Having trouble with c++ default constructor? - c++

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;