URL encoding is a way of representing certain characters in a URL by replacing them with a % symbol followed by their hexadecimal value. For example, the space character is encoded as %20 and the ampersand character is encoded as %26. In some cases, you may need to decode a URL encoded string back to its original form in your C++ program. In this tutorial, we will guide you through the process of URL decoding a string in C++. Let’s get started!

Step 1: Include necessary libraries

To perform URL decoding in C++, we need to include the <string> and <sstream> libraries. The <string> library provides functions for manipulating strings, while the <sstream> library allows us to perform stream operations on strings.

#include <string>
#include <sstream>

Step 2: Create a function to decode a URL encoded string

Next, let’s define a function called urlDecode that takes a URL encoded string as input and returns the decoded string.

std::string urlDecode(const std::string& urlEncodedString) {
    std::istringstream input(urlEncodedString);
    std::ostringstream output;

    char hex;
    int value;

    while (input >> std::noskipws >> hex) {
        if (hex == '%') {
            if (input >> std::hex >> value) {
                output << static_cast<char>(value);
        else {
            output << hex;

    return output.str();

In this function, we use std::istringstream to read characters from the URL encoded string. If a character is equal to ‘%’, it means that we have encountered an encoded character. We then read the hexadecimal value after ‘%’, convert it to an integer using std::hex, and append the corresponding character to the std::ostringstream called output. If the character is not ‘%’, we simply append it to the output.

Step 3: Decode a URL encoded string

Now that we have our urlDecode function, let’s see how we can use it to decode a URL encoded string.

int main() {
    std::string urlEncodedString = ""Hello%20World%21"";
    std::string decodedString = urlDecode(urlEncodedString);

    std::cout << ""Decoded string: "" << decodedString << std::endl;

    return 0;

In this example, we have a URL encoded string ““Hello%20World%21"” which represents ““Hello World!””. We pass this string to the urlDecode function, and the decoded string ““Hello World!”” is stored in the variable decodedString. Finally, we print the decoded string to the console using std::cout.

Step 4: Compile and run the program

To compile and run the program, save the code in a .cpp file (e.g., url_decode.cpp). Then, open a terminal or command prompt and navigate to the directory where the file is saved. Use a C++ compiler, such as g++, to compile the program.

g++ url_decode.cpp -o url_decode

After successful compilation, run the executable file.


You should see the following output:

Decoded string: Hello World!

Congratulations! You have successfully decoded a URL encoded string in C++. Feel free to use the urlDecode function in your projects to decode any URL encoded strings as needed.

I hope you found this tutorial helpful!