Value initialization
This is the initialization performed when an object is constructed with an empty initializer.
Syntax
T ()
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(1) | ||||||||
new T ()
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(2) | ||||||||
Class::Class(...) : member () { ... }
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(3) | ||||||||
T object {};
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(4) | (since C++11) | |||||||
T {}
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(5) | (since C++11) | |||||||
new T {}
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(6) | (since C++11) | |||||||
Class::Class(...) : member {} { ... }
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(7) | (since C++11) | |||||||
Explanation
Value initialization is performed in these situations:
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4) when a named object (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces.
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(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.
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If |
(since C++11) |
The effects of value initialization are:
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;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;T is an array type, each element of the array is value-initialized;Notes
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 }
Example
#include <string> #include <vector> #include <iostream> struct T1 { int mem1; std::string mem2; }; // 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 double f = double(); // scalar => zero-initialization, the value is 0.0 int* a = new int[10](); // array => value-initialization of each element // the value of each element is 0 T1 t1{}; // class with implicit default constructor => // t1.mem1 is zero-initialized, the value is 0 // t1.mem2 is default-initialized, the value is "" // T2 t2{}; // error: class with no default constructor T3 t3{}; // class with user-provided default constructor => // t3.mem1 is default-initialized to indeterminate value // t3.mem2 is default-initialized, the value is "" std::vector<int> v(3); // value-initialization of each element // the value of each element is 0 std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n'; std::cout << t1.mem1 << ' ' << t3.mem1 << '\n'; delete[] a; }
Possible output:
0 0 0 0 0 0 4199376
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 value-initialization |
| 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) |
zero-initializes 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 default-initialized |
| 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 zero-initialization |
| 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 |