Declaring and Defining Constructors

Now you need to build a Stock constructor. Because a Stock object has three values to be provided from the outside world, you should give the constructor three arguments. (The fourth value, the total_val member, is calculated from shares and share_val, so you don’t have to provide it to the constructor.) Possibly, you may want to provide just the company member value and set the other values to zero; you can do this by using default arguments (see Chapter 8, “Adventures in Functions.”). Thus, the prototype would look like this:

// constructor prototype with some default arguments
Stock(const string & co, long n = 0, double pr = 0.0);

The first argument is a pointer to the string that is used to initialize the company string member. The n and pr arguments provide values for the shares and share_val members. Note that there is no return type. The prototype goes in the public section of the class declaration.

Next, here’s one possible definition for the constructor:

// constructor definition
Stock::Stock(const string & co, long n, double pr)
{
company = co;

    if (n < 0)
    {
        std::cerr << "Number of shares can't be negative; "
                   << company << " shares set to 0. ";
        shares = 0;
    }
    else
        shares = n;
    share_val = pr;
    set_tot();
}

This is the same code that the acquire() function used earlier in this chapter. The difference is that in this case, a program automatically invokes the constructor when it declares an object.

Member Names and Parameter Names

Often those new to constructors try to use the class member names as parameter names in the constructor, as in this example:

// NO!
Stock::Stock(const string & company, long shares, double share_val)
{
...
}

This is wrong. The constructor arguments don’t represent the class members; they represent values that are assigned to the class members. Thus, they must have distinct names, or you end up with confusing code like this:

shares = shares;

One common coding practice to help avoid such confusion is to use an m_ prefix to identify data member names:

class Stock
{
private:
    string m_company;
    long m_shares;
    ...

Another common practice is to use an underbar suffix for member names:

class Stock
{
private:
    string company_;
    long shares_;
    ...

With either convention, you then can use company and shares as the parameter names in the public interface.

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