• If there are multiple constructors, all should use new the same way—either all with brackets or all without brackets. There’s only one destructor, so all constructors have to be compatible with that destructor. However, it is permissible to initialize a pointer with new in one constructor and with the null pointer (0, or, with C++11, nullptr) in another constructor because it’s okay to apply the delete operation (with or without brackets) to the null pointer.
NULL or 0 or nullptr?
Historically, the null pointer can be represented by 0 or by NULL, a symbolic constant defined as 0 in several header files. C programmers often use NULL instead of 0 as a visual reminder that the value is a pointer value, just as they use '\0' instead of 0 for the null character as a visual reminder that this value is a character. The C++ tradition, however, has favored a simple 0 instead of the equivalent NULL. And, as mentioned earlier, C++11 offers the nullptr keyword as a better alternative.
• You should define a copy constructor that initializes one object to another by doing deep copying. Typically, the constructor should emulate the following example:
String::String(const String & st)
{
num_strings++; // handle static member update if necessary
len = st.len; // same length as copied string
str = new char [len + 1]; // allot space
std::strcpy(str, st.str); // copy string to new location
}
In particular, the copy constructor should allocate space to hold the copied data, and it should copy the data, not just the address of the data. Also it should update any static class members whose value would be affected by the process.
• You should define an assignment operator that copies one object to another by doing deep copying. Typically, the class method should emulate the following example:
String & String::operator=(const String & st)
{
if (this == &st) // object assigned to itself
return *this; // all done
delete [] str; // free old string
len = st.len;
str = new char [len + 1]; // get space for new string
std::strcpy(str, st.str); // copy the string
return *this; // return reference to invoking object
}
In particular, the method should check for self-assignment; it should free memory formerly pointed to by the member pointer; it should copy the data, not just the address of the data; and it should return a reference to the invoking object.
Don’ts and Dos
The following excerpt contains two examples of what not to do and one example of a good constructor:
String::String()
{
str = "default string"; // oops, no new []
len = std::strlen(str);
}
String::String(const char * s)
{
len = std::strlen(s);
str = new char; // oops, no []
std::strcpy(str, s); // oops, no room
}
String::String(const String & st)
{
len = st.len;
str = new char[len + 1]; // good, allocate space
std::strcpy(str, st.str); // good, copy value
}
The first constructor fails to use new to initialize str. The destructor, when called for a default object, applies delete to str. The result of applying delete to a pointer not initialized by new is undefined, but it is probably bad. Any of the following would be okay:
String::String()
{
len = 0;
str = new char[1]; // uses new with []
str[0] = '\0';
}
String::String()
{
len = 0;
str = 0; // or, with C++11, str = nullptr;
}
String::String()
{
static const char * s = "C++"; // initialized just once
len = std::strlen(s);
str = new char[len + 1]; // uses new with []
std::strcpy(str, s);
}
Next, the second constructor in the original excerpt applies new, but it fails to request the correct amount of memory; hence, new returns a block containing space for just one character. Attempting to copy a longer string to that location is asking for memory problems. Also the use of new without brackets is inconsistent with the correct form of the other constructors.
The third constructor is fine.
Finally, here’s a destructor that
String::~String()
{
delete str; // oops, should be delete [] str;
}
The destructor uses delete incorrectly. Because the constructors request arrays of characters, the destructor should delete an array.
Memberwise Copying for Classes with Class Members
Suppose you use the String class, or, for that matter, the standard string class as a type for class members:
class Magazine
{
private:
String title;
string publisher;
...
};
