15.4. File output¶

std::ofstream writes to a file. Opening an output file normally truncates its old contents; use a different output name when copying a file. If you intend to append instead, open it with std::ios::app. Both file stream types are declared in <fstream>.

The following program copies whole lines. It checks input and output opening separately so its error messages identify the failed operation. It also checks writes and the explicit close: an output failure may be reported only when buffered output is flushed.

Example c192_copy_file

 1#include <fstream>
 2#include <iostream>
 3#include <string>
 4
 5int main() {
 6    std::ifstream input("c192_readings.txt");
 7    if (!input) {
 8        std::cerr << "Unable to open input\n";
 9        return 1;
10    }
11    std::ofstream output("readings_copy.txt");
12    if (!output) {
13        std::cerr << "Unable to open output\n";
14        return 1;
15    }
16    std::string line;
17    while (std::getline(input, line)) {
18        output << line << '\n';
19        if (!output) {
20            std::cerr << "Write failed\n";
21            return 1;
22        }
23    }
24    if (input.bad() || !input.eof()) {
25        std::cerr << "Read failed\n";
26        return 1;
27    }
28    output.close();
29    if (!output) {
30        std::cerr << "Unable to finish output\n";
31        return 1;
32    }
33    std::cout << "Copy complete\n";
34}

This copies text lines, adding a newline after each one. It is not a byte-for-byte copy when the original file has no final newline. If exact bytes matter, that is a different requirement.

Write the values in readings to readings.txt, one per line. Return a nonzero status from main if opening or writing fails. Assume the headers and readings are supplied.

```
    output << reading << '\n';
}
```
```
for (int reading : readings) {
```
```
if (!output) { return 1; }
```
```
if (!output) { return 1; }
```
```
output.close();
```
```
std::ofstream output("readings.txt");
```

The code from the previous problem checks whether the files open or not. It doesn't specify which one, if any, doesn't open. How could you specify which file does not open?

Create two "for" loops instead of an if-statement so that the statement loops through both conditions once.

Try again!

Create a "while" loop instead of an if-statement so that the statement loops through both conditions separately until the body of the loop is reached.

Try again!

Create two "if" statements, one that check whether in_file.good() is false, and another that checks whether out_file.good() is false, instead of putting them together in one "if" statement.

Correct!

Complete the type: output("results.txt");.