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

Concept​

When you use new, two things happen: memory is allocated and one or more constructors are called. When you use delete, two things happen: one or more destructors are called and memory is deallocated. The critical question is how many objects reside in the memory being deleted. If you used new[] (array form), you must use delete[]. If you used new (single object form), you must use delete. Using the wrong form results in undefined behavior because the memory layout for arrays includes the array size.

Code Example​

#include <string>

void correct_usage() {
    // Single object: new matched with delete
    std::string* stringPtr1 = new std::string;
    delete stringPtr1;

    // Array: new[] matched with delete[]
    std::string* stringPtr2 = new std::string[100];
    delete[] stringPtr2;
}

void incorrect_usage() {
    // WRONG: undefined behavior!
    std::string* stringPtr = new std::string[100];
    // delete stringPtr;   // Should be delete[]!
}

// Be especially careful with typedefs
typedef std::string AddressLines[4];  // array typedef

void typedef_example() {
    std::string* pal = new AddressLines;  // same as new std::string[4]
    // delete pal;    // WRONG! Undefined behavior!
    delete[] pal;    // Correct — it's an array
}

Full source code

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 mustn't use [] in the corresponding delete expression.
  • Be wary of typedefs for array types; prefer std::vector or std::array instead.
  • Item 13 — Using RAII to manage resources and avoid manual delete
  • Item 17 — Storing newed objects in smart pointers