operator delete, operator delete[]

void operator delete  ( void* ptr );

void operator delete[]( void* ptr );

void operator delete  ( void* ptr, std::align_val_t al );

void operator delete[]( void* ptr, std::align_val_t al );

void operator delete  ( void* ptr, std::size_t sz );

void operator delete[]( void* ptr, std::size_t sz );

void operator delete  ( void* ptr, std::size_t sz, std::align_val_t al );

void operator delete[]( void* ptr, std::size_t sz, std::align_val_t al );

void operator delete  ( void* ptr, const std::nothrow_t& tag );

void operator delete[]( void* ptr, const std::nothrow_t& tag );

void operator delete  ( void* ptr,
                        std::align_val_t al, const std::nothrow_t& tag );

void operator delete[]( void* ptr, 
                        std::align_val_t al, const std::nothrow_t& tag );

void operator delete  ( void* ptr, void* place );

void operator delete[]( void* ptr, void* place );

void operator delete  ( void* ptr, args... );

void operator delete[]( void* ptr, args... );

void T::operator delete  ( void* ptr );

void T::operator delete[]( void* ptr );

void T::operator delete  ( void* ptr, std::align_val_t al );

void T::operator delete[]( void* ptr, std::align_val_t al );

void T::operator delete  ( void* ptr, std::size_t sz );

void T::operator delete[]( void* ptr, std::size_t sz );

void T::operator delete  ( void* ptr, std::size_t sz, std::align_val_t al );

void T::operator delete[]( void* ptr, std::size_t sz, std::align_val_t al );

void T::operator delete  ( void* ptr, args... );

void T::operator delete[]( void* ptr, args... );

Deallocates storage previously allocated by a matching operator new. These deallocation functions are called by delete-expressions and by new-expressions to deallocate memory after destructing (or failing to construct) objects with dynamic storage duration. They may also be called using regular function call syntax.

In all cases, if ptr is a null pointer, the standard library deallocation functions do nothing. If the pointer passed to the standard library deallocation function was not obtained from the corresponding standard library allocation function, the behavior is undefined.

After the standard library deallocation function returns, all pointers referring to any part of the deallocated storage become invalid.

Parameters

Return value

(none).

Exceptions

Global replacements

The replaceable deallocation functions (1-10) are implicitly declared in each translation unit even if the <new> header is not included. These functions are replaceable: a user-provided non-member function with the same signature defined anywhere in the program, in any source file, replaces the corresponding implicit version for the entire program. Its declaration does not need to be visible.

The behavior is undefined if more than one replacement is provided in the program or if a replacement is defined with the inline specifier, the program is ill-formed if a replacement is defined in namespace other than global namespace, or if it is defined as a static non-member function at global scope.

#include <cstdio>
#include <cstdlib>
// replacement of a minimal set of functions:
void* operator new(std::size_t sz) {
    std::printf("global op new called, size = %zu\n",sz);
    return std::malloc(sz);
}
void operator delete(void* ptr) noexcept
{
    std::puts("global op delete called");
    std::free(ptr);
}
int main() {
     int* p1 = new int;
     delete p1;
 
     int* p2 = new int[10]; // guaranteed to call the replacement in C++11
     delete[] p2;
}

global op new called, size = 4
global op delete called
global op new called, size = 40
global op delete called

Overloads of operator delete and operator delete[] with additional user-defined parameters ("placement forms", (15,16)) may be declared at global scope as usual, and are called by the matching placement forms of new-expressions if a constructor of the object that is being allocated throws an exception.

The standard library placement forms of operator delete (13,14) cannot be replaced and can only be customized if the placement new-expression did not use the ::new syntax, by providing a class-specific placement delete (25,26) with matching signature: void T::operator delete(void*, void*) or void T::operator delete[](void*, void*).

Class-specific overloads

Deallocation functions (17-24) may be defined as static member functions of a class. These deallocation functions, if provided, are called by delete-expressions when deleting objects (17,19,21) and arrays (18,20,22) of this class, unless the delete expression used the form ::delete which bypasses class-scope lookup. The keyword static is optional for these function declarations: whether the keyword is used or not, the deallocation function is always a static member function.

The delete expression looks for appropriate deallocation function's name starting from the class scope (array form looks in the scope of the array element class) and proceeds to the global scope if no members are found as usual. Note, that as per name lookup rules, any deallocation functions declared in class scope hides all global deallocation functions.

If the static type of the object that is being deleted differs from its dynamic type (such as when deleting a polymorphic object through a pointer to base), and if the destructor in the static type is virtual, the single object form of delete begins lookup of the deallocation function's name starting from the point of definition of the final overrider of its virtual destructor. Regardless of which deallocation function would be executed at run time, the statically visible version of operator delete must be accessible in order to compile. In other cases, when deleting an array through a pointer to base, or when deleting through pointer to base with non-virtual destructor, the behavior is undefined.

If the single-argument overload (17,18) is not provided, but the size-aware overload taking std::size_t as the second parameter (21,22) is provided, the size-aware form is called for normal deallocation, and the C++ runtime passes the size of the object to be deallocated as the second argument. If both forms are defined, the size-unaware version is called.

#include <iostream>
// sized class-specific deallocation functions
struct X {
    static void operator delete(void* ptr, std::size_t sz)
    {
        std::cout << "custom delete for size " << sz << '\n';
        ::operator delete(ptr);
    }
    static void operator delete[](void* ptr, std::size_t sz)
    {
        std::cout << "custom delete for size " << sz << '\n';
        ::operator delete(ptr);
    }
};
int main() {
     X* p1 = new X;
     delete p1;
     X* p2 = new X[10];
     delete[] p2;
}

custom delete for size 1
custom delete for size 18

Overloads of operator delete and operator delete[] with additional user-defined parameters ("placement forms", (25,26)) may also be defined as class members. When the failed placement new expression looks for the corresponding placement delete function to call, it begins lookup at class scope before examining the global scope, and looks for the function with the signature matching the placement new:

#include <stdexcept>
#include <iostream>
struct X {
    X() { throw std::runtime_error(""); }
    // custom placement new
    static void* operator new(std::size_t sz, bool b) {
        std::cout << "custom placement new called, b = " << b << '\n';
        return ::operator new(sz);
    }
    // custom placement delete
    static void operator delete(void* ptr, bool b)
    {
        std::cout << "custom placement delete called, b = " << b << '\n';
        ::operator delete(ptr);
    }
};
int main() {
   try {
     X* p1 = new (true) X;
   } catch(const std::exception&) { }
}

custom placement new called, b = 1
custom placement delete called, b = 1

If class-level operator delete is a template function, it must have the return type of void, the first argument void*, and it must have two or more parameters. In other words, only placement forms can be templates. The specialization of the template operator delete is chosen with template argument deduction.

Notes

The call to the class-specific T::operator delete on a polymorphic class is the only case where a static member function is called through dynamic dispatch.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also