Write a C++ program that accepts a string from the user and then replaces all occurrences of the letter e with the letter x.

Here's a sample C++ program that replaces all occurrences of the letter "e" with "x" in a user-input string:

#include <iostream>
#include <string>

int main() {
    std::string input;
    std::cout << "Enter a string: ";
    std::getline(std::cin, input);

    for (auto& c : input) {
        if (c == 'e') {
            c = 'x';
        }
    }

    std::cout << "Modified string: " << input << std::endl;

    return 0;
}

In this program, we first ask the user to input a string using `std::getline()`. We then loop through each character in the string, and if we encounter the letter "e", we replace it with "x". Finally, we output the modified string using `std::cout`.