C ++默认构造函数不可用

Question

I'm currently learning C++ and reading through "C++ Primer 5th Edition". I just started learning about constructors and I'm having a bit of a problem that I can't figure out.

#ifndef SALES_DATA_H
#define SALES_DATA_H
#include <string>
struct Sales_data
{
    //default constructor
    Sales_data(const std::string &s, unsigned n, double p):
               bookNo(s), units_sold(n), revenue(p*n) { } 
    //new members: operations on Sales_data objects
    std::string isbn() const { return bookNo; }
    Sales_data& combine(const Sales_data&);
    double avg_price() const;
    //data members
    std::string bookNo;
    unsigned units_sold;
    double revenue;
};

I'm pretty sure that the default constructor I wrote is correct (considering it's the one written in the book), but obviously I'm missing something here. I don't see any syntax errors or anything and all of the built-in members are being initialized so I have no idea what's wrong.

EDIT :

I just found out that it's not my header file giving the error, it's actually my source file. When I create a Sales_data object like:

Sales_data total;

it gives me the "No appropriate default constructor available" error. I'm still unsure as to what's wrong considering the author gave three ways to write a default constructor, these are them:

struct Sales_data {
// constructors added
Sales_data() = default;   //Number 1
Sales_data(const std::string &s): bookNo(s) { }    //Number 2
Sales_data(const std::string &s, unsigned n, double p):    //Number 3
           bookNo(s), units_sold(n), revenue(p*n) { }

If those aren't default constructors, then what exactly are they/is their purpose?

Answer 1

A default constructor is a constructor that can be called without passing any argument.

This means that it must take no parameters or that all of them must have a default value.

The default constructor is needed for example when you write

MyClass x;

because the compiler must be able to generate code to build such an object without any arguments.

Also standard containers may require a default constructor depending on how you use them: for example if you use std::vector::resize the library can be asked to increase the size of a vector containing your class instances, thus it must be able to create elements without providing any argument.

Answer 2

The problem isn't related to default constructors. The problem indeed is in Sales_data total; . What's the book number, sales price and number sold for total ? You must provide them when constructing total .

Answer 3

The constructor of your class takes three parameters, those must be provided when you try to construct an object. Your current variable declaration doesn't provide any parameters:

Sales_data total;

Because there are no parameters provided, the compiler tries to use a constructor that doesn't take any parameters. Such a constructor is also called "default constructor". Your class doesn't have a constructor without parameters, so this doesn't work.

To use the existing constructor, you have to provide the parameters when you create the object:

Sales_data total("books", 28, 15.99);

Alternatively you could add a constructor to Sales_data that doesn't take any parameters (a default constructor), and initializes the class with some default values.