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C++ programs create, destroy, refer to, access, and manipulate objects.

An object, in C++, has

The following entities are not objects: value, reference, function, enumerator, type, non-static class member, template, class or function template specialization, namespace, parameter pack, and this.

A variable is an object or a reference that is not a non-static data member, that is introduced by a declaration.

Contents

[edit] Object creation

Objects can be explicitly created by definitions, new-expressions, throw-expressions, changing the active member of a union and evaluating expressions that require temporary objects. The created object is uniquely defined in explicit object creation.

Objects of implicit-lifetime types can also be implicitly created by

  • operations that begin lifetime of an array of type char, unsigned char, or std::byte, (since C++17) in which case such objects are created in the array,
  • call to following allocating functions, in which case such objects are created in the allocated storage:
(since C++17)
  • call to following object representation copying functions, in which case such objects are created in the destination region of storage or the result:
(since C++20)

Zero or more objects may be created in the same region of storage, as long as doing so would give the program defined behavior. If such creation is impossible, e.g. due to conflicting operations, the behavior of the program is undefined. If multiple such sets of implicitly created objects would give the program defined behavior, it is unspecified which such set of objects is created. In other words, implicitly created objects are not required to be uniquely defined.

After implicitly creating objects within a specified region of storage, some operations produce a pointer to a suitable created object. The suitable created object has the same address as the region of storage. Likewise, the behavior is undefined if only if no such pointer value can give the program defined behavior, and it is unspecified which pointer value is produced if there are multiple values giving the program defined behavior.

#include <cstdlib>
 
struct X { int a, b; };
 
X* MakeX()
{
    // One of possible defined behaviors:
    // the call to std::malloc implicitly creates an object of type X
    // and its subobjects a and b, and returns a pointer to that X object
    X* p = static_cast<X*>(std::malloc(sizeof(X)));
    p->a = 1;
    p->b = 2;
    return p;
}

Call to std::allocator::allocate or implicitly defined copy/move special member functions of union types can also create objects.

[edit] Object representation and value representation

For an object of type T:

  • its object representation is the sequence of sizeof(T) objects of type unsigned char (or, equivalently, std::byte) (since C++17) beginning at the same address as the T object,
  • its value representation is the set of bits that hold the value of its type T, and
  • its padding bits are the bits in the object representation that are not part of the value representation.

For trivially copyable types, value representation is a part of the object representation, which means that copying the bytes occupied by the object in the storage is sufficient to produce another object with the same value (except if the value is a trap representation of its type and loading it into the CPU raises a hardware exception, such as SNaN ("signalling not-a-number") floating-point values or NaT ("not-a-thing") integers).

The reverse is not necessarily true: two objects of a trivially copyable type with different object representations may represent the same value. For example, multiple floating-point bit patterns represent the same special value NaN. More commonly, padding bits may be introduced to satisfy alignment requirements, bit-field sizes, etc.

#include <cassert>
 
struct S
{
    char c;  // 1 byte value
             // 3 bytes of padding bits (assuming alignof(float) == 4)
    float f; // 4 bytes value (assuming sizeof(float) == 4)
 
    bool operator==(const S& arg) const // value-based equality
    {
        return c == arg.c && f == arg.f;
    }
};
 
void f()
{
    assert(sizeof(S) == 8);
    S s1 = {'a', 3.14};
    S s2 = s1;
    reinterpret_cast<unsigned char*>(&s1)[2] = 'b'; // modify some padding bits
    assert(s1 == s2); // value did not change
}

For the objects of type char, signed char, and unsigned char (unless they are oversize bit-fields), every bit of the object representation is required to participate in the value representation and each possible bit pattern represents a distinct value (no padding bits, trap bits, or multiple representations allowed).

[edit] Subobjects

An object can have subobjects. These include

  • member objects
  • base class subobjects
  • array elements

An object that is not a subobject of another object is called complete object.

A subobject is potentially overlapping if it is a base class subobject or a non-static data member declared with the [[no_unique_address]] attribute (since C++20).

Complete objects, member objects, and array elements are also known as most derived objects, to distinguish them from base class subobjects. The size of an object that is neither potentially overlapping nor a bit-field is required to be non-zero (the size of a base class subobject may be zero even without [[no_unique_address]] (since C++20): see empty base optimization).

An object can contain other objects, in which case the contained objects are nested within the former object. An object a is nested within another object b if

  • a is a subobject of b, or
  • b provides storage for a, or
  • there exists an object c where a is nested within c, and c is nested within b.

Any two objects with overlapping lifetimes (that are not bit-fields) are guaranteed to have different addresses unless one of them is nested within another, or if they are subobjects of different type within the same complete object, and one of them is a subobject of zero size.

static const char c1 = 'x';
static const char c2 = 'x';
assert(&c1 != &c2); // same values, different addresses

[edit] Polymorphic objects

Objects of a class type that declares or inherits at least one virtual function are polymorphic objects. Within each polymorphic object, the implementation stores additional information (in every existing implementation, it is one pointer unless optimized out), which is used by virtual function calls and by the RTTI features (dynamic_cast and typeid) to determine, at run time, the type with which the object was created, regardless of the expression it is used in.

For non-polymorphic objects, the interpretation of the value is determined from the expression in which the object is used, and is decided at compile time.

#include <iostream>
#include <typeinfo>
 
struct Base1
{
    // polymorphic type: declares a virtual member
    virtual ~Base1() {}
};
 
struct Derived1 : Base1
{
     // polymorphic type: inherits a virtual member
};
 
struct Base2
{
     // non-polymorphic type
};
 
struct Derived2 : Base2
{
     // non-polymorphic type
};
 
int main()
{
    Derived1 obj1; // object1 created with type Derived1
    Derived2 obj2; // object2 created with type Derived2
 
    Base1& b1 = obj1; // b1 refers to the object obj1
    Base2& b2 = obj2; // b2 refers to the object obj2
 
    std::cout << "Expression type of b1: " << typeid(decltype(b1)).name() << '\n'
              << "Expression type of b2: " << typeid(decltype(b2)).name() << '\n'
              << "Object type of b1: " << typeid(b1).name() << '\n'
              << "Object type of b2: " << typeid(b2).name() << '\n'
              << "Size of b1: " << sizeof b1 << '\n'
              << "Size of b2: " << sizeof b2 << '\n';
}

Possible output:

Expression type of b1: Base1
Expression type of b2: Base2
Object type of b1: Derived1
Object type of b2: Base2
Size of b1: 8
Size of b2: 1

[edit] Strict aliasing

Accessing an object using an expression of a type other than the type with which it was created is undefined behavior in many cases, see reinterpret_cast for the list of exceptions and examples.

[edit] Alignment

Every object type has the property called alignment requirement, which is an integer value (of type std::size_t, always a power of 2) representing the number of bytes between successive addresses at which objects of this type can be allocated.

The alignment requirement of a type can be queried with alignof or std::alignment_of. The pointer alignment function std::align can be used to obtain a suitably-aligned pointer within some buffer, and std::aligned_storage can be used to obtain suitably-aligned storage.

(since C++11)

Each object type imposes its alignment requirement on every object of that type; stricter alignment (with larger alignment requirement) can be requested using alignas (since C++11).

In order to satisfy alignment requirements of all non-static members of a class, padding bits may be inserted after some of its members.

#include <iostream>
 
// objects of type S can be allocated at any address
// because both S.a and S.b can be allocated at any address
struct S
{
    char a; // size: 1, alignment: 1
    char b; // size: 1, alignment: 1
}; // size: 2, alignment: 1
 
// objects of type X must be allocated at 4-byte boundaries
// because X.n must be allocated at 4-byte boundaries
// because int's alignment requirement is (usually) 4
struct X
{
    int n;  // size: 4, alignment: 4
    char c; // size: 1, alignment: 1
    // three bytes of padding bits
}; // size: 8, alignment: 4 
 
int main()
{
    std::cout << "sizeof(S) = " << sizeof(S)
              << " alignof(S) = " << alignof(S) << '\n';
    std::cout << "sizeof(X) = " << sizeof(X)
              << " alignof(X) = " << alignof(X) << '\n';
}

Possible output:

sizeof(S) = 2 alignof(S) = 1
sizeof(X) = 8 alignof(X) = 4

The weakest alignment (the smallest alignment requirement) is the alignment of char, signed char, and unsigned char, which equals 1; the largest fundamental alignment of any type is implementation-defined and equal to the alignment of std::max_align_t (since C++11).

If a type's alignment is made stricter (larger) than std::max_align_t using alignas, it is known as a type with extended alignment requirement. A type whose alignment is extended or a class type whose non-static data member has extended alignment is an over-aligned type.

Allocator types are required to handle over-aligned types correctly.

(since C++11)

It is implementation-defined if new-expressions and (until C++17) std::get_temporary_buffer support over-aligned types.

(since C++11)
(until C++20)

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 633 C++98 variables could only be objects they can also be references
CWG 734 C++98 it was unspecified whether variables defined
in the same scope that are guaranteed to have
the same value can have the same address
address is guauanteed to be
different if their lifetimes overlap,
regardless of their values
CWG 1189 C++98 two base class subobjects of the same
type could have the same address
they always have distinct addresses
CWG 1861 C++98 for oversize bit-fields of narrow character types, all bits of the
object representation still participated in the value representation
allowed padding bits
P0593R6 C++98 previous object model did not support many
useful idioms required by the standard library
and was not compatible with effective types in C
implicit object creation added

[edit] See also

C documentation for Object