Dev Notes

Software Development Resources by David Egan.

Arrow Operator in C++


C++
David Egan

In C++ . is the standard member access operator. It has higher precedence than the * dereference operator.

Accessing the member of an object through a pointer requires dereferencing to happen first, so the dereferencing operation must be wrapped in parentheses.

Example 1: Access Member Variable

int main()
{
    struct Resource {
        int id;
        const char * name;
    };
    struct Resource mainPerson{1, "Cornelius"};
    Resource *p = &mainPerson;

    // Dereference first, then access member variable:
    std::cout << "ID:\t" << (*p).id << std::endl;
    std::cout << "ID:\t" << (*p).name << std::endl;

    // Equivalent:
    std::cout << "ID:\t" << p->id << std::endl;
    std::cout << "Name:\t" << p->name << std::endl;
    return 0;
}

Example 2: Iterate over a Map

When you use iterators to loop through a container, you need to dereference the iterator to access each object. The arrow operator can provide a convenient shortcut for this:

int main()
{

    std::map<std::string, double> myMap{{"David", 99.9}, {"Elaine", 99.99}, {"Bill", 45}};
    std::map<std::string, double>::iterator myMapIt;

    for (myMapIt = myMap.begin(); myMapIt != myMap.end(); myMapIt++) {

        // Accessing the member requires dereferencing to happen first,
        // hence the placement of parentheses:
        std::cout << (*myMapIt).first << ": " << (*myMapIt).second << std::endl;

        // The arrow operator is shorthand for this, providing member access by an
        // indirect selector. This is equivalent to the line above:
        std::cout << myMapIt->first << ": " << myMapIt->second << std::endl;

    }
    return 0;
}
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