C ++入门练习1.20运行错误

Question

当我运行程序时，C ++ Primer 5th Edition一书中的练习1.20抛出错误。 我试过在3台不同的计算机上运行该程序，结果相同。 我一直在使用Visual Studio 2015作为我的IDE。 我怀疑Visual Studio是罪魁祸首，但是我对它的了解还不够多，我自己也无法搞清楚，而且搜索也没有运气。 谢谢！

错误消息如下：

调试断言失败！

程序：C：\\ Windows \\ system32 \\ MSVCP140D.dll文件：c：\\ program files（x86）\\ microsoft visual studio 14.0 \\ vc \\ include \\ xstring行：1175

表达式：无效的空指针

以下是我的源代码：

#include <iostream>
#include "Sales_item.h"

    int main()
    {
        Sales_item nonzero = 0;
        while (std::cin >> nonzero)
            std::cout << nonzero;
        return 0;
    }

头文件“ Sales_item.h”如下：

#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined 
#define SALESITEM_H

#include "Version_test.h" 

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
// these declarations are explained section 7.2.1, p. 270 
// and in chapter 14, pages 557, 558, 561
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool 
operator==(const Sales_item&, const Sales_item&);
public:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
#if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
    Sales_item() = default;
#else
    Sales_item(): units_sold(0), revenue(0.0) { }
#endif
    Sales_item(const std::string &book):
              bookNo(book), units_sold(0), revenue(0.0) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);

    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
// private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
#ifdef IN_CLASS_INITS
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
#else
    unsigned units_sold;  
    double revenue;       
#endif
};

// used in chapter 10
inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
{ return lhs.isbn() == rhs.isbn(); }

// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool 
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    return lhs.units_sold == rhs.units_sold &&
           lhs.revenue == rhs.revenue &&
           lhs.isbn() == rhs.isbn();
}

inline bool 
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

// assumes that both objects refer to the same ISBN
Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
{
    units_sold += rhs.units_sold; 
    revenue += rhs.revenue; 
    return *this;
}

// assumes that both objects refer to the same ISBN
Sales_item 
operator+(const Sales_item& lhs, const Sales_item& rhs) 
{
    Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
    ret += rhs;           // add in the contents of (|rhs|) 
    return ret;           // return (|ret|) by value
}

std::istream& 
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> s.units_sold >> price;
    // check that the inputs succeeded
    if (in)
        s.revenue = s.units_sold * price;
    else 
        s = Sales_item();  // input failed: reset object to default state
    return in;
}

std::ostream& 
operator<<(std::ostream& out, const Sales_item& s)
{
    out << s.isbn() << " " << s.units_sold << " "
        << s.revenue << " " << s.avg_price();
    return out;
}

inline bool operator<(const Sales_item &, const Sales_item &)
{
    return false;
}

double Sales_item::avg_price() const
{
    if (units_sold) 
        return revenue/units_sold; 
    else 
        return 0;
}
#endif

它引用了另一个名为“ Version_test.h”的头文件。

#ifndef VERSION_TEST_H
#define VERSION_TEST_H

/* As of the first printing of C++ Primer, 5th Edition (July 2012), 
 * the Microsoft Complier did not yet support a number of C++ 11 features.  
 *
 * The code we distribute contains both normal C++ code and 
 * workarounds for missing features.  We use a series of CPP variables to
 * determine whether a given features is implemented in a given release
 * of the MS compiler.  The base version we used to test the code in the book
 * is Compiler Version 17.00.50522.1 for x86.
 *
 * When new releases are available we will update this file which will
 * #define the features implmented in that release.
*/

#if _MSC_FULL_VER == 170050522 || _MSC_FULL_VER == 170050727 
// base version, future releases will #define those features as they are
// implemented by Microsoft

/* Code in this delivery use the following variables to control compilation

   Variable tests           C++ 11 Feature 
CONSTEXPR_VARS            constexpr variables
CONSTEXPR_FCNS            constexpr functions
CONSTEXPR_CTORS           constexpr constructors and other member functions
DEFAULT_FCNS              = default 
DELETED_FCNS              = delete  
FUNC_CPP                  __func__ local static
FUNCTION_PTRMEM           function template with pointer to member function
IN_CLASS_INITS            in class initializers 
INITIALIZER_LIST          library initializer_list<T> template
LIST_INIT                 list initialization of ordinary variables
LROUND                    lround function in cmath
NOEXCEPT                  noexcept specifier and noexcept operator
SIZEOF_MEMBER             sizeof class_name::member_name
TEMPLATE_FCN_DEFAULT_ARGS default template arguments for function templates
TYPE_ALIAS_DECLS          type alias declarations
UNION_CLASS_MEMS          unions members that have constructors or copy control
VARIADICS                 variadic templates
*/
#endif  // ends compiler version check

#ifndef LROUND
inline long lround(double d)
{
    return (d >= 0) ?  long(d + 0.5) : long(d - 0.5);
}
#endif

#endif  // ends header guard

Answer 1

您的问题是由以下行引起的：

Sales_item nonzero = 0;

由于这个构造函数：

Sales_item(const std::string &book)

之所以使用它，是因为0可以表示一个空char* ，并且std::string可以从char*构造。

发生您的问题是因为从空指针构造std::string是无效的。

该行应该是

Sales_item nonzero;

（顺便说一句，这些工具有过错的可能性非常少。最可能的罪魁祸首总是您自己编写的代码。）