How to remove comments with a custom function in C++

Comments can be vital in the readability and documentation of code. It enables developers to provide explanations inside their code. However, when processing code for various purposes, such as analysis or formatting, comments can become an obstacle. In this Answer, we’ll demonstrate how to remove comments from code efficiently.

Code example

The C++ code given below removes comments from the source code provided in its parameters. The commentsremoval function removes comments from a given array of strings representing the code. It iterates through each line, character by character, and tracks whether it’s inside a comment. After execution, it returns the code with the comments removed.

Note: For this problem, we only check for comments that begin with //.

vector<string> commentsremoval(vector<string>& source) {
    vector<string> output;
    bool comment = false;
    
    string curr = "";
    
    for (const string& line : source) {
        for (int i = 0; i < line.size(); ++i) {
            if (!comment && i + 1 < line.size() && line[i] == '/' && line[i + 1] == '/') {
                break;
            } else if (!comment && i + 1 < line.size() && line[i] == '/' && line[i + 1] == '*') {
                comment = true;
                i++;
            } else if (comment && i + 1 < line.size() && line[i] == '*' && line[i + 1] == '/') {
                comment = false;
                i++;
            } else if (!comment) {
                curr += line[i];
            }
        }
        
        if (!comment && !curr.empty()) {
            output.push_back(curr);
            curr = "";
        }
    }
    
    return output;
}
int main() {
    vector<string> code = {
        "int main() {",
        " int a;",
        "// This is a comment",
        "}"
    };
    vector<string> output = commentsremoval(code);
    for (const string& line : output) {
        cout << line << endl;
    }
}

Code explanation

The code can be explained below:

Line 1: The commentremoval function takes a vector of strings source representing the source code to remove comments from.
Line 2: We define output which stores the code without comments that we’ll return at the end.
Line 5: We define curr, which is a string accumulator to build the current line of code.
Lines 7–8: We iterate through each line of the code, and then, for each line, we iterate through each character.
Lines 9–20: The steps to remove comments in the code are:
- If we’re not inside a comment and encounter a double slash, we break out of the loop.
- If we’re not inside a comment and we find a forward slash followed by an asterisk, we set comment to true and increments i to skip the asterisk.
- If we’re inside a comment and we find an asterisk followed by a forward slash, we set comment to false and increments i to skip the forward slash character.
- If none of the above conditions are met, it appends the character to curr.
Lines 22–25: If we’re not in a comment and curr is not empty, we add curr to the output vector and reset curr for the next line.
Lines 31–43: This is the main function to test this code.

Note: You can change the inputs in the main function to test other inputs.

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