A string formatting library for C++
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ggformat v1.0

ggformat is a liberally licensed string formatting library for C++ that supports user defined types without blowing up your compile times. It is meant to be used as a replacement for printf and friends.

ggformat saves you time by reducing the amount of tedious boilerplate code you have to write, without adding that time back onto the build. ggformat has a trivial API and does not mind being mixed with other formatting libraries, so it's easy to incrementally integrate with existing software. In particular, it's easy to integrate ggformat with your favourite string class, adding an extra convenience API on top of the functionality you already use. ggformat does not allocate memory unless you want it to, and it's easy to use your own allocators, making it appropriate for use in long-lived applications and games where memory fragmentation is a concern.

ggformat requires C++11 (variadic templates), and supports VS2015, GCC and clang out of the box. It should also work with VS2013 and VS2017 but I don't test against them.

I wrote ggformat because the existing string formatting options for C++ either do not support user defined types or bloat compile times too much. printf doesn't support user defined types. Streams and std::string are slow to compile and IO manipulators are unreadable. Other C++ formatting libraries include STL headers and also hurt compile times.

Version history

  • v1.0 29th Oct 2017: variadic arguments are now passed by const reference. You can now use ggformat with types that have deleted copy constructors/assignment operators.


size_t ggformat( char * buf, size_t len, const char * fmt, ... );
bool ggprint_to_file( FILE * file, const char * fmt, ... );
bool ggprint( const char * fmt, ... );

In short, ggformat replaces s(n)printf, ggprint_to_file replaces fprintf, and ggprint replaces printf.

ggformat writes at most len bytes to buf, and that always includes a null terminator. It returns the number of bytes that would have been written if buf were large enough, not including the null terminator, and can be larger than len (just like sprintf).

ggprint_to_file does what you would expect, and ggprint writes to standard output. Both return true on success, or false if the write fails.

ggformat does not allocate memory, the other functions allocate if you print lots of data. All functions abort if you mess up the formatting string or don't pass the right number of arguments. You should not pass user defined strings as format strings, and I believe it's more helpful to fail hard on programmer typos.

Basic usage looks like this:

#include "ggformat.h"

int main() {
	ggprint( "hello {}\n", 1.23 ); // hello 1.23000
	return 0;

and you can see more examples in basic_examples.cc and string_examples.cc.

Format options

You can add format specifiers between the braces to change how things are printed. The following options are supported:

  • Plus sign ({+}): Prints a leading + for positive numeric types.
  • Width ({x}): left pads the output with spaces to be x characters wide. When used on floats, it left pads the output so the left side of the decimal point is x characters wide (I chose this because I think it makes {x.y} more intuitive). If the output is already wider than x characters, it doesn't do anything.
  • Width with zero padding ({0x}): as above, but pads with zeroes instead of spaces.
  • Width with left alignment ({-x} or {-0x}): same again but puts the spaces/zeroes on the right.
  • Precision ({.x}): specifies the number of digits that appear after the decimal point when printing floats.
  • Number format ({x} or {b}): prints integers in hexadecimal/binary.

These can all be combined, but should be kept in the order they were just listed in.

If you want to print a literal { or }, use {{ and }}.

User defined types

If you want to print your own types with ggformat, you need to define void format( FormatBuffer * fb, T x, const FormatOpts & opts );. FormatBuffer is a wrapper around a char * and length and its exact definition is not important. FormatOpts holds parsed format options and is defined as:

struct FormatOpts {
        enum NumberFormat { DECIMAL, HEX, BINARY };

        int width = -1;
        int precision = -1;
        bool plus_sign = false;
        bool left_align = false;
        bool zero_pad = false;
        NumberFormat number_format = DECIMAL;

format implementations are typically quite simple:

#include "ggformat.h"

struct v3 {
	explicit v3( float x_, float y_, float z_ ) { x = x_; y = y_; z = z_; }
	float x, y, z;

v3 operator+( const v3 & lhs, const v3 & rhs ) {
	return v3( lhs.x + rhs.x, lhs.y + rhs.y, lhs.z + rhs.z );

void format( FormatBuffer * fb, const v3 & v, const FormatOpts & opts ) {
	format( fb, "v3(" );
	format( fb, v.x, opts );
	format( fb, ", " );
	format( fb, v.y, opts );
	format( fb, ", " );
	format( fb, v.z, opts );
	format( fb, ")" );

int main() {
        v3 a = v3( 1, 2, 3 );
        v3 b = v3( 4, 5, 6 );
	// a = v3(1.00000, 2.00000, 3.00000). b = v3( 4.00,  5.00,  6.00).
	// a + b = v3(+5.00000, +7.00000, +9.00000)
        ggprint( "a = {}. b = {2.2}.\na + b = {+}\n", a, b, a + b );

        return 0;

If you have a huge type and don't feel like writing a wall of format, see Thing in basic_examples.cc.

Dynamic allocation (std::string, asprintf, etc)

ggformat( NULL, 0, ... ); returns the number of bytes required to hold the formatted string. With that it's easy to integrate ggformat with your favourite dynamic string solution. For example, ggformat with std::string:

template< typename... Rest >
std::string ggformat_to_string( const char * fmt, const Rest & ... rest ) {
	size_t space_required = ggformat( nullptr, 0, fmt, rest... );

	if( space_required + 1 < space_required )
		throw std::overflow_error( "formatted string is too long" );

	std::string result;
	result.resize( space_required + 1 ); // + 1 so there's space for the null terminator...
	ggformat( &result[ 0 ], space_required + 1, fmt, rest... );
	result.resize( space_required ); // ...and then trim it off

	return result;

Other stuff

Since this is C++ you can and should wrap ggformat in a string class to make it more convenient to use. You can see an example in string_examples.cc.

ggformat uses sprintf under the hood. sprintf can be pretty slow at runtime, but compiles quickly.

In general ggformat is short enough that you can easily modify it to fit your needs, and will be updated infrequently enough that doing so isn't a huge pain. For example, it's very easy to replace malloc/free with your own allocators, and if you don't like aborting on errors it's pretty easy to change that too.