Can't write to txt file [closed] - c++

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I am trying to write to a .txt file within a program, and I am using the following code snippet. I want to output a line of the form:
a(space)b
but I get nothing on the txt file.
#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;
int main(){
string a = "string";
int b = 0;
fstream f;
f.open("filename.txt",ios::in | ios::out );
f << a << ' ' << b << endl;
f.close();
return 0;
}

Answer 1

If you try this piece of code:
f.open("filename.txt", ios::in | ios::out);
if (!f.is_open()) {
cout << "error";
}
you will see that the file is never opened. ios::in requires an already existing file (std::fstream::open()):
It should work if you should only pass std::ios::out to f.open():
f.open("filename.txt", ios::out);
Read here why you shouldn't be using namespace std;.

Answer 2

Try this version. Few changes:
Include the correct header <string> instead of <cstring>
No using namespace std;
Use std::ofstream for output
No .open(): pass the filename in the constructor
Check if the file is valid after opening
No .close(). Let the destructor do its job.
No std::endl if '\n' is enough.
#include <iostream>
#include <fstream>
#include <string>
int main()
{
std::string a = "string";
int b = 0;
std::ofstream f("filename.txt");
if (!f) {
return EXIT_FAILURE;
}
f << a << ' ' << b << '\n';
return EXIT_SUCCESS;
}