~/Building a REPL in C++

Apr 17, 2022

A REPL or Read Eval Print Loop in C++ allows users to enter commands, evaluate them, and see results immediately. Implementing a basic REPL is straightforward.

Basic steps

1. Read input from stdin.
2. Evaluate or process the input.
3. Print results to stdout.
4. Loop until exit.

Example code

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#include <iostream>
#include <string>

int main() {
    std::string line;
    while (true) {
        std::cout << "cpprepl> ";
        if (!std::getline(std::cin, line)) {
            break;
        }
        if (line == "exit" || line == "quit") {
            break;
        }
        // Replace with actual evaluation as needed
        std::cout << "You entered: " << line << std::endl;
    }
    return 0;
}

Features Add features by parsing the input, evaluating C++ expressions, or using a library such as Cling to interpret input as C++ code.

Considerations

• For advanced usage, use Readline for keyboard input.
• To evaluate C++ code directly, integrate with Cling.
• Always handle input errors gracefully.

A REPL pattern is valuable for CLI tools and interpreters.