FBB::ReadLineBuf(3) std::streambuf offering line-editing and history


#include <bobcat/readlinebuf>
Linking option: -lreadline -lbobcat


The FBB::ReadLineBuf object may be used as a std::streambuf of std::istream objects, allowing line-editing and history manipulation.

The ReadLineBuf class uses Gnu's readline library to allow editing of input lines. The ReadLineBuf object can be used to construct a std::istream allowing in-line editing of lines read from the terminal. All lines may be preceded by a configurable prompt.

Since Gnu's readline library operates on global data there can only be one ReadLineBuf object. Therefore ReadLineBuf is a singleton class: in any program there can only be one ReadLineBuf object (Gnu's readline library does, however, offer functions allowing programs to use multiple histories. So it would in principle be possible to design a non-singleton ReadLineBuf class. Since programs normally only interact with a single terminal, there is probably little use for non-singleton ReadLineBuf class).

ReadLineBuf offers editing capabilities while the user is entering lines. Like Gnu's readline(3) function, the line editing commands are by default similar to those of emacs(1), but can easily be reconfigured, e.g. to offer vi(1)-like characteristics.

History manipulation is provided as an option. The collected history may be accessed for reading using an FBB::ReadLineHistory object.

Specific information about the facilities offered by the Gnu software used by ReadLineBuf is provided in the GNU Readline Library documentation (http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html).

Gnu's readline function reads its information from the standard input file. Programs using ReadLineBuf should normally not extract information from std::cin. However, as the standard input file has a file descriptor (0), redirection should be possible (e.g., using FBB::Redirector).

When the command line is kept, history expansion is offered as an option. History expansion introduces words from the history list into the input stream, making it easy to repeat commands, to insert elements of a previous input line into the current input line, or to fix errors in previous command lines.

History expansion is usually performed immediately after a complete line is read.

The line selected from the history is called the event, and the portions of that line that are processed are called words. Various modifiers are available to manipulate selected words. This is comparable to the way a program like bash(1) breaks up its input line into `words'.

History expansion is introduced by the use of the history expansion character, by default equal to the !-character. Only backslash (\) and single quotes can change the history expansion character into a normal character.

The remainder of this section is copied almost verbatim from the history(3) man-page. The reader is referred to that man-page or to the Gnu History Library documentation for further details.

The following event designators are supported:

  • !: starts a history substitution, except when followed by a blank, newline, = or (.
  • !n: refers to command line n.
  • !-n: refers to the current command line minus n.
  • !! refers to the previous command. This is a synonym for `!-1'.
  • !string refers to the most recent command starting with string.
  • !?string[?] refers to the most recent command containing string. The trailing ? may be omitted if string is followed immediately by a newline.
  • ^string1^string2^ (quick substitution) repeats the last command, replacing string1 with string2. Equivalent to !!:s/string1/string2/.
  • !# the entire command line typed so far.

Word Designators

Word designators are used to select desired words from the event. A : separates the event specification from the word designator. It may be omitted if the word designator begins with a ^, $, *, -, or %. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces.

  • 0 (zero) The zeroth word. For the shell, this is the command word.
  • n The nth word.
  • ^ The first argument. That is, word 1.
  • $ The last argument.
  • % The word matched by the most recent ?string? search.
  • x-y A range of words; `-y' abbreviates `0-y'.
  • * All of the words but the zeroth. This is a synonym for 1-$. It is not an error to use * if there is just one word in the event; the empty string is returned in that case.
  • x* Abbreviates x-$.
  • x- Abbreviates x-$ like x*, but omits the last word. If a word designator is supplied without an event specification, the previous command is used as the event.


After the optional word designator, there may appear a sequence of one or more of the following modifiers, each preceded by a :.

  • h removes a trailing file name component, leaving only the head.
  • t removes all leading file name components, leaving the tail.
  • r removes a trailing suffix of the form .xxx, leaving the basename.
  • e removes all but the trailing suffix.
  • p prints the new command but does not execute it.
  • q quotes the substituted words, escaping further substitutions.
  • x quotes the substituted words as with q, but break into words at blanks and newlines.
  • s/old/new/ substitutes new for the first occurrence of old in the event line. Any delimiter can be used in place of /. The final delimiter is optional if it is the last character of the event line. The delimiter may be quoted in old and new with a single backslash. If & appears in new, it is replaced by old. A single backslash will quote the &. If old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last string in a !?string[?] search.
  • & repeats the previous substitution.
  • g Causes changes to be applied over the entire event line. This is used in conjunction with :s (e.g., :gs/old/new/) or :&. If used with :s, any delimiter can be used in place of /, and the final delimiter is optional if it is the last character of the event line. An a may be used as a synonym for g.
  • G Apply the following s modifier once to each word in the event line.


All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.




The enum Type defines the following value:

  • DONT_EXPAND_HISTORY: history expansion is not requested;
  • EXPAND_HISTORY: history expansion is requested.

The enum Expansion provides meaningful return values for the history expansion process. Its values are:

  • DONT_EXEC: history expansion succeeded, but the expanded line should not be executed. E.g., after entering the line
        ls *
    the line
    should cause the using program to display, rather than exectute ls *. Note that interpretation of this expansion return value is not the task of the ReadLineBuf object, but of the program using the ReadLineBuf object.
  • ERROR: the history expansion failed. See also the member expansionError below;
  • EXPANDED: the history expansion succeeded;
  • NO_EXPANSION: no history expansion took place.


  • ReadLineBuf &initialize(std::string const &prompt = "", Type type = NO_EXPANSION):
    This static member returns the ReadLineBuf using an initial prompt, using a history of at most std::numeric_limits<int>::max() lines, and by default not using history expansion. If the object has already been initialized a logic_error exception is thrown.
  • ReadLineBuf &initialize(std::string const &prompt, size_t historySize, Type type = NO_EXPANSION):
    This static member initializes the ReadLineBuf using an initial prompt, an initial history of a predefined maximum size, and by default not using history expansion. Specifying a history size 0 results in no history being kept, any value equal to or exceeding the predefined constant std::numeric_limits<int>::max() results in a history of at most std::numeric_limits<int>::max() lines. If no history is requested but type is specified as EXPAND_HISTORY a logic_error exception is thrown. A logic_error is also thrown if the object has already been initialized.
  • ReadLineBuf &instance():
    This static member returns the already initialized ReadLineBuf object. If the object has not yet been initialized a logic_error exception is thrown.


As the class ReadLineBuf is a singleton class it offers no public constructors.


All members of std::streambuf are available, as FBB::ReadLineBuf inherits from this class.
  • ReadLineBuf::Expansion expansion() const:
    The status of the history expansion after retrieving a line from the terminal is returned. Its value is determined after each line retrieved from the terminal. If no history expansion is requested it returns Expansion::ERROR.
  • std::string const &expansionError() const:
    A short textual description of the nature of the error when expansion returns Expansion::ERROR is returned. If no history expansion is requested an empty string is returned.
  • bool setExpansion(Type type):
    History expansion can be activated or stopped using this member. When history expansion is requested but the ReadLineBuf object maintains no history the function returns false. Otherwise it returns true.
  • void setPrompt(std::string const &prompt = ""):
    The prompt that is displayed in front of the next line read from the terminal can be modified by this member. When called without arguments no prompt will be displayed. setPrompt can be called while input lines are being received. The new prompt will be active after the current line has been read from the terminal.
  • bool useTimestamps(std::string (*timestamp)() = 0):
    When initialized with the address of a function returning a std::string the entered commands will be given a timestamp equal to the text returned by the function pointed to by timestamp. The timestamps can be retrieved using the ReadLineHistory(3) object. By default or after passing an explicit 0-pointer to useTimestamps no timestamps are stored. The value false is returned when no history is kept, otherwise true is returned.


#include <iostream>
#include <istream>
#include <cstdio>
#include <sstream>
#include <iomanip>
#include <bobcat/readlinebuf>
using namespace std;
using namespace FBB;
int main()
    ReadLineBuf &readlineBuf = 
            ReadLineBuf::initialize("", 10, ReadLineBuf::EXPAND_HISTORY);
    istream in(&readlineBuf);
    size_t count = 0;
    string line;
    while (true)
        ostringstream prompt;
        prompt << setw(2) << ++count << ": ";
        if (!getline(in, line))          // uses the last-set prompt
        cout << "Retrieved: " << line << "\n"
                "Expansion status: ";
        switch (readlineBuf.expansion())
            case ReadLineBuf::ERROR:
                cout << "ERROR: " << readlineBuf.expansionError() << '\n';
            case ReadLineBuf::NO_EXPANSION:
                cout << "no expansion performed\n";
            case ReadLineBuf::DONT_EXEC:
                cout << "don't execute the expanded line\n";
            case ReadLineBuf::EXPANDED:
                cout << "expansion successfully performed\n";


bobcat/readlinebuf - defines the class interface


None Reported.


  • bobcat_4.02.00-x.dsc: detached signature;
  • bobcat_4.02.00-x.tar.gz: source archive;
  • bobcat_4.02.00-x_i386.changes: change log;
  • libbobcat1_4.02.00-x_*.deb: debian package holding the libraries;
  • libbobcat1-dev_4.02.00-x_*.deb: debian package holding the libraries, headers and manual pages;
  • http://sourceforge.net/projects/bobcat: public archive location;


Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.


This is free software, distributed under the terms of the GNU General Public License (GPL).


Frank B. Brokken ([email protected]).