有c ++默认构造函数的问题？

Question

I already have a constructor in my .h and .cpp file which takes some arguments but I also need a default argument which I'm not sure how to make because the way I tried it compiles but I get errors when I run my testfile. This is my .h file

public:
Class();
Class(const std::string &, const int)
void getInfo();
std::string listItem();

private:

std::string name;
int quantity;

This is my .cpp file

Class::Class()
: name(0), quantity(0)
{
Class::Class(const string &nam, const int quant)
: name(nam),quantity(quant)
{
}
void Class::getInfo()
{
cout << "Enter Name: ";
cin >> name
cout << "Enter quantity: ";
cin >> quantity;
}
string Class::listItem()
{
ostringstream outputString;
outputString << getName() << getQuantity();
return outputString.str();
}

And this is the part of my test causing trouble:

const int shortList = 2;
array<Class*, shortList> newList;

 for (int i=0; i< 2; i++){
        Class *p = new Class();
        p->getInfo();
        newList[i] = p;
 }
 cout << "newList contains: " << endl;
 for (Class* p : newList)
            cout << p->listItem() << endl;

I get : terminate called after throwing an instance of 'std::logic_error' what(): basic_string::_S_construct null not valid

Is it a constructor issue or is it some syntax error?

Answer 1

The problem is in the default constructor's initialiser list:

name(0)

This attempts to initialise the string using the constructor taking a C-style string pointer, char const* , with a null pointer value. You then get a runtime error since you're not allowed to pass a null pointer to that constructor.

To initialise the string to be empty, either specify default initialisation (or, pedantically, value-initialisation, which amounts to the same thing for this type)

name()

or leave it out of the initialiser list.

Answer 2

Assuming there are no intentional typos in the above code, there are semi-colons missing in very key locations, as well as curly brace use looking incorrect and missing declarations in the header.

The new lines and/or characters are noted with a comment starting with // added ...

Starting with the header file:

class Class        // added
{                  // added
public:
    Class();
    Class(const std::string &, const int); // added semi-colon
    void getInfo();
    std::string listItem();

private:
    std::string name;
    int quantity;
};   // added closing curly brace and semi-colon

The .cpp source file:

Class::Class()
: name(""), quantity(0) // modified name initial value to the empty string
{
}  // added curly brace

Class::Class(const string &nam, const int quant)
: name(nam),quantity(quant)
{
}

void Class::getInfo()
{
    cout << "Enter Name: ";
    cin >> name;    // added semi-colon
    cout << "Enter quantity: ";
    cin >> quantity;
}
string Class::listItem()
{
    ostringstream outputString;
    outputString << getName() << getQuantity();
    return outputString.str();
}

Later on the code that is causing fits is:

const int shortList = 2;
array<Class*, shortList> newList;

for (int i=0; i< shortList; i++){ // changed bounds check for i to the const int shortList
     Class *p = new Class();
     p->getInfo();
     newList[i] = p;
}
cout << "newList contains: " << endl;

//
// changed to declare auto instead.  
// As a pointer declaration, there is a chance the Class copy constructor is being called
// inside the loop body prior to the dereference.  It should not be, but... 
// In my opinion, it is much more likely that setting the name to a zero initial value
// in the Class() constructor is the real problem cause as Mike says above.
//
for (auto p : newList)
     cout << p->listItem() << endl;