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delete expression

From cppreference.com
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Destructs object(s) previously allocated by the new expression and releases obtained memory area

Contents

[edit] Syntax

::(optional)    delete    expression (1)
::(optional)    delete [] expression (2)
1) Destroys one non-array object created by a new-expression
2) Destroys an array created by a new[]-expression

[edit] Explanation

For the first (non-array) form, expression must be a pointer to a complete object type or a class type contextually implicitly convertible to such pointer, and its value must be either null or pointer to a non-array object created by a new-expression, or a pointer to a base subobject of a non-array object created by a new-expression. If expression is anything else, including if it is a pointer obtained by the array form of new-expression, the behavior is undefined.

For the second (array) form, expression must be a null pointer value or a pointer value previously obtained by an array form of new-expression. If expression is anything else, including if it's a pointer obtained by the non-array form of new-expression, the behavior is undefined.

If expression is not a null pointer, the delete expression invokes the destructor (if any) for the object that's being destroyed, or for every element of the array being destroyed (proceeding from the last element to the first element of the array).

After that, unless the matching new-expression was combined with another new-expression (since C++14) the delete expression invokes the deallocation function, either operator delete (for the first version of the expression) or operator delete[] (for the second version of the expression).

The deallocation function's name is looked up in the scope of the dynamic type of the object pointed to by expression, which means class-specific deallocation functions, if present, are found before the global ones. If :: is present in the delete expression, only the global namespace is examined by this lookup.

If lookup finds more than one deallocation function, the function to be called is selected as follows (see deallocation function for a more detailed description of these functions and their effects):

  • If the type's alignment requirement exceeds __STDCPP_DEFAULT_NEW_ALIGNMENT__, alignment-aware deallocation functions (with a parameter of type std::align_val_t) are preferred. For other types, the alignment-unaware deallocation functions (without a parameter of type std::align_val_t) are preferred.
  • If exactly one preferred function is found, that function is selected.
  • If more than one preferred functions are found, only preferred functions are considered in the next step.
  • If no preferred functions are found, the non-preferred ones are considered in the next step.
(since C++17)
  • If the deallocation functions that were found are class-specific, size-unaware class-specific deallocation function (without a parameter of type std::size_t) is preferred over size-aware class-specific deallocation function (with a parameter of type std::size_t)
  • Otherwise, lookup reached global scope, and:
  • If the type is complete and if, for the delete[] only, the operand is a pointer to a class type with a non-trivial destructor or a (possibly multi-dimensional) array thereof, the global size-aware global function (with a parameter of type std::size_t) is selected
  • Otherwise, it is unspecified whether the global size-aware deallocation function (with a parameter of type std::size_t) or the global size-unaware deallocation function (without a parameter of type std::size_t) is selected
(since C++14)

The pointer to the block of storage to be reclaimed is passed to the deallocation function that was selected by the process above as the first argument. The size of the block is passed as the optional std::size_t argument. The alignment requirement is passed as the optional std::align_val_t argument. (since C++17)

If expression evaluates to a null pointer value, no destructors are called, and the deallocation function may or may not be called (it's implementation-defined), but the default deallocation functions are guaranteed to do nothing when handed a null pointer.

(until C++14)

If expression evaluates to a null pointer value, no destructors are called, and the deallocation function is not called.

(since C++14)

If expression evaluates to a pointer to the base class subobject of the object that was allocated with new, the destructor of the base class must be virtual, otherwise the behavior is undefined.

[edit] Notes

A pointer to void cannot be deleted because it is not a pointer to a complete object type.

Because a pair of brackets following the keyword delete is always interpreted as the array form of delete, a lambda-expression with empty capture list immediately after delete must be enclosed in parentheses.

// delete []{return new int; }(); // parse error
delete ([]{return new int; })(); // OK
(since C++11)

[edit] Keywords

delete