operators (new[])

void* operator new  ( std::size_t count );

void* operator new[]( std::size_t count );

void* operator new  ( std::size_t count, const std::nothrow_t& tag);

void* operator new[]( std::size_t count, const std::nothrow_t& tag);

void* operator new  ( std::size_t count, void* ptr );

void* operator new[]( std::size_t count, void* ptr );

void* operator new  ( std::size_t count, user-defined-args... );

void* operator new[]( std::size_t count, user-defined-args... );

void* T::operator new  ( std::size_t count );

void* T::operator new[]( std::size_t count );

void* T::operator new  ( std::size_t count, user-defined-args... );

void* T::operator new[]( std::size_t count, user-defined-args... );

Allocates requested number of bytes. These allocation functions are called by new-expressions to allocate memory in which new object would then be initialized. They may also be called using regular function call syntax.

Parameters

Return value

non-null pointer to suitably aligned memory of size at least size.

Exceptions

Global replacements

The versions (1-4) 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 implicit version. Its declaration does not need to be visible.

The behavior is undefined if more than one replacement is provided in the program for any of the four implicit allocation function, 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 new and operator new[] with additional user-defined parameters ("placement forms", version 7-8) may be declared at global scope as usual, and are called by the matching placement forms of new-expressions.

The standard library placement forms of operator new (5-6) 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 new (11-12) with matching signature: void* T::operator new(size_t, void*) or void* T::operator new[](size_t, void*).

Class-specific overloads

Both single-object and array allocation functions may be defined as public static member functions of a class (versions 9-10). If defined, these allocation functions are called by new-expressions to allocate memory for single objects and arrays of this class, unless the new expression used the form ::new which bypasses class-scope lookup. The keyword static is optional for these functions: whether used or not, the allocation function is a static member function.

The new expression looks for appropriate allocation function's name firstly in the class scope, and after that in the global scope. Note, that as per name lookup rules, any allocation functions declared in class scope hides all global allocation functions for the new-expressions that attempt to allocate objects of this class.

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

custom new for size 1
custom new for size 10

Overloads of operator new and operator new[] with additional user-defined parameters ("placement forms", versions 11 and 12) may also be defined as class members. When the placement new expression with the matching signature looks for the corresponding allocation function to call, it begins at class scope before examining the global scope, and if the class-specific placement new is provided, it is called.

#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 new is a template function, it must have the return type of void*, the first argument size_t, and it must have two or more parameters. In other words, only placement forms can be templates.

Notes

Even though placement new (overloads 5 and 6) cannot be replaced, a function with the same signature may be defined at class scope as described above. In addition, global overloads that look like placement new but take a non-void pointer type as the second argument are allowed, so the code that wants to ensure that the true placement new is called (e.g. std::allocator::construct), must use ::new and also cast the pointer to void*.

Example

#include <iostream>
#include <algorithm>
 
void* operator new[](std::size_t sz, char c)
{
    void* p = operator new[](sz);
    std::fill_n(reinterpret_cast<char*>(p), sz, c);
    return p;
}
 
int main()
{
    char* p = new('*') char[6];
    p[5] = '\0';
    std::cout << p << '\n';
    delete[] p;
}

*****

See also

References

• C++11 standard (ISO/IEC 14882:2011):
  • 18.6 Dynamic memory management [support.dynamic]