C++ Linux basic_string::_M_construct null not valid error during runtime

Question

This is C++ on Linux. I'm getting the error during runtime, and I've narrowed down the code to be here, which is a constructor of a custom object. What I do is create a new thread and pass a function to it. In this function, I call the constructor like this:

ColorNinja cn(gameData->difficulty);

gameData is a struct that's also passed to the thread, where difficulty is an int member variable.

I don't fully understand the error or exactly what's causing it. Does anyone have any insight?

Here is the constructor. I can provide more code if necessary.

ColorNinja::ColorNinja(int difficulty) {
    // create the engine that will generate random numbers
    random_device rand;
    mt19937 engine(rand());

    int randomNumber = 0;

    // possible colors that can be present on the randomly-generated game board
    vector<string> possibleColors = {"red", "blue", "yellow", "green"};

    uniform_int_distribution<int> distribution2(0, possibleColors.size());

    // build the game board by choosing and inserting random colors
    for (int i = 0; i < 4; i++) {
        randomNumber = distribution2(engine);
        gameBoard.push_back(possibleColors[randomNumber]);
    }

    // print the game board
    cout << "gameBoard.size(): " << gameBoard.size() << endl;
    for (string s : gameBoard) {
        cout << s << endl;
    }
}

Remy Lebeau · Accepted Answer · 2019-12-03 23:03:05Z

1

You need to use possibleColors.size()-1 when initializing distribution2:

std::uniform_int_distribution<int> distribution2(0, possibleColors.size()-1);

The two constructor parameters for std::uniform_int_distribution are the minimum and maximum values to be generated. By using possibleColors.size() as the maximum value, you allow the generator to return an index that may be out of bounds of the array. Had you used possibleColors.at(randomNumber), you would have gotten a std::out_of_range error thrown if that actually happened. Using possibleColors[randomNumber] does not perform any bounds checking, so it will happily take an invalid index, and then your code will have undefined behavior. So it is important that you initialize your generator to produce only valid indexes.

On a side note: since possibleColors is a fixed array, consider using std::array instead of std::vector for it:

#include <array>
std::array<string, 4> possibleColors{"red", "blue", "yellow", "green"};

edited Dec 3, 2019 at 23:03

answered Dec 3, 2019 at 22:51

Remy Lebeau

609k36 gold badges516 silver badges875 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Tyler M. Over a year ago

Perfect, that was it! Thank you so much!

Collectives™ on Stack Overflow

C++ Linux basic_string::_M_construct null not valid error during runtime

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related