Missing example of a Member Initializer List on p. 184 of Programming Principles and Practice Using C++, 2nd ed

问题

I'm currently having trouble with part of Chapter 6 of "Programming: Principles and Practice Using C++" (2nd ed, 3rd printing).

According to the book's index, an example of a member initializer list is on page 184.

Part of page 184 reads as follows:

"Here, we'll just provide two member functions to give us a more convenient way of initializing Tokens:

 class Token {
    public: 
    char kind; // what kind of token
    double value; // for numbers: a value
    };

We can now initialize ("construct") Token objects. For example:

Token t1 {'+'}; // initialize t1 so that t1.kind = '+'
Token t2 {'8,' 11.5}; // initialize t2 so that t2.kind = '8' and t2.value = 11.5

Is one of these sets of code an example of a member initializer list as the index would indicate? I'm somewhat confused because, based on another Stackoverflow answer (What is this weird colon-member (" : ") syntax in the constructor? ), I thought a member initializer list would look something more like:

Name_value(string n, int v)
                :name(n), value(v) { }

(found within the following code by Chrinkus on Github: https://github.com/Chrinkus/stroustrup-ppp/blob/master/chapter06/ex04_Name_value.cpp )

class Name_value {
    public:
        string name;
        int value;
        Name_value(string n, int v)
            :name(n), value(v) { }
};

I think it's far more likely that I'm mistaken than that there is a typo on the book, but nonetheless, I'm finding this part of the book confusing. I appreciate any feedback you may be able to offer.

Update: I find it interesting that the drill for chapter 6 includes this code:

Token(char ch) // make a Token from a char
        : kind(ch), value(0)
    {}
    Token(char ch, double val) // make a Token from a char and a double
        : kind(ch), value(val)
    {}

Those are member initializers, are they not? I wonder if the author meant to refer to those two functions when he wrote "Here, we'll just provide two member functions to give us a more convenient way of initializing Tokens:"

回答1:

I reached out to Bjarne by email, and he promptly informed me that this was a bug that would be fixed in the next printing. So my guess is that that section of page 184 was meant to read more or less as follows:

"Here, we'll just provide two member functions to give us a more convenient way of initializing Tokens:

 class Token {
    public: 
    char kind; // what kind of token
    double value; // for numbers: a value
    Token(char ch)  // make a Token from a char
        :kind(ch), value(0) { }    
    Token(char ch, double val) // make a Token from a char and a double
        :kind(ch), value(val) { }

};

We can now initialize ("construct") Token objects . . ."

The two member initializer lists in the above code were taken from the sample code for the chapter 6 drill, which is available here: http://www.stroustrup.com/Programming/calculator02buggy.cpp (Please note that this code contains some intentional bugs that the reader is meant to solve within the drill exercise.)

I hope this will be useful to anyone else who was confused about member initializer lists. The book has been very useful for me so far and I plan to continue to read through it.

来源：https://stackoverflow.com/questions/61150964/missing-example-of-a-member-initializer-list-on-p-184-of-programming-principles

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