A small C++ library for formatted stream output of standard library objects

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0. Preface and wording

A 'decoration' in terms of this project is a set of prefix-string, delimiter-string and a postfix-string. Where the prefix string is inserted into a stream before and the postfix string after the values of a container (see 2. Target containers). The delimiter string is inserted between the values of the respective container.

Note: May have minor bugs since it is not yet well tested, yet.

1. General idea / usage

Zero additional code required for usage

It is to be kept as easy as

#include <vector>
#include "pretty.h"

int main()
  std::cout << std::vector<int>{1,2,3,4,5}; // prints 1, 2, 3, 4, 5
  return 0;

Easy customization ...

... with respect to specific stream object

#include <vector>
#include "pretty.h"

int main()
  // set decoration for std::vector<int> for cout object
  std::cout << pretty::decoration<std::vector<int>>("(", ",", ")");
  std::cout << std::vector<int>{1,2,3,4,5}; // prints (1,2,3,4,5)
  return 0;

or with respect to all streams:

#include <vector>
#include "pretty.h"

// set decoration for std::vector<int> for all ostream objects
PRETTY_DEFAULT_DECORATION(std::vector<int>, "{", ", ", "}")

int main()
  std::cout << std::vector<int>{1,2,3,4,5}; // prints {1, 2, 3, 4, 5}
  std::cout << pretty::decoration<std::vector<int>>("(", ",", ")");
  std::cout << std::vector<int>{1,2,3,4,5}; // prints (1,2,3,4,5)
  return 0;

Rough description

  • The code includes a class template providing a default decoration for any type
  • which can be specialized to change the default decoration for (a) certain type(s) and it is
  • using the private storage provided by ios_base using xalloc/pword in order to save a pointer to a pretty::decor object specifically decorating a certain type on a certain stream.

If no pretty::decor<T> object for this stream has been set up explicitly pretty::defaulted<T, charT, chartraitT>::decoration() is called to obtain the default decoration for the given type. The class pretty::defaulted is to be specialized to customize default decorations.

2. Target objects / containers

Target objects obj for the 'pretty decoration' of this code are objects having either

  • overloads std::begin and std::end defined (includes C-Style arrays),
  • having begin(obj) and end(obj) available via ADL,
  • are of type std::tuple
  • or of type std::pair.

The code includes a trait for identification of classes with range features (begin/end). (There's no check included, whether begin(obj) == end(obj) is a valid expression, though.)

The code provides operator<<s in the global namespace that only apply for classes not having a more specialized version of operator<< available. Therefore, in example std::string is not printed using the operator in this code although having a valid begin/end pair.

3. Utilization and customization

Decorations can be imposed seperately for every type (except different tuples) and stream (not stream type!). (I.e. a std::vector<int> can have a different decorations for different stream objects.)

A) Default decoration

The default prefix is "" (nothing) as is the default postfix, while the default delimiter is ", " (comma+space).

B) Customized default decoration of a type by specializing the pretty::defaulted class template

The struct defaulted has a static member function decoration() returning a decor object which includes the default values for the given type.

Example using array:

Cutomize default array printing:

namespace pretty
  template<class T, std::size_t N>
  struct defaulted<T[N]>
    static decor<T[N]> decoration()
      return{ { "(" }, { ":" }, { ")" } };

Print an arry array:

float e[5] = { 3.4f, 4.3f, 5.2f, 1.1f, 22.2f };
std::cout << e << '\n'; // prints (3.4:4.3:5.2:1.1:22.2)

Using the PRETTY_DEFAULT_DECORATION(TYPE, PREFIX, DELIM, POSTFIX, ...) macro for char streams

The macro expands to

namespace pretty { 
  template< __VA_ARGS__ >
  struct defaulted< TYPE > {
    static decor< TYPE > decoration() {
      return { PREFIX, DELIM, POSTFIX };

enabling the above partial specialization to be rewritten to

PRETTY_DEFAULT_DECORATION(T[N], "", ";", "", class T, std::size_t N)

or inserting a full specialization like

PRETTY_DEFAULT_DECORATION(std::vector<int>, "(", ", ", ")")

Another macro for wchar_t streams is included: PRETTY_DEFAULT_WDECORATION.

C) Impose decoration on streams

The function pretty::decoration is used to impose a decoration on a certain stream. There are overloads taking either

  • one string argument being the delimiter (adopting prefix and postfix from the defaulted class)
  • or three string arguments assembling the complete decoration

Complete decoration for given type and stream

float e[3] = { 3.4f, 4.3f, 5.2f };
std::stringstream u;
// add { ; } decoration to u
u << pretty::decoration<float[3]>("{", "; ", "}");

// use { ; } decoration
u << e << '\n'; // prints {3.4; 4.3; 5.2}

// uses decoration returned by defaulted<float[3]>::decoration()
std::cout << e; // prints 3.4, 4.3, 5.2

Customization of delimiter for given stream

PRETTY_DEFAULT_DECORATION(float[3], "{{{", ",", "}}}")

std::stringstream v;
v << e; // prints {{{3.4,4.3,5.2}}}

v << pretty::decoration<float[3]>(":");
v << e; // prints {{{3.4:4.3:5.2}}}

v << pretty::decoration<float[3]>("((", "=", "))");
v << e; // prints ((3.4=4.3=5.2))

4. Special handling of std::tuple

Instead of allowing a specialization for every possible tuple type, this code applies any decoration available for std::tuple<void*> to all kind of std::tuple<...>s.

5. Remove custom decoration from stream

To go back to the defaulted decoration for a given type use pretty::clear function template on the stream s.

s << pretty::clear<std::vector<int>>();

5. Further examples

Printing "matrix-like" with newline delimiter

std::vector<std::vector<int>> m{ {1,2,3}, {4,5,6}, {7,8,9} };
std::cout << pretty::decoration<std::vector<std::vector<int>>>("\n");
std::cout << m;


1, 2, 3
4, 5, 6
7, 8, 9