My problem is really petty, but nevertheless I have not found any answer by asking google or by asking peers. The problem can be shown by the following code:
std::ostringstream oss("I am a ");
oss << "donkey";
std::cout << oss.str();
Expected Output: "I am a donkey"
Actual Output: "donkey"
What happens here? Is the initial string the stringstream had to begin with been discarded?
You have to add std::ios_base::ate to the constructor, otherwise it would overwrite from the beginning:
#include <iostream>
#include <sstream>
using namespace std;
int main() {
std::ostringstream oss("I am a ", std::ios_base::ate);
oss << "donkey";
std::cout << oss.str();
return 0;
}
https://ideone.com/IIfOkB
More information: https://en.cppreference.com/w/cpp/io/basic_ostringstream
Example: https://en.cppreference.com/w/cpp/io/basic_ostringstream/str
Related
How can I store the output from cout into a variable of string or character type?
I have written following code but it doesn't work:
#include<iostream>
#include<stdio.h>
using namespace std;
int main(){
string n;
n = (cout<<"\nHello world");
cout<<n;
return 0;
}
#include <sstream>
std::ostringstream a;
a << "Hello, world!";
std::string b = a.str(); // Or better, `std::move(a).str()`.
std::cout << b;
Other answers have shown you how to capture formatted output using a std::(o)stringstream object directly. But, if for some reason, you really need to capture the output of std::cout, then you can temporarily redirect std::cout to use a std::ostringstream's buffer, eg:
#include <iostream>
#include <sstream>
using namespace std;
int main(){
ostringstream oss;
auto cout_buff = cout.rdbuf(oss.rdbuf());
cout << "\nHello world";
cout.rdbuf(cout_buff);
string n = oss.str();
cout << n;
return 0;
}
Online Demo
Of course there's a way! But you have to use a different kind of stream:
std::ostringstream ss;
ss << "\nHello world";
std::string result = ss.str();
Also, in C++20, you can simply use std::format:
std::string n = std::format("Hello {}! I have {} cats\n", "world", 3);
// n == "Hello world! I have 3 cats\n"
using namespace std;
string str{ "text" };
stringstream ss{ str };
cout.rdbuf(ss.rdbuf());
cout.flush(); //cout<<endl;
This code is expected to print text,but showing nothing.
I associate ss to stdout then flush it,but I don't know why it didn't work even if I refer to many illustration.
cout<<rdbuf(ss);
this is OK but where's the different? : (
I think you misunderstood this concept. The line
cout.rdbuf(ss.rdbuf());
sets the pointer to the buffer. It redirects cout to ss and not to console. If you write into cout it will be written to ss.
I hope this clears it for you
#include <iostream>
#include <string>
#include <sstream>
int main() {
std::streambuf *backup;
backup = std::cout.rdbuf();
std::stringstream ss;
std::cout.rdbuf(ss.rdbuf());
std::cout << "Test" << std::endl;
std::cout.rdbuf(backup);
std::cout << ss.rdbuf();
return 0;
}
In the example code I create a copy of the current pointer of cout's buffer. Next I redirect the cout to ss. When I write into cout it is written into ss's buffer. Next I redirect cout back to console and print the content of ss.
If you want to manipulate the buffer you need somethink like
#include <iostream>
#include <string>
#include <sstream>
#include <cstring>
int main() {
std::stringstream ss{ "test" };
std::stringbuf *pbuf = ss.rdbuf();
char buffer[80];
pbuf->sgetn (buffer,80);
std::streambuf *pbuf2 = std::cout.rdbuf();
pbuf2->sputn (buffer, std::strlen(buffer));
return 0;
}
As a complement to #Thomas Sablik answer, here is a code example for printing "text" by redirecting streams:
std::stringstream ss;
ss.basic_ios<char>::rdbuf(std::cout.rdbuf());
ss << "text";
std::cout.flush();
If you want to copy the content of ss's streambuf to std::cout, you can use inserter directly:
std::string str{ "text" };
std::stringstream ss{ str };
std::cout << ss.rdbuf();
std::cout.flush();
http://wordaligned.org/articles/cpp-streambufs
In c++ we have std::to_string which converts int/float/double to strings. So just to test my understanding of templates I tried the code below:
#include "iostream"
#include "sstream"
#include "string"
using std::cout;
template <typename T>
std::string getString(const T& data){
std::stringstream ss;
cout << '\n' << data << '\n';
ss << data;
std::string s;
ss >> s;
return s;
}
int main(int argc , char** argv){
cout << getString(1.0000011);
cout <<' '<<std::to_string(1.0000011);
return 0;
}
However, the output doesn't make sense, to_string gives me 1.0000011, whereas getString gets 1 and gives me 1. As I am using templates shouldn't getString get 1.0000011 as well and give me 1.0000011 too?
You can use std::setprecision in the <iomanip> header to set the precision that std::stringstream will use when formatting numeric data.
For example:
std::stringstream ss;
ss << std::setprecision(9) << data;
cout << ss.str();
Will print:
1.0000011
Here's a quick demo online: cpp.sh/9v7xf
As a side note, you don't have to create a string and output from the stringstream - you can replace the last 3 lines in getString() with:
return ss.str();
Numeric values are often truncated for appearance. You can supply the std::fixed manipulator from the iomanip standard header to avoid this issue.
#include "iomanip" // <- Add this header
#include "iostream"
#include "sstream"
#include "string"
using std::cout;
template <typename T>
std::string getString(const T& data)
{
std::stringstream ss;
cout << '\n' << data << '\n';
ss << std::fixed << data;
// ^^^^^^^^^^^^^ Add this
std::string s;
ss >> s;
return s;
}
int main(int argc, char** argv)
{
cout << getString(1.0000011);
cout << ' ' << std::to_string(1.0000011);
return 0;
}
<iomanip> needs included, and std::setprecision must be used when outputting float values to streams. Using your example, this looks like:
#include <iostream>
#include <iomanip> //include this.
#include <sstream>
#include <string>
template <typename T>
std::string getString(const T& data){
std::ostringstream ss;
ss << std::setprecision(8);
std::cout << std::setprecision(8);
std::cout << data << '\n';
ss << data;
return ss.str();
}
int main(int argc , char** argv){
std::cout << getString(1.0000011) << "\n";
std::cout << std::to_string(1.0000011) << std::endl;
return 0;
}
Which prints:
1.0000011
1.0000011
1.000001
Program ended with exit code: 0
Note how to_string alone truncates the floating point number!!! I suspect this is undesired behavior, but to_string cannot be manipulated directly, so...
If desired, you can fix this with the solution found here.
Otherwise, just use std:set_precision() when inserting into streams for precisely converted strings.
I have a string (of some fixed length), which I need to compress and then compare the compressed lengths (as a proxy for redundancy in the data or as a rough approximation to the Kolmogorov complexity). Currently, I am using boost::iostreams for compression, which seems working well. However, I don't know how to obtain the size of the compressed data. Can someone help, please?
The code snippet is
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/filesystem.hpp>
#include <string>
#include <sstream>
namespace io = boost::iostreams;
int main() {
std::string memblock;
std::cout << "Input the string to be compressed:";
std::cin >> memblock;
std::cout << memblock << std::endl;
io::filtering_ostream out;
out.push(io::gzip_compressor());
out.push(io::file_descriptor_sink("test.gz"));
out.write (memblock.c_str(), memblock.size());
std::cout << out.size() << std::endl;
return 0;
}
You can try adding boost::iostreams::counter to you chain between the compressor and sink and then calling it's characters() member to get number of bytes that went through it.
This works for me:
#include <boost/iostreams/filter/counter.hpp>
...
io::filtering_ostream out;
out.push(io::counter());
out.push(io::gzip_compressor());
out.push(io::counter());
out.push(io::file_descriptor_sink("test.gz"));
out.write (memblock.c_str(), memblock.size());
io::close(out); // Needed for flushing the data from compressor
std::cout << "Wrote " << out.component<io::counter>(0)->characters() << " bytes to compressor, "
<< "got " << out.component<io::counter>(2)->characters() << " bytes out of it." << std::endl;
I figured out yet another (and slightly slicker) way to achieve the compressed length of a string. I thought sharing it here, but basically it is simply passing the uncompressed string to a filtered buffer and copying the output back to a string:
template<typename T>
inline std::string compressIt(std::vector<T> s){
std::stringstream uncompressed, compressed;
for (typename std::vector<T>::iterator it = s.begin();
it != s.end(); it++)
uncompressed << *it;
io::filtering_streambuf<io::input> o;
o.push(io::gzip_compressor());
o.push(uncompressed);
io::copy(o, compressed);
return compressed.str();
}
Later one can easily get the size of the compressed string as
compressIt(uncompressedString).size()
I feel it is better for it does not required me to create an output file as previously.
cheers,
Nikhil
one other way would be
stream<array_source> input_stream(input_data,input_data_ize);
stream<array_sink> compressed_stream(compressed_data,alloc_compressed_size);
filtering_istreambuf out;
out.push(gzip_compressor());
out.push(input_stream);
int compressed_size = copy(out,compressed_stream);
cout << "size of compressed_stream" << compressed_size << endl;
I should know this already but... printf is to sprintf as cout is to ____? Please give an example.
It sounds like you are looking for std::ostringstream.
Of course C++ streams don't use format-specifiers like C's printf()-type functions; they use manipulators.
Example, as requested:
#include <sstream>
#include <iomanip>
#include <cassert>
std::string stringify(double x, size_t precision)
{
std::ostringstream o;
o << std::fixed << std::setprecision(precision) << x;
return o.str();
}
int main()
{
assert(stringify(42.0, 6) == "42.000000");
return 0;
}
#include <iostream>
#include <sstream>
using namespace std;
int main()
{
ostringstream s;
s.precision(3);
s << "pi = " << fixed << 3.141592;
cout << s.str() << endl;
return 0;
}
Output:
pi = 3.142
Here's an example:
#include <sstream>
int main()
{
std::stringstream sout;
sout << "Hello " << 10 << "\n";
const std::string s = sout.str();
std::cout << s;
return 0;
}
If you want to clear the stream for reuse, you can do
sout.str(std::string());
Also look at the Boost Format library.
std::ostringstream
You can use this to create something like the Boost lexical cast:
#include <sstream>
#include <string>
template <typename T>
std::string ToString( const T & t ) {
std::ostringstream os;
os << t;
return os.str();
}
In use:
string is = ToString( 42 ); // is contains "42"
string fs = ToString( 1.23 ) ; // fs contains something approximating "1.23"
You have a little misunderstanding for the concept of cout. cout is a stream and the operator << is defined for any stream. So, you just need another stream that writes to string in order to output your data. You can use a standard stream like std::ostringstream or define your own one.
So your analogy is not very precise, since cout is not a function like printf and sprintf