Question

Newbie to C++ learning by converting a java program to c++. The following code results in a segmentation fault (SIGSEGV) when executed.

//add web page reference to pages queue (STL)
void CrawlerQueue::addWebPage(WebPage & webpage) {
    pagesBuffer.push(webpage);
}

//remove and return web page reference from pages queue
WebPage & CrawlerQueue::getWebPage() {
    if (pagesBuffer.size() > 0) {
        WebPage & page = pagesBuffer.front();
        pagesBuffer.pop();
        return page;
    } else
        throw "Web pages queue is empty!";
}

//code that results in segmentation fault when called
void PageParser::extractLinks(){ 
    try {
        WebPage &  page =  crawlerqueue.getWebPage();
    }catch (const char * error) {
       return;
    }
}

The changes to the above code that fix the segmentation fault issue are highlighted(<====):

//return a const WebPage object instead of a WebPage reference
const WebPage CrawlerQueue::getWebPage() {          <====
    if (pagesBuffer.size() > 0) {
        WebPage page = pagesBuffer.front();         <==== 
        pagesBuffer.pop();
        return page;
    } else
        throw "Web pages queue is empty!";
}

//no segmentation fault thrown with modifications
void PageParser::extractLinks(){ 
    try {
        WebPage page =  crawlerqueue.getWebPage(); <====
    }catch (const char * error) {
       return;
    }
}

What gives? I’m still trying to understand references and pointers

Answer 1

pagesBuffer.pop();

This line invalidate your reference.

Remember that standard container works with values, not “references”, so when you add an object using a reference to it, in fact you add a copy of the object in the container.

Then using pop(), you destroy this object, making any reference or pointer pointing to it invalid.

Maybe you should store (shared) pointers instead of the objects.