Java to C++ - method modifiers

9/26/2015

Like in Java, you can add modifiers to methods, such as specifying what they can throw or declaring them as const. These are very powerful and fit right into the type system.

Let's look at how to apply const properly to methods. Last time we showed two methods that retrieve elements from the Array:

// unchecked const access
T& operator[](int index) {
    return array_start[index];
}

// checked access - throws exception if out of bounds
T& at(int index) {
    if(index < 0 || index >= length)
        throw std::out_of_range("Index out of range");
    return array_start[index];
}

None of the methods on the Array class have a const flag so a const Array is useless! Looking at these methods, it makes sense to make a const operator[] method that will return a const reference rather than a normal reference. Let's implement that:

// unchecked const access
const T& operator[](int index) const {
    return array_start[index];
}

// checked const access
const T& at(int index) const {
    if(index < 0 || index >= length)
        throw std::out_of_range("Index out of range");
    return array_start[index];
}

Now we can read elements from our const Array without changing it! Each element returned will be a const reference so cannot be modified, ensuring the constness of the Array.

I'll showing template meta programming next time and how to make your classes work with the standard library functions.

Chris Gregory

Java to C++ - method modifiers