15.3. File input¶

std::ifstream, declared in <fstream>, reads from a file. Construct the stream with a string containing the file name. Modern C++ accepts a std::string directly; no conversion to a C string is required.

std::string file_name = "readings.txt";
std::ifstream input(file_name);

Check that the file opened, then check each read. A stream converts to a Boolean value that is false after a failed operation. Reading in a loop's condition avoids processing a value that was never successfully read.

c192_readings.txt

12
7
19

Example c192_file_input_readings

 1#include <fstream>
 2#include <iostream>
 3
 4int main() {
 5    std::ifstream input("c192_readings.txt");
 6    if (!input) {
 7        std::cerr << "Unable to open readings\n";
 8        return 1;
 9    }
10    int reading = 0;
11    while (input >> reading) {
12        std::cout << reading << '\n';
13    }
14    if (input.bad() || !input.eof()) {
15        std::cerr << "Unable to read an integer\n";
16        return 1;
17    }
18}

For whole lines, use while (std::getline(input, line)). A final line without a newline is still a valid line and must be processed. Testing eof() immediately after a successful getline can incorrectly discard that line. Testing !eof() before reading can process stale data after a failed read. Check the read itself instead.

std::string line;
while (std::getline(input, line)) {
    std::cout << line << '\n';
}

Formatted extraction (>>) usually skips leading whitespace. getline reads up to the next newline. If you mix them, remember that >> can leave the newline in the stream, so the next getline may read an empty line. One option is to read whole lines and parse each line with a separate string stream, as we will do for the city records.

A file stream closes its file when it goes out of scope. This is an example of resource ownership: the object's lifetime controls the resource's lifetime.

We need to use the function c_str() to convert a string to a native C string because...

the ifstream constructor expects a C string as an argument.

Correct!

you need to make sure you have permission to read to/from the file.

Incorrect! Try reading again!

it will check whether you have an infinite loop or not.

Incorrect! Try reading again!

strings are not supported by C++.

Incorrect! strings are allowed in C++.

The standard-library type used to read from a file is .