Item 16 - Use the same form in corresponding uses of new and delete

What's wrong with this picture?

std::string *stringArray = new std::string[100];
...
delete stringArray;

Here, the new is matched with a delete, Still, something is quite wrong. 99 of the 100 string objects pointer to by stringArray are unlikely to be properly destroyed, because their destructors will probably never be called.

Background process of new: When you employ a new expression (i.e., dynamic creation of an object via a use of new), two things happen.

• memory is allocated (via function named operator new)
• One or more constructors are called for that memory.

Background process of delete: When you employ a delete expression (i.e., use delete), two things happen

• one or more destructors are called for the memory.
• memory is deallocated (via function named operator delete)

Example:

std::string *stringPtr1 = new std::string;
std::string *stringPtr2 = new std::string[100];
...
delete stringPtr1;        // delete an object
delete[] stringPtr2;      // delete an array of objects

The rule is simple: if you use [] in a new expression, you must use [] in the corresponding delete expression. If you don't use [] in a new expression, don't use [] in the matching delete expression.

This rule is also noteworthy for the typedef-inclined, because it means that a typedef's author must document which form of delete should be employed when new is used to conjure up objects of the typedef type. For example, consider this typedef:

// a person's address has 4 lines, each of which is a string
typedef std::string AddressLines[4];

Because AddressLines is an array, this use of new,

// note that "new AddressLines" returns a string*, just like "new string[4]" would
std::string* pal = new AddressLines;

must be matched with the array form of delete

delete pal;                // undefined!
delete[] pal;              // fine

Things to Remember

• if you use[] in a new expression, you must use [] in the corresponding delete expression. If you don't use [] in a new expression, you must not use [] in the corresponding delete expression.