Store cout output into variable - c++

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"

Related

istringstream-function string_to_int cannot take c-str, why?

I learned a helper function that can convert strings to integers:
int string_to_int(string s)
{
istringstream instr(s);
int n;
instr>>n;
return n;
}
It's mentioned that the argument s cannot be c-str string, why is this the case?
But you can pass a C style string.
The reason for that is because the std::string constructor can implicitly accept a CharT* (Char type, which is char in this case) as a parameter. Thus, something like the following would work:
#include <iostream>
#include <sstream>
using namespace std;
int string_to_int(string s)
{
istringstream instr(s);
int n;
instr>>n;
return n;
}
int main()
{
const char* test = "12345";
std::cout << string_to_int(test) << "\n"; // Outputs 12345
std::cout << string_to_int("122") << "\n"; // Outputs 122
}

Assign a integer to char pointer using stringstream

I want to assign integer to a char pointer using stringstream. But I am getting error while running this program at line ss >> p. Please help me here i want integer to go into the buffer first and the it must be assigned to a char*.
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
using namespace std;
int main ()
{
stringstream ss;
int n=100;
char *p;
ss << n;
ss >> p; //not working
cout << ss;
return 0;
}
Use stringstream::str to get a C++ string, then use .c_str() on the string:
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
using namespace std;
int main ()
{
stringstream ss;
int n = 100;
char* p;
ss << n;
string tmp = ss.str();
p = const_cast<char*>(tmp.c_str());
cout << "p: " << p << '\n';
return 0;
}
Beware that the char pointer becomes invalid as soon as the string goes out of scope. If you need some kind of factory function behavior, return a string by value, use strlcpy or maybe new and shared_ptr.
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
using namespace std;
int main ()
{
stringstream ss;
int n=100;
char buffer[100];
char *p = buffer;
ss << n;
ss >> p;
cout << p;
return 0;
}
This is fixing only the problem you directly encountered - there's no storage behind p so it will crash. Stylistically there are many other things to improve / fix, but this should show you what part of this was actually wrong.

What's the equivalent of cout for output to strings?

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

converting from strings to ints

I would like to know what is the easiest way to convert an int to C++ style string and from C++ style string to int.
edit
Thank you very much. When converting form string to int what happens if I pass a char string ? (ex: "abce").
Thanks & Regards,
Mousey
Probably the easiest is to use operator<< and operator>> with a stringstream (you can initialize a stringstream from a string, and use the stream's .str() member to retrieve a string after writing to it.
Boost has a lexical_cast that makes this particularly easy (though hardly a paragon of efficiency). Normal use would be something like int x = lexical_cast<int>(your_string);
You can change "%x" specifier to "%d" or any other format supported by sprintf. Ensure to appropriately adjust the buffer size 'buf'
int main(){
char buf[sizeof(int)*2 + 1];
int x = 0x12345678;
sprintf(buf, "%x", x);
string str(buf);
int y = atoi(str.c_str());
}
EDIT 2:
int main(){
char buf[sizeof(int)*2 + 1];
int x = 42;
sprintf(buf, "%x", x);
string str(buf);
//int y = atoi(str.c_str());
int y = static_cast<int>(strtol(str.c_str(), NULL, 16));
}
This is to convert string to number.
#include "stdafx.h"
#include <iostream>
#include <string>
#include <sstream>
int convert_string_to_number(const std::string& st)
{
std::istringstream stringinfo(st);
int num = 0;
stringinfo >> num;
return num;
}
int main()
{
int number = 0;
std::string number_as_string("425");
number = convert_string_to_number(number_as_string);
std::cout << "The number is " << number << std::endl;
std::cout << "Number of digits are " << number_as_string.length() << std::endl;
}
Like wise, the following is to convert number to string.
#include "stdafx.h"
#include <iostream>
#include <string>
#include <sstream>
std::string convert_number_to_string(const int& number_to_convert)
{
std::ostringstream os;
os << number_to_convert;
return (os.str());
}
int main()
{
int number = 425;
std::string stringafterconversion;
stringafterconversion = convert_number_to_string(number);
std::cout << "After conversion " << stringafterconversion << std::endl;
std::cout << "Number of digits are " << stringafterconversion.length() << std::endl;
}
Use atoi to convert a string to an int. Use a stringstream to convert the other way.

how do i add a int to a string

i have a string and i need to add a number to it i.e a int. like:
string number1 = ("dfg");
int number2 = 123;
number1 += number2;
this is my code:
name = root_enter; // pull name from another string.
size_t sz;
sz = name.size(); //find the size of the string.
name.resize (sz + 5, account); // add the account number.
cout << name; //test the string.
this works... somewhat but i only get the "*name*88888" and... i don't know why.
i just need a way to add the value of a int to the end of a string
There are no in-built operators that do this. You can write your own function, overload an operator+ for a string and an int. If you use a custom function, try using a stringstream:
string addi2str(string const& instr, int v) {
stringstream s(instr);
s << v;
return s.str();
}
Use a stringstream.
#include <iostream>
#include <sstream>
using namespace std;
int main () {
int a = 30;
stringstream ss(stringstream::in | stringstream::out);
ss << "hello world";
ss << '\n';
ss << a;
cout << ss.str() << '\n';
return 0;
}
You can use string streams:
template<class T>
std::string to_string(const T& t) {
std::ostringstream ss;
ss << t;
return ss.str();
}
// usage:
std::string s("foo");
s.append(to_string(12345));
Alternatively you can use utilities like Boosts lexical_cast():
s.append(boost::lexical_cast<std::string>(12345));
Use a stringstream.
int x = 29;
std::stringstream ss;
ss << "My age is: " << x << std::endl;
std::string str = ss.str();
you can use lexecal_cast from boost, then C itoa and of course stringstream from STL