C++ custom stream manipulator that changes next item on stream - c++

Question

In C++, to print a number in hexadecimal you do this:
int num = 10;
std::cout << std::hex << num; // => 'a'
I know I can create a manipulator that just adds stuff to the stream like so:
std::ostream& windows_feed(std::ostream& out)
{
out << "\r\n";
return out;
}
std::cout << "Hello" << windows_feed; // => "Hello\r\n"
However, how can I create a manipulator that, like 'hex', modifies items to come on the stream? As a simple example, how would I create the plusone manipulator here?:
int num2 = 1;
std::cout << "1 + 1 = " << plusone << num2; // => "1 + 1 = 2"
// note that the value stored in num2 does not change, just its display above.
std::cout << num2; // => "1"

Answer 1

First, you have to store some state into each stream. You can do that with the function iword and an index you pass to it, given by xalloc:
inline int geti() {
static int i = ios_base::xalloc();
return i;
}
ostream& add_one(ostream& os) { os.iword(geti()) = 1; return os; }
ostream& add_none(ostream& os) { os.iword(geti()) = 0; return os; }
Having that in place, you can already retrieve some state in all streams. Now, you just have to hook into the respective output operation. Numeric output is done by a facet, because it potentially is locale dependent. So you can do
struct my_num_put : num_put<char> {
iter_type
do_put(iter_type s, ios_base& f, char_type fill, long v) const {
return num_put<char>::do_put(s, f, fill, v + f.iword(geti()));
}
iter_type
do_put(iter_type s, ios_base& f, char_type fill, unsigned long v) const {
return num_put<char>::do_put(s, f, fill, v + f.iword(geti()));
}
};
Now, you can test the stuff.
int main() {
// outputs: 11121011
cout.imbue(locale(locale(),new my_num_put));
cout << add_one << 10 << 11
<< add_none << 10 << 11;
}
If you want that only the next number is incremented, just set the word to 0 again after each call to do_put.

Answer 2

I totally agree with Neil Butterworth on this one, however in the specific case you are using you could do this totally horrible hack. Do not do this in any production code. It has lots of bugs. For one thing it only works in your one-liner above, it does not change the state of the underlying stream.
class plusone_stream : public std::ostream
{
public:
std::ostream operator<<(int i)
{
_out << i+1;
return *this;
}
};
std::ostream& plusone(std::ostream& out)
{
return plusone_stream(out);
}

Answer 3

It's not a direct answer to your question, but don't you think that using a plain old function is both simpler to implement and clearer to use than writing a full blown manipulator?
#include <sstream>
template<typename T>
std::string plusone(T const& t) {
std::ostringstream oss;
oss << (t + 1);
return oss.str();
}
Usage:
cout << plusone(42);
By "clear to use", I mean that the user doesn't need to ask themselves, "Does it affect only the next item, or all subsequent items?" It's obvious from inspection that only the argument of the function is affected.
(For the plusone() example, you could simplify even further by just returning a T instead, but returning a std::string serves the general case.)

Answer 4

I created a simple solution for your test case without using <iomanip>. I can't promise that the same approach will work in real life.
The basic approach is that cout << plusone returns a temporary auxiliary object (PlusOnePlus), which in turn has the overloaded operator << that performs the addition.
I've tested it on Windows:
PlusOne plusone;
cout << plusone << 41
produces "42", as expected. Here's the code:
class PlusOnePlus {
public:
PlusOnePlus(ostream& os) : m_os(os) {}
// NOTE: This implementation relies on the default copy ctor,
// assignment, etc.
private:
friend ostream& operator << (PlusOnePlus& p, int n);
ostream& m_os;
};
class PlusOne {
public:
static void test(ostream& os);
};
PlusOnePlus operator << (ostream& os, const PlusOne p)
{
return PlusOnePlus(os);
}
ostream& operator << (PlusOnePlus& p, int n)
{
return p.m_os << n + 1;
}
void PlusOne::test(ostream& os)
{
PlusOne plusone;
os << plusone << 0 << endl;
os << plusone << 41 << endl;
}
EDIT: Commented the code to point out that I'm relying on the default copy constructor (etc.) for PlusOnePlus. A robust implementation would probably define these

Answer 5

You'll have to play with streamstates. I've bookmarked the following links on the subject:
A discussion on boost ML
Maciej Sobczak's article on CUJ/DDJ
As Maciej Sobczak library is no longer available online, and as the licence permits me to do so, (correct me if I'm wrong), here is a copy of its main file that I've managed to salvage from oblivion:
// streamstate.h
//
// Copyright (C) Maciej Sobczak, 2002, 2003
//
// Permission to copy, use, modify, sell and distribute this software is
// granted provided this copyright notice appears in all copies. This software
// is provided "as is" without express or implied warranty, and with no claim
// as to its suitability for any purpose.
//
// <http://lists.boost.org/Archives/boost/2002/10/38275.php>
// <http://www.ddj.com/dept/cpp/184402062?pgno=1>
// <http://www.msobczak.com/prog/publications.html>
#ifndef STREAMSTATE_H_INCLUDED
#define STREAMSTATE_H_INCLUDED
#include <ios>
#include <istream>
#include <ostream>
// helper exception class, thrown when the source of error
// was in one of the functions managing the additional state storage
class StreamStateException : public std::ios_base::failure
{
public:
explicit StreamStateException()
: std::ios_base::failure(
"Error while managing additional IOStream state.")
{
}
};
// State should be:
// default-constructible
// copy-constructible
// assignable
// note: the "void *" slot is used for storing the actual value
// the "long" slot is used to propagate the error flag
template
<
class State,
class charT = char,
class traits = std::char_traits<charT>
>
class streamstate
{
public:
// construct with the default state value
streamstate() {}
// construct with the given stream value
streamstate(const State &s) : state_(s) {}
// modifies the stream
std::basic_ios<charT, traits> &
modify(std::basic_ios<charT, traits> &ios) const
{
long *errslot;
void *&p = state_slot(ios, errslot);
// propagate the error flag to the real stream state
if (*errslot == std::ios_base::badbit)
{
ios.setstate(std::ios_base::badbit);
*errslot = 0;
}
// here, do-nothing-in-case-of-error semantics
if (ios.bad())
return ios;
if (p == NULL)
{
// copy existing state object if this is new slot
p = new State(state_);
ios.register_callback(state_callback, 0);
}
else
*static_cast<State*>(p) = state_;
return ios;
}
// gets the current (possibly default) state from the slot
static State & get(std::basic_ios<charT, traits> &ios)
{
long *errslot;
void *&p = state_slot(ios, errslot);
// propagate the error flag to the real stream state
if (*errslot == std::ios_base::badbit)
{
ios.setstate(std::ios_base::badbit);
*errslot = 0;
}
// this function returns a reference and therefore
// the only sensible error reporting is via exception
if (ios.bad())
throw StreamStateException();
if (p == NULL)
{
// create default state if this is new slot
p = new State;
ios.register_callback(state_callback, 0);
}
return *static_cast<State*>(p);
}
private:
// manages the destruction and format copying
// (in the latter case performs deep copy of the state)
static void state_callback(std::ios_base::event e,
std::ios_base &ios, int)
{
long *errslot;
if (e == std::ios_base::erase_event)
{
// safe delete if state_slot fails
delete static_cast<State*>(state_slot(ios, errslot));
}
else if (e == std::ios_base::copyfmt_event)
{
void *& p = state_slot(ios, errslot);
State *old = static_cast<State*>(p);
// Standard forbids any exceptions from callbacks
try
{
// in-place deep copy
p = new State(*old);
}
catch (...)
{
// clean the value slot and
// set the error flag in the error slot
p = NULL;
*errslot = std::ios_base::badbit;
}
}
}
// returns the references to associated slot
static void *& state_slot(std::ios_base &ios, long *&errslot)
{
static int index = std::ios_base::xalloc();
void *&p = ios.pword(index);
errslot = &(ios.iword(index));
// note: if pword failed,
// then p is a valid void *& initialized to 0
// (27.4.2.5/5)
return p;
}
State state_;
};
// partial specialization for iword functionality
template
<
class charT,
class traits
>
class streamstate<long, charT, traits>
{
public:
// construct with the default state value
streamstate() {}
// construct with the given stream value
streamstate(long s) : state_(s) {}
// modifies the stream
// the return value is not really useful,
// it has to be downcasted to the expected stream type
std::basic_ios<charT, traits> &
modify(std::basic_ios<charT, traits> &ios) const
{
long &s = state_slot(ios);
s = state_;
return ios;
}
static long & get(std::basic_ios<charT, traits> &ios)
{
return state_slot(ios);
}
private:
static long & state_slot(std::basic_ios<charT, traits> &ios)
{
static int index = std::ios_base::xalloc();
long &s = ios.iword(index);
// this function returns a reference and we decide
// to report errors via exceptions
if (ios.bad())
throw StreamStateException();
return s;
}
long state_;
};
// convenience inserter for ostream classes
template
<
class State,
class charT,
class traits
>
std::basic_ostream<charT, traits> &
operator<<(std::basic_ostream<charT, traits> &os,
const streamstate<State> &s)
{
s.modify(os);
return os;
}
// convenience extractor for istream classes
template
<
class State,
class charT,
class traits
>
std::basic_istream<charT, traits> &
operator>>(std::basic_istream<charT, traits> &is,
const streamstate<State> &s)
{
s.modify(is);
return is;
}
// the alternative if there is a need to have
// many different state values of the same type
// here, the instance of streamstate_value encapsulates
// the access information (the slot index)
template
<
class State,
class charT = char,
class traits = std::char_traits<char>
>
class streamstate_value
{
public:
streamstate_value()
: index_(-1)
{
}
// returns a reference to current (possibly default) state
State & get(std::basic_ios<charT, traits> &ios)
{
long *errslot;
void *&p = state_slot(ios, errslot, index_);
// propagate the error flag to the real stream state
if (*errslot == std::ios_base::badbit)
{
ios.setstate(std::ios_base::badbit);
*errslot = 0;
}
// this function returns a reference and the only
// sensible way of error reporting is via exception
if (ios.bad())
throw StreamStateException();
if (p == NULL)
{
// create default state if this is new slot
p = new State;
ios.register_callback(state_callback, index_);
}
return *static_cast<State*>(p);
}
private:
// manages the destruction and format copying
// (in the latter case performs deep copy of the state)
static void state_callback(std::ios_base::event e,
std::ios_base &ios, int index)
{
long *errslot;
if (e == std::ios_base::erase_event)
{
// safe delete if state_slot fails
delete static_cast<State*>(state_slot(ios, errslot, index));
}
else if (e == std::ios_base::copyfmt_event)
{
void *& p = state_slot(ios, errslot, index);
State *old = static_cast<State*>(p);
// Standard forbids any exceptions from callbacks
try
{
// in-place deep copy
p = new State(*old);
}
catch (...)
{
// clean the value slot and set the error flag
// in the error slot
p = NULL;
*errslot = std::ios_base::badbit;
}
}
}
// returns the references to associated slot
static void *& state_slot(std::ios_base &ios,
long *& errslot, int & index)
{
if (index < 0)
{
// first index usage
index = std::ios_base::xalloc();
}
void *&p = ios.pword(index);
errslot = &(ios.iword(index));
// note: if pword failed,
// then p is a valid void *& initialized to 0
// (27.4.2.5/5)
return p;
}
int index_;
};
// partial specialization for iword functionality
template
<
class charT,
class traits
>
class streamstate_value<long, charT, traits>
{
public:
// construct with the default state value
streamstate_value()
: index_(-1)
{
}
long & get(std::basic_ios<charT, traits> &ios)
{
if (index_ < 0)
{
// first index usage
index_ = std::ios_base::xalloc();
}
long &s = ios.iword(index_);
if (ios.bad())
throw StreamStateException();
return s;
}
private:
long index_;
};
#endif // STREAMSTATE_H_INCLUDED

Answer 6

The hex, dec and oct manipulators simply change the basefield property of the existing stream.
See C++ Reference for more deatail about these manipulators.
As posted in Neil Butterworth's answer, you would need to extend the existing stream classes, or create your own, in order to have manipulators that affect future values inserted into the stream.
In the example of your plusone manipulator, the stream object would have to have an internal flag to indicate that one should be added to all inserted values. The plusone manipulator would simply set that flag, and the code to handle stream insertion would check that flag before inserting numbers.

Answer 7

litb's approach is "the right way" and necessary for complicated stuff, but something like this can be good enough. Add privacy and friendship to taste.
struct PlusOne
{
PlusOne(int i) : i_(i) { }
int i_;
};
std::ostream &
operator<<(std::ostream &o, const PlusOne &po)
{
return o << (po.i_ + 1);
}
std::cout << "1 + 1 = " << PlusOne(num2); // => "1 + 1 = 2"
In this simple example creating and streaming a temporary object doesn't seem much more helpful than defining a function plusOne() as someone already suggested. But suppose you wanted it to work like this:
std::ostream &
operator<<(std::ostream &o, const PlusOne &po)
{
return o << po.i_ << " + 1 = " << (po.i_ + 1);
}
std::cout << PlusOne(num2); // => "1 + 1 = 2"