copy initialization

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor Comments Keywords	ASCII chart Escape sequences History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
default initialization value initialization zero initialization copy initialization direct initialization	aggregate initialization list initialization (C++11) constant initialization reference initialization

Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
Class-specific function properties
virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
default constructor copy constructor move constructor (C++11)	copy assignment move assignment (C++11) destructor
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly

 

Initializes an object from another object

Syntax

T object = other ;	(1)
f(other);	(2)
return other;	(3)
catch ( T other) ;	(4)
T array [ N ] = { other };	(5)

Explanation

Copy initialization is performed in the following situations:

1) when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of an equals sign followed by an expression.

2) when passing an argument to a function by value

3) when returning from a function that returns by value

4) when catching an exception by value

5) as part of aggregate initialization, to initialize each element for which an initializer is provided

The effects of copy initialization are:

• If T is a class type and the type of other is cv-unqualified version of T or a class derived from T, the constructors of T are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.

• If T is a class type, and the type of other is different, or if T is non-class type, but the type of other is a class type, user-defined conversion sequences that can convert from the type of other to T are examined and the best one is selected through overload resolution. The result of the conversion, which is a prvalue temporary of the destination type, is then used to direct-initialize the object. The last step is usually eliminated and the result of the conversion function is constructed directly in the memory allocated for the target object, but the copy constructor is required to be accessible even though it's not used.

• Otherwise (if neither T nor the type of other are class types), standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version of T.

Notes

Copy-initialization is less permissive than direct-initialization: copy-initialization only considers non-explicit constructors and user-defined conversion functions.

If other is an rvalue expression, move constructor will be selected by overload resolution and called during copy-initialization.

Implicit conversion is defined in terms of copy-initialization: if an object of type T can be copy-initialized with expression E, then E is implicitly convertible to T.

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.

Example

run this code

#include <string>
#include <utility>
#include <memory>
 
int main()
{
    std::string s = "test"; // OK: constructor is non-explicit
    std::string s2 = std::move(s); // this copy-initialization performs a move
 
//  std::unique_ptr<int> p = new int(1); // error: constructor is explicit
    std::unique_ptr<int> p(new int(1)); // OK: direct-initialization
 
    int n = 3.14;    // floating-integral conversion
    const int b = n; // const doesn't matter
    int c = b;       // ...either way
}

See also

• default initialization
• direct initialization
• aggregate initialization
• list initialization