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Declaring Parameters as Constant
The function display() shows the contents of the hero’s inventory. In the function’s header I specify one parameter—a constant reference to a vector of string objects named vec.
void display(const vector<string>& vec)
A constant reference is a restricted reference. It acts like any other reference, except you can’t use it to change the value to which it refers. To create a constant reference, simply put the keyword const before the type in the reference declaration.
What does this all mean for the function display()? Because the parameter vec is a constant reference, it means display() can’t change vec. In turn, this means that inventory is safe; it can’t be changed by display(). In general, you can efficiently pass an argument to a function as a constant reference so it’s accessible, but not changeable. It’s like providing the function read-only access to the argument. Although constant references are very useful for specifying function parameters, you can use them anywhere in your program.
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A constant reference comes in handy in another way. If you need to assign a constant value to a reference, you have to assign it to a constant reference. (A non-constant reference won’t do.)
