Value initialization

This is the initialization performed when an object is constructed with an empty initializer.


T () (1)
new T () (2)
Class::Class(...) : member () { ... } (3)
T object {}; (4) (since C++11)
T {} (5) (since C++11)
new T {} (6) (since C++11)
Class::Class(...) : member {} { ... } (7) (since C++11)


Value initialization is performed in these situations:

1,5) when a nameless temporary object is created with the initializer consisting of an empty pair of parentheses or braces (since C++11);
2,6) when an object with dynamic storage duration is created by a new-expression with the initializer consisting of an empty pair of parentheses or braces (since C++11);
3,7) when a non-static data member or a base class is initialized using a member initializer with an empty pair of parentheses or braces (since C++11);
4) when a named object (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces. (since C++11)

In all cases, if the empty pair of braces {} is used and T is an aggregate type, aggregate-initialization is performed instead of value-initialization.

If T is a class type that has no default constructor but has a constructor taking std::initializer_list, list-initialization is performed.

(since C++11)

The effects of value initialization are:

1) if T is a class type with no default constructor or with a user-declared (until C++11)user-provided or deleted (since C++11) default constructor, the object is default-initialized;
2) if T is a class type with a default constructor that is not user-declared (until C++11)neither user-provided nor deleted (since C++11) (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;
3) if T is an array type, each element of the array is value-initialized;
4) otherwise, the object is zero-initialized.


The syntax T object(); does not initialize an object; it declares a function that takes no arguments and returns T. The way to value-initialize a named variable before C++11 was T object = T();, which value-initializes a temporary and then copy-initializes the object: most compilers optimize out the copy in this case.

References cannot be value-initialized.

As described in functional cast, the syntax T() (1) is prohibited for arrays, while T{} (5) is allowed.

All standard containers (std::vector, std::list, etc.) value-initialize their elements when constructed with a single size_type argument or when grown by a call to resize(), unless their allocator customizes the behavior of construct.

The standard specifies that zero-initialization is not performed when the class has a user-provided or deleted default constructor, which implies that whether said default constructor is selected by overload resolution is not considered. All known compilers performs additional zero-initialization if a non-deleted defaulted default constructor is selected.

struct A
    A() = default;
    template<class = void>
    A(int = 0) {} // A has a user-provided default constructor, which is not selected
    int x;
constexpr int test(A a)
    return a.x; // the behavior is undefined if a's value is indeterminate
constexpr int zero = test(A());
// ill-formed: the parameter is not zero-initialized according to the standard,
// which results in undefined behavior that makes the program ill-formed in contexts 
// where constant evaluation is required.
// However, such code is accepted by all known compilers.
void f()
    A a = A(); // not zero-initialized according to the standard
               // but implementations generate code for zero-initialization nonetheless


#include <cassert>
#include <iostream>
#include <string>
#include <vector>
struct T1
    int mem1;
    std::string mem2;
    virtual void foo() {} // make sure T1 is not an aggregate
}; // implicit default constructor
struct T2
    int mem1;
    std::string mem2;
    T2(const T2&) {} // user-provided copy constructor
};                   // no default constructor
struct T3
    int mem1;
    std::string mem2;
    T3() {} // user-provided default constructor
std::string s{}; // class => default-initialization, the value is ""
int main()
    int n{};                // scalar => zero-initialization, the value is 0
    assert(n == 0);
    double f = double();    // scalar => zero-initialization, the value is 0.0
    assert(f == 0.0);
    int* a = new int[10](); // array => value-initialization of each element
    assert(a[9] == 0);      //          the value of each element is 0
    T1 t1{};                // class with implicit default constructor =>
    assert(t1.mem1 == 0);   //     t1.mem1 is zero-initialized, the value is 0
    assert(t1.mem2 == "");  //     t1.mem2 is default-initialized, the value is ""
//  T2 t2{};                // error: class with no default constructor
    T3 t3{};                // class with user-provided default constructor =>
    std::cout << t3.mem1;   //     t3.mem1 is default-initialized to indeterminate value
    assert(t3.mem2 == "");  //     t3.mem2 is default-initialized, the value is ""
    std::vector<int> v(3);  // value-initialization of each element
    assert(v[2] == 0);      // the value of each element is 0
    std::cout << '\n';
    delete[] a;

Possible output:


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 178 C++98 there was no value-initialization; empty initializer invoked default-
initialization (though new T() also performs zero-initialization)
empty initializer invoke
CWG 543 C++98 value-initialization for a class object without any
user-provided constructors was equivalent to value-
initializing each subobject (which need not zero-
initialize a member with user-provided default constructor)
the entire object,
then calls the
default constructor
CWG 1301 C++11 value-initialization of unions with deleted
default constructors led to zero-initialization
they are
CWG 1368 C++98 any user-provided constructor caused
zero-initialization to be skipped
only a user-provided
default constructor
skips zero-initialization
CWG 1502 C++11 value-initializing a union without a user-provided
default constructor only zero-initialized the
object, despite default member initializers
performs default-
initialization after
CWG 1507 C++98 value-initialization for a class object without any
user-provided constructors did not check the validity
of the default constructor when the latter is trivial
the validity of trivial
default constructor
is checked

See also

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