C++ program to append content of one text file to another

Given a source file and a destination file, we can append the contents of the source file to the end of the destination file. C++ provides file-stream classes and C-style file functions to perform this operation.

• The source file is opened in read mode to access its contents.
• The destination file is opened in append mode so its existing content is preserved.

Example:

Consider two text files with the following contents:

file.txt : geeks
file2.txt : geeks for

After appending the contents of file.txt to file2.txt, the destination file becomes:

file2.txt : geeks for geeks

Approaches to Append One File to Another

The following approaches can be used to append the contents of one text file to another:

1. Using C++ File Streams

The ifstream class can be used to read the source file, while ofstream can open the destination file in append mode using ios::app.

Steps

• Open the source file using ifstream.
• Open the destination file using ofstream with ios::app.
• Copy the contents of the source file to the destination file.
• Open the destination file in read mode and display its contents.

#include <fstream>
#include <iostream>
using namespace std;

int main()
{
    ifstream source("file.txt");
    ofstream destination("file2.txt", ios::app);

    if (!source || !destination) {
        cout << "Unable to open file";
        return 1;
    }

    destination << source.rdbuf();

    source.close();
    destination.close();

    ifstream file("file2.txt");
    string content;

    while (getline(file, content))
        cout << content << endl;

    file.close();

    return 0;
}

output

geeks for geeks

Explanation: ifstream opens file.txt for reading, while ofstream opens file2.txt in append mode andrdbuf() copies the source content to the destination, which is then opened and displayed.

2. Using C-Style File Functions

The C standard library provides functions such as fopen(), fgetc(), fprintf(), and fclose() that can also be used in a C++ program to work with files.

Steps:

• Open the source file in read mode using fopen().
• Open the destination file in append mode using fopen().
• Read each character from the source file using fgetc().
• Write each character to the destination file using fprintf().
• Close both files using fclose().
• Open the destination file in read mode and display its contents.

#include <cstdio>
#include <iostream>
using namespace std;

int main()
{
    FILE* source = fopen("file.txt", "r");
    FILE* destination = fopen("file2.txt", "a");

    if (source == nullptr || destination == nullptr) {
        cout << "Unable to open file";
        
        if (source)
            fclose(source);
        if (destination)
            fclose(destination);

        return 1;
    }

    int ch;

    while ((ch = fgetc(source)) != EOF)
        fprintf(destination, "%c", ch);

    fclose(source);
    fclose(destination);

    FILE* file = fopen("file2.txt", "r");

    if (file == nullptr) {
        cout << "Unable to open destination file";
        return 1;
    }

    while ((ch = fgetc(file)) != EOF)
        printf("%c", ch);

    fclose(file);

    return 0;
} 

Output

geeks for geeks

Explanation

• fopen() opens the source file in read mode and the destination file in append mode, while fgetc() reads the source file character by character.
• fprintf() appends each character to the destination file, which is then reopened for display, and fclose() closes the opened file streams.

File Opening Modes Used