so I want to create a file but the name of it will be dependent on the user input e.g. if the user types "shrek" the file must be named "shrek.txt". Thats what I came up with but it doesn't work.
int main(){
ofstream file;
string name = "abc";
file.open(name + ".txt");
file.close();
}
I guess you are using an old C++ standard. If that's the case, fstream::open won't accept a std::string, only a C string (char*). You can use c_str in your string to obtain a const char* that will be accepted:
int main(){
ofstream file;
string name = "abc";
string file_name = name + ".txt";
file.open(file_name.c_str()); // <- here
file.close();
}
However, it's recommendable to switch to a more modern standard, as your code actually works for C++11 and newer.
Do you have the libraries required? In this case #include fstream. Does the same issue happens to another complier? Check that out. I attempted this myself and your code surely works. Attempt to use my code which I confirm works and see if you have any issues.
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main() {
string name;
cin>> name;
ofstream file(name +".txt");
file.close();
}
Related
I found some code online to write a binary file, but when I want to execute it, the compiler gives me an error
unknown type name 'fs'
How can I fix this?
#include <fstream>
namespace std{
string token = "token";
std::ofstream fs("example.bin", ios::out | ios::binary | ios::app);
fs.write(token, sizeof token);
fs.close();
}
You can't have non-declaration statements inside of a namespace block. The code to use the std::string and std::ofstream objects needs to be inside of a function instead, which can then be declared inside of a namespace if desired. And besides, it is illegal to add new things to the std namespace, anyway.
Also, you can't write() a std::string object in the manner the code is trying to. Not only will it not compile to begin with (the 1st parameter needs a char* pointer), but it is logically wrong anyway since a std::string stores its character data elsewhere in memory, so you would be writing the std::string's internal pointers to that data, not the actual data itself.
Try this instead:
#include <fstream>
#include <string>
namespace my_ns{
void save() {
std::string token = "token";
std::ofstream fs("example.bin", std::ios::binary | std::ios::app);
fs.write(token.c_str(), token.size());
fs.close();
}
}
int main() {
my_ns::save();
return 0;
}
Though, I suspect the original code was actually trying to do something more like this instead:
#include <fstream>
#include <string>
using namespace std; // <--
int main() {
string token = "token";
ofstream fs("example.bin", ios::binary | ios::app);
fs.write(token.c_str(), token.size());
fs.close();
return 0;
}
I am using a third party library which needs a ifstream object as input and I have a stringstream object which contains everything they need. Right now, I have to write the content of stringstream to a file and then send a ifstream object to the library. I am wondering if it is possible directly convert stringstream to ifstream in memory so I don't need to write a file on disk? thanks.
I your library is really accepting std::ifstream instead of std::istream, then I found the following hack:
#include <fstream>
#include <iostream>
#include <sstream>
#include <string>
void foo(std::ifstream& fs)
{
std::string h;
fs >> h;
std::cout << h << std::endl;
}
int main()
{
std::istringstream s1("hello");
std::ifstream s2;
s2.basic_ios<char>::rdbuf(s1.rdbuf());
foo(s2);
return 0;
}
I am not sure how safe it is, however, so you might investigate the topic further.
I want to be able to take text input and store it in a string variable like so:
#include <fstream>
int main()
{
string fileInput = "filetoinput.txt";
ifstream inputFile (fileInput);
}
But it will only accept creating an ifstream type variable like so:
#include <fstream>
int main()
{
ifstream inputFile ("filetoinput.txt");
}
Is there a way to make a string variable act like text in quotes?
With C++11 the original example should work:
#include <fstream>
#include <string>
std::string fileInput = "filetoinput.txt";
std::ifstream inputFile(fileInput);
If you're not up to C++11, then fileInput.c_str() gives you a C-style string that you can use for the call:
std::ifstream inputFile(fileInput.c_str());
#include <fstream>
int main()
{
ifstream inputFile (fileInput.c_str());
}
c_str() is what you want.
Yes, use .c_str() method:
ifstream inputFile (fileInput.c_str());
ifstream is using explicit constructor
explicit ifstream (const char* filename, ios_base::openmode mode = ios_base::in)
So you need to use strings' const char* c_str() const function to pass parameter.
When using ifstream class to read words from an input file, I have used the following expression:
#include <iostream>
#include <fstream>
int main(int argc, char *argv[])
{
std::ifstream inputStream(myFile.txt);
std::string myString;
myFile.open()
while(myFile.good())
{
myFile >> myString;
printf("%s \n", myString);
}
return 0;
}
The contents of myFile.txt are:
" This is a simple program. "
The compiles and executes as expected using g++ compiler.
However, the same code when compiled using msvc 2008, returns error at the extraction operator (>>) requiring me to replace the std::string with either an initialized character array or any of the supported native types.
This threw me off as I was expecting the usage of the standard library to be same across implementations.
I understand the compile error and know the way to fix it via using c_str().
But, it would help me a great deal, if someone could clarify why the usage for the standard library is different across platforms.
To me it is not starndard anymore !!
EDIT: Code updated to be complete. Content of myFile.txt updated.
Chances are that you forgot to #include <string>. Without it, Microsoft's version of <iostream> (and such) include enough of a declaration of std::string for some things to work, but other parts are missing, so you get strange, seemingly inexplicable failures.
One of the things that's missing is most of the operator overloads for std::string, which is exactly what you seem to be missing.
As an aside, while (myfile.good()) ... is pretty much a guaranteed bug -- you probably want:
while (myfile>>myString)
std::cout << myString << " \n";
Alternatively, you could do the job with a standard algorithm:
#include <string>
#include <algorithm>
#include <iterator>
#include <fstream>
#include <iostream>
int main() {
std::ifstream myfile("input.txt");
std::copy(std::istream_iterator<std::string>(myfile),
std::istream_iterator<std::string>(),
std::ostream_iterator<std::string>(std::cout, " \n"));
return 0;
}
The following compiles fine for me on MSVC 2010:
std::ifstream inputStream;
std::string myString;
inputStream.open("myFile.txt", std::ifstream::in);
while(inputStream.good())
{
inputStream >> myString;
}
Note: without using std::ifstream::in as my open mode, I got the same error as you. I suggest you check what value you have for this parameter.
I have really strange problem. In Visual C++ express, I have very simple code, just:
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt");
file<<"Hello";
file.close();
}
This same code works OK in my one project, but when I create now project and use this same lines of code, no file test.txt is created. Please, what is wrong?ยจ
EDIT: I expect to see test.txt in VS2008/project_name/debug - just like the first functional project does.
Canonical code to write to a file:
#include <fstream>
#include <iostream>
using namespace std;
int main() {
ofstream file;
file.open("test.txt");
if ( ! file.is_open() ) {
cerr << "open error\n";
}
if ( ! ( file << "Hello" ) ) {
cerr << "write error\n";
}
file.close();
}
Whenever you perform file I/O you must test every single operation, with the possible exception of closing a file, which it is not usually possible to recover from.
As for the file being created somewhere else - simply give it a weird name like mxyzptlk.txt and then search for it using Windows explorer.
Perhaps the executable is run in a different directory than it was before, making test.txt appear somewhere else. Try using an absolute path, such as "C:\\Users\\NoName\\Desktop\\test.txt" (The double backslashes are needed as escape characters in C strings).
fstream::open() takes two arguments: filename and mode. Since you are not providing the second, you may wish to check what the default argument in fstream is or provide ios_base::out yourself.
Furthermore, you may wish to check whether the file is open. It is possible that you do not have write permissions in the current working directory (where 'test.txt' will be written since you don't provide an absolute path). fstream provides the is_open() method as one way of checking this.
Lastly, think about indenting your code. While you only have a few lines there, code can soon become difficult to read without proper indentation. Sample code:
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt", ios_base::out);
if (not file.is_open())
{
// Your error-handling code here
}
file << "Hello";
file.close();
}
You can use Process Monitor and filter on file access and your process to determine whether the open/write is succeeding and where on disk it's happening.
Theres two ways to fix this. Either do:
file.open("test.txt", ios::out)
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt", ios::out);
file<<"Hello";
file.close();
}
Or you can create an ofstream instead of fstream.
#include <fstream>
using namespace std;
int main()
{
ofstream file;
file.open("test.txt");
file<<"Hello";
file.close();
}