fstream .open and appending .txt or .dat to filename

Question

This has worked fine on some compilers... Is there a way of doing this were it will just work without it being a problem with different compilers on c++11 or c++14?

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

void save_file() {
    string file;
    ofstream os;
    cout << "Save As: ";
    getline(cin, file, '\n');
    os.open(file + ".dat");
    //rest of code
}

error: no viable conversion from 'basic_string, std::allocator >' to 'const char *'

So I google it, found some answers, or in this case, canswers (cancers), tried

os.open(file.c_str() + ".dat");

error: invalid operands to binary expression ('const char *' and 'const char *')

always provide an [SSCCE](http://sscce.org/). Also, where is the error, what line? — VP., Jul 13 '15 at 17:10

score 1 · Answer 1 · answered Jul 13 '15 at 17:01

Accoding to the C++11 standard 27.9.1.10 one of the constructors for a basic_ofstream is:

explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out);

This means that any standard compliant compiler should be able to compile:

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

int main()
{
    string file = "temp";
    ofstream os;
    os.open(file + ".dat");
}

Live Example

Don't forget that you need to use the -std=c++11 or higher flag when compiling.

And pre-C++11, you could still concatenate and call `.c_str()` on the results. — James Kanze, Jul 13 '15 at 18:00

score 1 · Answer 2 · answered Jul 13 '15 at 17:01

1

"+" operator cannot be used for the C-style strings. Try this:

string name = file+".dat";
os.open(name.c_str());

You create the std::string type as a concatenation in c++ style and then pass it to open as a c string.

answered Jul 13 '15 at 17:01

Hitokage

733
8
19

no problem ^^, mark this as an answer please so it's clear for the others – Hitokage Jul 13 '15 at 17:30
apparently I don't have the rep to do that! sorry. – StackAttack Jul 13 '15 at 21:33

score 1 · Answer 3 · answered Jul 13 '15 at 17:58

1

In C++11, os.open( file + ".dat" ) works just fine. Pre C++11, there was no std::ofstream::open which took a string, so you had to write os.open( (file + ".dat").c_str() ). Note the parentheses and where the .c_str() goes---you have to concatenate with the std::string first, and only call .c_str() on the results.

answered Jul 13 '15 at 17:58

James Kanze

150,581
18
184
329

confirmed: this worked os.open((file + ".dat").c_str()); – StackAttack Jul 13 '15 at 18:02

fstream .open and appending .txt or .dat to filename

3 Answers3