value initialization
This is the initialization performed when a variable is constructed with an empty initializer.
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[edit] 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) | |||||||
[edit] Explanation
Value initialization is performed in these situations:
4) when a named variable (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.
If T
is a class type that has no default constructor but has a constructor taking std::initializer_list, list-initialization is performed.
The effects of value initialization are:
1) If
T is a class type with at least one user-provided constructor of any kind, the default constructor is called.
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(until C++11) |
1) If
T is a class type with no default constructor or with a user-provided default constructor or with a deleted default constructor, the object is default-initialized.
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(since C++11) |
2) If
T is an non-union class type without any user-provided constructors, then every non-static data member and base-class component of T is value-initialized
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(until C++11) |
2) If
T is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with a defaulted default constructor or with an implicitly-defined one) then the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor
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(since C++11) |
T
is an array type, each element of the array is value-initialized[edit] Notes
A constructor is user-provided if it is user-declared and not explicitly defaulted on its first declaration.
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.
In C++98 prior to C++03 (which introduced value initialization), the expression new T() was classified as default initialization and specified zero-initialization.
References cannot be value-initialized.
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().
Since C++11, value-initializing a class without a user-provided constructor, which has a member of a class type with a user-provided constructor zeroes out the member before calling its constructor:
struct A { int i; A() {} // user-provided default ctor, does not initialize i }; struct B { A a; }; // implicitly-defined default ctor std::cout << B().a.i << '\n'; // value-initializes a B temporary // leaves b.a.i uninitialized in C++03 // sets b.a.i to zero in C++11 // (note that B{}.a.i leaves b.a.i uninitialized in C++11, but for // a different reason: in post-DR1301 C++11, B{} is aggregate-initialization, // which then value-initializes A, which has a user-provided ctor)
[edit] Example
#include <string> #include <vector> #include <iostream> struct T1 { int mem1; std::string mem2; }; // no constructors struct T2 { int mem1; std::string mem2; T2(const T2&) {} // a constructor, but no default }; struct T3 { int mem1; std::string mem2; T3() {} // user-provided default ctor }; std::string s{}; // calls default ctor, the value is "" (empty string) int main() { int n{}; // non-class value-initialization, value is 0 double f = double(); // non-class value-init, value is 0.0 int* a = new int[10](); // array of 10 zeroes T1 t1{}; // no ctors: zero-initialized // t1.mem1 is zero-initialized // t1.mem2 is default-initialized // T2 t2{}; // error: has a ctor, but no default ctor T3 t3{}; // user-defined default ctor: // t3.mem1 is default-initialized (the value is indeterminate) // t3.mem2 is default-initialized std::vector<int> v(3); // value-initializes three ints std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n'; std::cout << t1.mem1 << ' ' << t3.mem1 << '\n'; delete[] a; }
Output:
0 0 0 0 0 0 4199376
[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 |
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CWG 1301 | C++11 | defaulted default constructor skipped zero-init before construction | zero-init performed |