Initializes an object from another object.

[edit]Syntax

[edit]Explanation

Copy-initialization is performed in the following situations:

The effects of copy-initialization are:

• First(until C++17)Otherwise(since C++17), ifT is a class type and the cv-unqualified version of the type ofother isT or a class derived fromT, thenon-explicit constructors ofT are examined and the best match is selected by overload resolution. That constructor is then called to initialize the object.

• Otherwise, ifT is a class type, and the cv-unqualified version of the type ofother is notT or derived fromT, or ifT is non-class type, but the type ofother is a class type,user-defined conversion sequences that can convert from the type ofother toT (or to a type derived fromT ifT is a class type and a conversion function is available) are examined and the best one is selected through overload resolution. The result of the conversion, which is arvalue temporary(until C++11)prvalue temporary(since C++11)(until C++17)prvalue expression(since C++17) of the cv-unqualified version ofT if aconverting constructor was used, is then used todirect-initialize the object.The last step is usuallyoptimized out and the result of the conversion is constructed directly in the memory allocated for the target object, but the appropriate constructor (move or copy) is required to be accessible even though it's not used.(until C++17)

• Otherwise (if neitherT nor the type ofother are class types),standard conversions are used, if necessary, to convert the value ofother to the cv-unqualified version ofT.

[edit]Notes

Copy-initialization is less permissive than direct-initialization:explicit constructors are notconverting constructors and are not considered for copy-initialization.

struct Exp{explicit Exp(constchar*){}};// not convertible from const char*Exp e1("abc");// OKExp e2="abc";// Error, copy-initialization does not consider explicit constructor struct Imp{ Imp(constchar*){}};// convertible from const char*Imp i1("abc");// OKImp i2="abc";// OK

In addition, the implicit conversion in copy-initialization must produceT directly from the initializer, while, e.g. direct-initialization expects an implicit conversion from the initializer to an argument ofT's constructor.

struct S{ S(std::string){}};// implicitly convertible from std::stringS s("abc");// OK: conversion from const char[4] to std::stringS s="abc";// Error: no conversion from const char[4] to SS s="abc"s;// OK: conversion from std::string to S

Ifother is an rvalue expression, amove constructor will be selected by overload resolution and called during copy-initialization. This is still considered copy-initialization; there is no special term (e.g., move-initialization) for this case.

Implicit conversion is defined in terms of copy-initialization: if an object of typeT can be copy-initialized with expressionE, thenE is implicitly convertible toT.

The equals sign,=, in copy-initialization of a named variable is not related to the assignment operator. Assignment operator overloads have no effect on copy-initialization.

[edit]Example

#include <memory>#include <string>#include <utility> struct A{    operatorint(){return12;}}; struct B{    B(int){}}; int main(){std::string s="test";// OK: constructor is non-explicitstd::string s2= std::move(s);// this copy-initialization performs a move //  std::unique_ptr<int> p = new int(1); // error: constructor is explicitstd::unique_ptr<int> p(newint(1));// OK: direct-initialization int n=3.14;// floating-integral conversionconstint b= n;// const doesn't matterint c= b;// ...either way     A a;    B b0=12;//  B b1 = a;       // < error: conversion from 'A' to non-scalar type 'B' requested    B b2{a};// < identical, calling A::operator int(), then B::B(int)    B b3={a};// <auto b4= B{a};// < //  b0 = a;         // < error, assignment operator overload needed [](...){}(c, b0, b3, b4);// pretend these variables are used}

[edit]Defect reports

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

[edit]See also

• copy elision
• converting constructor
• copy assignment
• copy constructor
• default constructor
• destructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • default initialization
  • direct initialization
  • initializer list
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• move assignment
• move constructor
• new

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