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

[edit]Syntax

[edit]Explanation

Value-initialization is performed in these situations:

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

The effects of value-initialization are:

• IfT is a (possibly cv-qualified) class type:

• If the default-initialization forT selects aconstructor, and the constructor is notuser-declared(until C++11)user-provided(since C++11), the object is firstzero-initialized.
• In any case, the object isdefault-initialized.

• Otherwise, ifT is an array type, each element of the array is value-initialized.
• Otherwise, the object is zero-initialized.

[edit]Notes

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

References cannot be value-initialized.

As described infunction-style cast, the syntaxT()(1) is prohibited ifT names an array type, whileT{}(5) is allowed.

All standard containers (std::vector,std::list, etc.) value-initialize their elements when constructed with a singlesize_type argument or when grown by a call toresize(), unless their allocator customizes the behavior ofconstruct.

[edit]Example

#include <cassert>#include <iostream>#include <string>#include <vector> struct T1{int mem1;std::string mem2;virtualvoid 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 0assert(n==0);double f=double();// scalar => zero-initialization, the value is 0.0assert(f==0.0);int* a= newint[10]();// array => value-initialization of each elementassert(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 0assert(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 valueassert(t3.mem2=="");//     t3.mem2 is default-initialized, the value is ""std::vector<int> v(3);// value-initialization of each elementassert(v[2]==0);// the value of each element is 0std::cout<<'\n';    delete[] a;}

42

[edit]Defect reports

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

[edit]See also

• default constructor
• explicit
• aggregate initialization
• list-initialization

• In other languages

• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文