File open check

Learn file open checking with is_open().

Safe opening with is_open

After opening a file you check whether it succeeded with is_open(). If the file does not exist or permissions are missing, is_open() returns false.

Example: std::ifstream in("missing.txt"); if (in.is_open()) { /* read */ } else { std::cout << "Could not open"; }. This pattern lets the program report an error instead of crashing. If you call >> without checking, reading silently fails when the file is not open and the program behaves wrongly. That is why checking is_open() on every file open is a solid habit.

Handle a file that did not open

When is_open() returns false the program should print an error message and not read the file. Otherwise >> fails silently.

Example: if (!in.is_open()) { std::cout << "Error"; return 1; }. This ends the program early when the file is not found. The ! operator inverts the is_open() result. Some programs write if (in) to query the stream state; it gives a check similar to is_open(). Whichever you use, reading a file without an open check is not safe.

Exercise Check a missing file

The starting code tries to open missing.txt (this file does not exist). Check with is_open(): print "Open" if open, otherwise "Closed".

Show example solution

#include <iostream>
#include <fstream>

int main() {
    std::ifstream in("missing.txt");
    if (in.is_open()) {
        std::cout << "Open";
    } else {
        std::cout << "Closed";
    }
    return 0;
}

Test yourself

What does it mean when in.is_open() returns false?
```
std::ifstream in("none.txt");
if (in.is_open()) { /* read */ }
```
- The file is too large
- The file is empty
- The file could not be opened
- The file is already closed
Show answer

The file could not be opened — false means the file could not be opened.

Complete the is_open call

if (in.____()) { std::cout << "Open"; }

Show answer

if (in.is_open()) { std::cout << "Open"; }

Match each piece to its role

is_open()

Open check
if (in.is_open())

Safe open
return 1

Exit on error
!in.is_open()

Not open check

Next lesson: Line reading

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