (default)
o o Octal integer std::oct
x x Hexadecimal integer std::hex
X X Hexadecimal integer, uppercase std::hex << std::uppercase
p p Pointer (same as #x) std::hex << std::showbase
f f Fixed float std::fixed
F F Fixed float, uppercase std::fixed << std::uppercase
e e Scientific float std::scientific
E E Scientific float, uppercase std::scientific << std::uppercase
g g Automatic (floats) (default)
G G Automatic (floats), uppercase std::uppercase
a a Hexadecimal float [1] [2] std::hexfloat
A A Hexadecimal float [1] [2], uc std::hexfloat << std::uppercase
b (no equivalent) Print true/false as strings [1] std::boolalpha
# # Alternate format std::showpoint (floats) / std::showbase (ints)
+ + Show sign even if positive std::showpos
. N . N Set float precision to N std::setprecision(N)
- - Align left, pad right std::left
L - Align left, pad right std::left
R (default) Align right, pad left std::right
I (no equivalent) Align left and right, pad middle std::internal
= C 0 (only C = '0') Use C as padding character std::setfill(C)
N N Set format width to N chars std::setw(N)
& (no equivalent) Flush after each I/O operation std::unitbuf
> (no equivalent) Skip whitespace on input std::skipws
! (no equivalent) Invert meaning of next &/> std::nounitbuf or std::noskipws
[1](1, 2, 3) Hexadecimal floats (aA) and booleans as strings (b) are a new feature from C++11. These format characters are only effective when compiling in C++11 mode (e.g. -std=c++11 or -std=gnu++11).
[2](1, 2) Hexadecimal floats are not yet supported in libstdc++ and libc++ as of G++ 4.8 and Clang++ 3.3, although they are mandated by the ISO C++ standard. Although io::fmt recognizes the syntax the desired effect will not be obtained unless the underlying C++ library also supports the feature. Complain to your C++ library provider if needed.

Each use of io::fmt first resets all formatting flags on the I/O stream. This means that any unspecified parameter other than & and > revert to their C++ defaults: decimal, automatic width, fill using spaces, float precision 6, automatic float format, show sign only if negative, no prefix.

In particular,

s << io::fmt("");

is equivalent to:

s << std::setw(0)
  << std::dec
  << std::setfill(' ')
  << std::setprecision(6)
  << std::noshowpos
  << std::noshowbase;
s.setf(std::ios_base::fmtflags(), std::ios_base::floatfield);

For & and > there is no default setting: each use of io::fmt will keep the previous configuration. This is why a syntax is also provided to disable these flags when needed: !& (std::noflush) and !> (std::noskipws).


Using the enclosed on a Core 2 Duo @ 2.4GHz and optimization level -O3, the following was measured:

C++ toolchain Formatter Performance (fmt/sec) Slowdown
g++ 4.8 / libstdc++ none 13796503 N/A
g++ 4.8 / libstdc++ io::fmt 12920048 7% (1.07x slower)
g++ 4.8 / libstdc++ boost w/ pre-parse 603934 95% (22x slower)
g++ 4.8 / libstdc++ boost w/o pre-parse 523598 96% (26x slower)
clang++ 3.3 / libc++ none 9001542 N/A
clang++ 3.3 / libc++ io::fmt 7752862 7% (1.16x slower)
clang++ 3.3 / libc++ boost w/ pre-parse 531301 95% (17x slower)
clang++ 3.3 / libc++ boost w/o pre-parse 401781 96% (22x slower)

In short, in this test program io::fmt incurs less than 10% overhead compared to no formatting; whereas boost::format incurs at least 95% overhead. YMMV.


io::fmt -- printf-like formatting for C++ streams







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