Question

I’m trying to better understand the differences between pointers and references in C++. Coming from a Java background, I was expecting references in C++ to be similar; I expected a pointer minus the pointer arithmetic. However, I have been very disappointed and, at times, confused. After some reading I thought I understood references to be pointers without pointer arithmetic and that can never be set to NULL. To test what I’ve learned I decided to start coding. However, I came across this problem and I do not understand why my code does not compile.

Here is what I was trying:

  3 void test(biNode*& bn)
  4 {
  5    string& s("another test");
  6    bn = new biNode(s);
  7    printf("Just Checking: %s\n", bn->getObject().c_str());
  8 }
  9
 10 int main()
 11 {
 12    biNode* bn;
 13    test(bn);
 14    printf("Just Checking: %s\n", bn->getObject().c_str());
 15 }

And here is the my ‘biNode’ header:

  1 #include <string>
  2 #include <iostream>
  3
  4 using namespace std;
  5
  6 class biNode
  7 {
  8    public:
  9       biNode(string& s);
 10       string& getObject();
 11    private:
 12       string& obj;
 13 };

With corresponding definitions:

  1 biNode::biNode(string& s) : obj(s)
  2 {}
  3 string& biNode::getObject()
  4 {
  5    return this->obj;
  6 }

Attempting to compile this produces the following error:

./test.cpp: In function `void test(biNode*&)':
./test.cpp:5: error: invalid initialization of non-const reference of type 'std::string&' from a temporary of type 'const char*'

I do not understand how ‘string& s(“another test”);’ is not valid. Can anyone explain this please?

Thanks in advance!

Answer 1

You can’t initialize a non-const reference with anything but an existing object of the same type, which the reference will then alias. But your class biNode contains a reference member, too, and so you must only initialize a biNode instance with an object that exists for as least as long as the node instance itself!

Here’s an example that demonstrates how you might use the biNode in a sane way:

int main()
{
    std::string s("Hello");

    for (int i = 0; i != 10; ++i)
    {
        biNode bn(s);
        // more stuff
    }
}

A sensible version of your test function might look like this:

biNode test(std::string & s)
{
    return biNode(s);
}

int main()
{
    std::string s("World");
    auto bn = test(s);
}