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Outputting more things than a Polymorphic Text Archive
(1 answer)
Closed 6 years ago.
I'm trying to use boost library to serialize std::map so that is possible to store it in a file. However I'm having so weird behaviours (i guess). So here is my code:
#include <map>
#include <fstream>
#include <iostream>
#include <bitset>
#include <boost/serialization/map.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
std::map<int,int> map = {{65,2}, {69,1}, {75,1} ,{77,1}, {82,1}, {84,2}, {89,2}};
void saveMapToFile(std::ofstream& f);
int main()
{
std::ofstream f("test.txt", std::ios::binary);
saveMapToFile(f);
std::cout << "position: " << f.tellp() << std::endl;
}
void saveMapToFile(std::ofstream& f)
{
std::cout << "position : " << f.tellp() << std::endl;
boost::archive::text_oarchive oarch(f);
std::cout << "position : " << f.tellp() << std::endl;
oarch << map;
std::cout << "position : " << f.tellp() << std::endl;
}
And here is the above code's output:
position : 0
position : 28
position : 75
position: 76
So can someone explain to me what is happening here? Why position after insterting map (in function) in different outside of it? I don't do any additional opperations, yet that pointer goes one byte further... Am I missing something? Thanks for your help in advance.
I don't see why you would be making assumptions about the implementation of the archive format.
Archives write headers, and can write "trailers" (think XML archives).
The destructor of oarch wrote another byte, finishing the stream. It could be a sentinel, a checksum, a newline etc.
Related
I'm self learning C++ and for some reason "double" doesn't print more than 6 significant digits even after std::setprecision. Do I need to do something else? Most recent version of codeblocks if that helps. This is all the code:
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
std::setprecision(9);
double A = 654321.987;
cout << A << endl;
return 0;
}
You need to feed the result of std::setprecision(9) to std::cout. Otherwise it has no way of knowing what output stream it applies to (and so it won't apply to anything).
std::cout << std::setprecision(9) << A << std::endl;
Or if you prefer you can do it separately:
std::cout << std::setprecision(9);
std::cout << A << std::endl;
This question already has answers here:
Restore the state of std::cout after manipulating it
(9 answers)
Closed 4 years ago.
C++ steam objects have state. If one write a piece of code like
using namespace std;
cout << hex << setw(8) << setfill('0') << x << endl;
forgetting setting the stream state back. This will cause problems in some other unrelated codes. It's tedious to do "set" and "set back" pair matching. Besides from that, it seems to me it's also against convention behind RAII.
My question is: is it possible, with only a thin layer of wrapping, to make those state manipulations RAII-like. That is, right after the end of an expression by semicolon, stream state is automatically set back to default.
Update: Following the link provided by #0x499602D2, one workaround might be something like
#include <boost/io/ios_state.hpp>
#include <ios>
#include <iostream>
#include <ostream>
#define AUTO_COUT(x) {\
boost::io::ios_all_saver ias( cout );\
x;\
}while(0)
Then one can use the macro like
AUTO_COUT(cout << hex << setw(8) << setfill('0') << x << endl);
BTW, it might be a good idea to add a lock field to those saver class of boost::io::ios_state, in case funny things occur in a multi-threading program. Or they have already done so?
I'm going to suggest an alternative approach. The manipulators apply to the std::[i|o]stream instance, but they do nothing with regards to the std::[i|o]streambuf which is managed by that std::[i|o]stream.
Therefore, you can create your own std::[i|o]stream, which will have its own formatting state, but writing to the same buffer std::cout uses:
#include <iostream>
#include <iomanip>
int main()
{
std::cout << std::hex << 32 << "\n";
std::ostream os(std::cout.rdbuf());
os << 32 << "\n" << std::hex;
std::cout << std::dec;
os << 32 << "\n";
std::cout << 32 << "\n";
}
Output:
20
32
20
32
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This uses only features from standard library, and since the original stream is not touched, applying manipulators is trivially thread safe (because each thread operates on a different stream). Now, the actual writes and reads' thread safety depends on the thread safety of the managed stream buffer.
I once wrote a utility class for my personal use. (I don't know whether it is as perfect as the boost code probably is but it worked for me – so, I dare to share.)
#include <iostream>
#include <iomanip>
/** provides a helper class to work with streams.
*
* It stores current format states of a stream in constructor and
* recovers these settings in destructor.
*
* Example:
* <pre>
* { // backup states of std::cout
* IOSFmtBackup stateCOut(std::cout);
* // do some formatted output
* std::cout
* << "dec: " << std::dec << 123 << std::endl
* << "hex: " << std::hex << std::setw(8) << std::setfill('0')
* << 0xdeadbeef << std::endl;
* } // destruction of stateCOut recovers former states of std::cout
* </pre>
*/
class IOSFmtBackup {
// variables:
private:
/// the concerning stream
std::ios &_stream;
/// the backup of formatter states
std::ios _fmt;
// methods:
public:
/// #name Construction & Destruction
//#{
/** constructor.
*
* #param stream the stream for backup
*/
explicit IOSFmtBackup(std::ios &stream):
_stream(stream), _fmt(0)
{
_fmt.copyfmt(_stream);
}
/// destructor.
~IOSFmtBackup() { _stream.copyfmt(_fmt); }
// disabled:
IOSFmtBackup(const IOSFmtBackup&) = delete;
IOSFmtBackup& operator=(const IOSFmtBackup&) = delete;
//#}
};
int main()
{
{ // backup states of std::cout
IOSFmtBackup stateCOut(std::cout);
// do some formatted output
std::cout
<< "dec: " << std::dec << 123 << std::endl
<< "hex: " << std::hex << std::setw(8) << std::setfill('0')
<< 0xdeadbeef << std::endl
<< "123 in current: " << 123 << std::endl;
} // destruction of stateCOut recovers former states of std::cout
// check whether formatting is recovered
std::cout << "123 after recovered: " << 123 << std::endl;
return 0;
}
Compiled and tested on ideone (life demo).
Output:
dec: 123
hex: deadbeef
123 in current: 7b
123 after recovered: 123
Suppose I have a code like this:
void printHex(std::ostream& x){
x<<std::hex<<123;
}
..
int main(){
std::cout<<100; // prints 100 base 10
printHex(std::cout); //prints 123 in hex
std::cout<<73; //problem! prints 73 in hex..
}
My question is if there is any way to 'restore' the state of cout to its original one after returning from the function? (Somewhat like std::boolalpha and std::noboolalpha..) ?
Thanks.
you need to #include <iostream> or #include <ios> then when required:
std::ios_base::fmtflags f( cout.flags() );
//Your code here...
cout.flags( f );
You can put these at the beginning and end of your function, or check out this answer on how to use this with RAII.
Note that the answers presented here won't restore the full state of std::cout. For example, std::setfill will "stick" even after calling .flags(). A better solution is to use .copyfmt:
std::ios oldState(nullptr);
oldState.copyfmt(std::cout);
std::cout
<< std::hex
<< std::setw(8)
<< std::setfill('0')
<< 0xDECEA5ED
<< std::endl;
std::cout.copyfmt(oldState);
std::cout
<< std::setw(15)
<< std::left
<< "case closed"
<< std::endl;
Will print:
case closed
rather than:
case closed0000
The Boost IO Stream State Saver seems exactly what you need. :-)
Example based on your code snippet:
void printHex(std::ostream& x) {
boost::io::ios_flags_saver ifs(x);
x << std::hex << 123;
}
I've created an RAII class using the example code from this answer. The big advantage to this technique comes if you have multiple return paths from a function that sets flags on an iostream. Whichever return path is used, the destructor will always be called and the flags will always get reset. There is no chance of forgetting to restore the flags when the function returns.
class IosFlagSaver {
public:
explicit IosFlagSaver(std::ostream& _ios):
ios(_ios),
f(_ios.flags()) {
}
~IosFlagSaver() {
ios.flags(f);
}
IosFlagSaver(const IosFlagSaver &rhs) = delete;
IosFlagSaver& operator= (const IosFlagSaver& rhs) = delete;
private:
std::ostream& ios;
std::ios::fmtflags f;
};
You would then use it by creating a local instance of IosFlagSaver whenever you wanted to save the current flag state. When this instance goes out of scope, the flag state will be restored.
void f(int i) {
IosFlagSaver iosfs(std::cout);
std::cout << i << " " << std::hex << i << " ";
if (i < 100) {
std::cout << std::endl;
return;
}
std::cout << std::oct << i << std::endl;
}
You can create another wrapper around the stdout buffer:
#include <iostream>
#include <iomanip>
int main() {
int x = 76;
std::ostream hexcout (std::cout.rdbuf());
hexcout << std::hex;
std::cout << x << "\n"; // still "76"
hexcout << x << "\n"; // "4c"
}
In a function:
void print(std::ostream& os) {
std::ostream copy (os.rdbuf());
copy << std::hex;
copy << 123;
}
Of course if performance is an issue this is a bit more expensive because it's copying the entire ios object (but not the buffer) including some stuff that you're paying for but unlikely to use such as the locale.
Otherwise I feel like if you're going to use .flags() it's better to be consistent and use .setf() as well rather than the << syntax (pure question of style).
void print(std::ostream& os) {
std::ios::fmtflags os_flags (os.flags());
os.setf(std::ios::hex);
os << 123;
os.flags(os_flags);
}
As others have said you can put the above (and .precision() and .fill(), but typically not the locale and words-related stuff that is usually not going to be modified and is heavier) in a class for convenience and to make it exception-safe; the constructor should accept std::ios&.
C++20 std::format will be a superior alternative to save restore in most cases
Once you can use it, you will e.g. be able to write hexadecimals simply as:
#include <format>
#include <string>
int main() {
std::cout << std::format("{:x} {:#x} {}\n", 16, 17, 18);
}
Expected output:
10 0x11 18
This will therefore completely overcome the madness of modifying std::cout state.
The existing fmt library implements it for before it gets official support: https://github.com/fmtlib/fmt Install on Ubuntu 22.04:
sudo apt install libfmt-dev
Modify source to replace:
<format> with <fmt/core.h>
std::format to fmt::format
main.cpp
#include <iostream>
#include <fmt/core.h>
int main() {
std::cout << fmt::format("{:x} {:#x} {}\n", 16, 17, 18);
}
and compile and run with:
g++ -std=c++11 -o main.out main.cpp -lfmt
./main.out
Output:
10 0x11 18
Related: std::string formatting like sprintf
With a little bit of modification to make the output more readable :
void printHex(std::ostream& x) {
ios::fmtflags f(x.flags());
x << std::hex << 123 << "\n";
x.flags(f);
}
int main() {
std::cout << 100 << "\n"; // prints 100 base 10
printHex(std::cout); // prints 123 in hex
std::cout << 73 << "\n"; // problem! prints 73 in hex..
}
Instead of injecting format into cout, the << way, adopting setf and unsetf could be a cleaner solution.
void printHex(std::ostream& x){
x.setf(std::ios::hex, std::ios::basefield);
x << 123;
x.unsetf(std::ios::basefield);
}
the ios_base namespace works fine too
void printHex(std::ostream& x){
x.setf(std::ios_base::hex, std::ios_base::basefield);
x << 123;
x.unsetf(std::ios_base::basefield);
}
Reference: http://www.cplusplus.com/reference/ios/ios_base/setf/
I would like to generalize the answer from qbert220 somewhat:
#include <ios>
class IoStreamFlagsRestorer
{
public:
IoStreamFlagsRestorer(std::ios_base & ioStream)
: ioStream_(ioStream)
, flags_(ioStream_.flags())
{
}
~IoStreamFlagsRestorer()
{
ioStream_.flags(flags_);
}
private:
std::ios_base & ioStream_;
std::ios_base::fmtflags const flags_;
};
This should work for input streams and others as well.
PS: I would have liked to make this simply a comment to above answer, stackoverflow however does not allow me to do so because of missing reputation. Thus make me clutter the answers here instead of a simple comment...
Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 8 years ago.
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I need to write/read a file that contains a std::map. The file must be read at the program start up (if it exists). Im using boost's fstream, but im getting this:
"terminate called after throwing an instance of 'boost::archive::archive_exception'
what(): input stream error"
Well, i really dont know what is happening.. these are my lines:
map<int64_t, int64_t> foo;
filesystem::path myFile = GetWorkingDirectory() / "myfile.dat";
[...............] // some code
filesystem::ifstream ifs(myFile);
archive::text_archive ta(ifs);
if (filesystem::exists(myFile)
{
ta >> foo; // foo is empty until now, it's fed by myFile
ifs.close();
}
What im doing wrong? Any idea?
Thanks.
P.S. Note that some lines after, i need to do the reverse action: write into myfile.dat the std::map foo.
EDIT: all works if i use std::ifstream, saving the file in the same dir where im running the Application. But using boost and his path, something goes wrong.
I'm a bit miffed. You're clearly using Boost Serialization (the archive/ headers are part of this library) but somehow you're not saying anything about that. Because it's so easy to demonstrate:
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#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <boost/serialization/map.hpp>
#include <boost/filesystem.hpp>
#include <boost/filesystem/fstream.hpp>
using namespace boost;
int main() {
std::map<int64_t, int64_t> foo;
filesystem::path myFile = filesystem::current_path() / "myfile.dat";
if (filesystem::exists(myFile))
{
filesystem::ifstream ifs(myFile/*.native()*/);
archive::text_iarchive ta(ifs);
ta >> foo; // foo is empty until now, it's fed by myFile
std::cout << "Read " << foo.size() << " entries from " << myFile << "\n";
} else {
for (int i=0; i<100; ++i) foo.emplace(rand(), rand());
filesystem::ofstream ofs(myFile/*.native()*/);
archive::text_oarchive ta(ofs);
ta << foo; // foo is empty until now, it's fed by myFile
std::cout << "Wrote " << foo.size() << " random entries to " << myFile << "\n";
}
}
Prints
Wrote 100 random entries to "/tmp/myfile.dat"
And on second run:
Read 100 entries from "/tmp/myfile.dat"
This question already has answers here:
Restore the state of std::cout after manipulating it
(9 answers)
Closed 4 years ago.
If I apply an arbitrary number of manipulators to a stream, is there a way to undo the application of those manipulators in a generic way?
For example, consider the following:
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
cout << "Hello" << hex << 42 << "\n";
// now i want to "roll-back" cout to whatever state it was in
// before the code above, *without* having to know
// what modifiers I added to it
// ... MAGIC HAPPENS! ...
cout << "This should not be in hex: " << 42 << "\n";
}
Suppose I want to add code at MAGIC HAPPENS that will revert the state of the stream manipulators to whatever it was before I did cout << hex. But I don't know what manipulators I added. How can I accomplish this?
In other words, I'd like to be able to write something like this (psudocode/fantasy code):
std::something old_state = cout.current_manip_state();
cout << hex;
cout.restore_manip_state(old_state);
Is this possible?
EDIT:
In case you're curious, I'm interested in doing this in a custom operator<<() I'm writing for a complex type. The type is a kind of discriminated union, and different value types will have different manips applied to the stream.
EDIT2:
Restriction: I cannot use Boost or any other 3rd party libraries. Solution must be in standard C++.
Yes.
You can save the state and restore it:
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
std::ios state(NULL);
state.copyfmt(std::cout);
cout << "Hello" << hex << 42 << "\n";
// now i want to "roll-back" cout to whatever state it was in
// before the code above, *without* having to know what modifiers I added to it
// ... MAGIC HAPPENS! ...
std::cout.copyfmt(state);
cout << "This should not be in hex: " << 42 << "\n";
}
If you want to get back to the default state you don't even need to save the state you can extract it from a temporary object.
std::cout.copyfmt(std::ios(NULL));
The standard manipulators all manipulate a stream's format flags, precision and width settings. The width setting is reset by most formatted output operations anyway. These can all be retrieved like this:
std::ios_base::fmtflags saveflags = std::cout.flags();
std::streamsize prec = std::cout.precision();
std::streamsize width = std::cout.width();
and restored:
std::cout.flags( saveflags );
std::cout.precision( prec );
std::cout.width( width );
Turning this into an RAII class is an exercise for the reader...
Saving and restoring state is not exception-safe. I would propose to shuffle everything into a stringstream, and finally you put that on the real stream (which has never changed its flags at all).
#include <iostream>
#include <iomanip>
#include <sstream>
int main()
{
std::ostringstream out;
out << "Hello" << std::hex << 42 << "\n";
std::cout << out.str();
// no magic necessary!
std::cout << "This should not be in hex: " << 42 << "\n";
}
Of course this is a little less performant. The perfect solutions depends on your specific needs.
Boost IO State saver might be of help.
http://www.boost.org/doc/libs/1_40_0/libs/io/doc/ios_state.html
I know that is an old question, but for future generations:
You can also write a simple state saver yourself (it will certainly help you avoid leaving the state changed). Just use the solution suggested by #loki and run it from the constructor/destructor of an object (in short: RAII) along these lines:
class stateSaver
{
public:
stateSaver(ostream& os): stream_(os), state_(nullptr) { state_.copyfmt(os); }
~stateSaver() { stream_.copyfmt(state_); }
private:
std::ios state_;
ostream& stream_;
};
Then, you will use it like this:
void myFunc() {
stateSaver state(cout);
cout << hex << 42 << endl; // will be in hex
}
int main() {
cout << 42 << endl; // will be in dec
myFunc();
cout << 42 << endl; // will also be in dec
}