Is there a difference between copy initialization and direct initialization?

Question

C++17 Update

In C++17, the meaning of A_factory_func() changed from creating a temporary object (C++<=14) to just specifying the initialization of whatever object this expression is initialized to (loosely speaking) in C++17. These objects (called "result objects") are the variables created by a declaration (like a1), artificial objects created when the initialization ends up being discarded, or if an object is needed for reference binding (like, in A_factory_func();. In the last case, an object is artificially created, called "temporary materialization", because A_factory_func() doesn't have a variable or reference that otherwise would require an object to exist).

As examples in our case, in the case of a1 and a2 special rules say that in such declarations, the result object of a prvalue initializer of the same type as a1 is variable a1, and therefore A_factory_func() directly initializes the object a1. Any intermediary functional-style cast would not have any effect, because A_factory_func(another-prvalue) just "passes through" the result object of the outer prvalue to be also the result object of the inner prvalue.

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A a1 = A_factory_func();
A a2(A_factory_func());

Depends on what type A_factory_func() returns. I assume it returns an A - then it's doing the same - except that when the copy constructor is explicit, then the first one will fail. Read 8.6/14

double b1 = 0.5;
double b2(0.5);

This is doing the same because it's a built-in type (this means not a class type here). Read 8.6/14.

A c1;
A c2 = A();
A c3(A());

This is not doing the same. The first default-initializes if A is a non-POD, and doesn't do any initialization for a POD (Read 8.6/9). The second copy initializes: Value-initializes a temporary and then copies that value into c2 (Read 5.2.3/2 and 8.6/14). This of course will require a non-explicit copy constructor (Read 8.6/14 and 12.3.1/3 and 13.3.1.3/1 ). The third creates a function declaration for a function c3 that returns an A and that takes a function pointer to a function returning a A (Read 8.2).

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Delving into Initializations Direct and Copy initialization

While they look identical and are supposed to do the same, these two forms are remarkably different in certain cases. The two forms of initialization are direct and copy initialization:

T t(x);
T t = x;

There is behavior we can attribute to each of them:

• Direct initialization behaves like a function call to an overloaded function: The functions, in this case, are the constructors of T (including explicit ones), and the argument is x. Overload resolution will find the best matching constructor, and when needed will do any implicit conversion required.
• Copy initialization constructs an implicit conversion sequence: It tries to convert x to an object of type T. (It then may copy over that object into the to-initialized object, so a copy constructor is needed too - but this is not important below)

As you see, copy initialization is in some way a part of direct initialization with regard to possible implicit conversions: While direct initialization has all constructors available to call, and in addition can do any implicit conversion it needs to match up argument types, copy initialization can just set up one implicit conversion sequence.

I tried hard and got the following code to output different text for each of those forms, without using the "obvious" through explicit constructors.

#include <iostream>
struct B;
struct A { 
  operator B();
};

struct B { 
  B() { }
  B(A const&) { std::cout << "<direct> "; }
};

A::operator B() { std::cout << "<copy> "; return B(); }

int main() { 
  A a;
  B b1(a);  // 1)
  B b2 = a; // 2)
}
// output: <direct> <copy>

How does it work, and why does it output that result?

1. Direct initialization

   It first doesn't know anything about conversion. It will just try to call a constructor. In this case, the following constructor is available and is an exact match:

   B(A const&)
   

   There is no conversion, much less a user defined conversion, needed to call that constructor (note that no const qualification conversion happens here either). And so direct initialization will call it.

2. Copy initialization

   As said above, copy initialization will construct a conversion sequence when a has not type B or derived from it (which is clearly the case here). So it will look for ways to do the conversion, and will find the following candidates

   B(A const&)
   operator B(A&);
   

   Notice how I rewrote the conversion function: The parameter type reflects the type of the this pointer, which in a non-const member function is to non-const. Now, we call these candidates with x as argument. The winner is the conversion function: Because if we have two candidate functions both accepting a reference to the same type, then the less const version wins (this is, by the way, also the mechanism that prefers non-const member function calls for non-const objects).

   Note that if we change the conversion function to be a const member function, then the conversion is ambiguous (because both have a parameter type of A const& then): The Comeau compiler rejects it properly, but GCC accepts it in non-pedantic mode. Switching to -pedantic makes it output the proper ambiguity warning too, though.

I hope this helps somewhat to make it clearer how these two forms differ!

Assignment is different from initialization.

Both of the following lines do initialization. A single constructor call is done:

A a1 = A_factory_func();  // calls copy constructor
A a1(A_factory_func());   // calls copy constructor

but it's not equivalent to:

A a1;                     // calls default constructor
a1 = A_factory_func();    // (assignment) calls operator =

I don't have a text at the moment to prove this but it's very easy to experiment:

#include <iostream>
using namespace std;

class A {
public:
    A() { 
        cout << "default constructor" << endl;
    }

    A(const A& x) { 
        cout << "copy constructor" << endl;
    }

    const A& operator = (const A& x) {
        cout << "operator =" << endl;
        return *this;
    }
};

int main() {
    A a;       // default constructor
    A b(a);    // copy constructor
    A c = a;   // copy constructor
    c = b;     // operator =
    return 0;
}

double b1 = 0.5; is implicit call of constructor.

double b2(0.5); is explicit call.

Look at the following code to see the difference:

#include <iostream>
class sss { 
public: 
  explicit sss( int ) 
  { 
    std::cout << "int" << std::endl;
  };
  sss( double ) 
  {
    std::cout << "double" << std::endl;
  };
};

int main() 
{ 
  sss ddd( 7 ); // calls int constructor 
  sss xxx = 7;  // calls double constructor 
  return 0;
}

If your class has no explicit constuctors than explicit and implicit calls are identical.

You can see its difference in explicit and implicit constructor types when you initialize an object :

Classes :

class A
{
    A(int) { }      // converting constructor
    A(int, int) { } // converting constructor (C++11)
};

class B
{
    explicit B(int) { }
    explicit B(int, int) { }
};

And in the main function :

int main()
{
    A a1 = 1;      // OK: copy-initialization selects A::A(int)
    A a2(2);       // OK: direct-initialization selects A::A(int)
    A a3 {4, 5};   // OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1;   // OK: explicit cast performs static_cast

//  B b1 = 1;      // error: copy-initialization does not consider B::B(int)
    B b2(2);       // OK: direct-initialization selects B::B(int)
    B b3 {4, 5};   // OK: direct-list-initialization selects B::B(int, int)
//  B b4 = {4, 5}; // error: copy-list-initialization does not consider B::B(int,int)
    B b5 = (B)1;   // OK: explicit cast performs static_cast
}

By default, a constructor is as implicit so you have two way to initialize it :

A a1 = 1;        // this is copy initialization
A a2(2);         // this is direct initialization

And by defining a structure as explicit just you have one way as direct :

B b2(2);        // this is direct initialization
B b5 = (B)1;    // not problem if you either use of assign to initialize and cast it as static_cast

Of note:

[12.2/1] Temporaries of class type are created in various contexts: ... and in some initializations (8.5).

I.e., for copy-initialization.

[12.8/15] When certain criteria are met, an implementation is allowed to omit the copy construction of a class object ...

In other words, a good compiler will not create a copy for copy-initialization when it can be avoided; instead it will just call the constructor directly -- ie, just like for direct-initialization.

In other words, copy-initialization is just like direct-initialization in most cases <opinion> where understandable code has been written. Since direct-initialization potentially causes arbitrary (and therefore probably unknown) conversions, I prefer to always use copy-initialization when possible. (With the bonus that it actually looks like initialization.)</opinion>

Technical goriness: [12.2/1 cont from above] Even when the creation of the temporary object is avoided (12.8), all the semantic restrictions must be respected as if the temporary object was created.

Glad I'm not writing a C++ compiler.

First grouping: it depends on what A_factory_func returns. The first line is an example of copy initialization, the second line is direct initialization. If A_factory_func returns an A object then they are equivalent, they both call the copy constructor for A, otherwise the first version creates an rvalue of type A from an available conversion operators for the return type of A_factory_func or appropriate A constructors, and then calls the copy constructor to construct a1 from this temporary. The second version attempts to find a suitable constructor that takes whatever A_factory_func returns, or that takes something that the return value can be implicitly converted to.

Second grouping: exactly the same logic holds, except that built in types don't have any exotic constructors so they are, in practice, identical.

Third grouping: c1 is default initialized, c2 is copy-initialized from a value initialized temporary. Any members of c1 that have pod-type (or members of members, etc., etc.) may not be initialized if the user supplied default constructors (if any) do not explicitly initialize them. For c2, it depends on whether there is a user supplied copy constructor and whether that appropriately initializes those members, but the members of the temporary will all be initialized (zero-initialized if not otherwise explicitly initialized). As litb spotted, c3 is a trap. It's actually a function declaration.

Answering with respect to this part:

A c2 = A(); A c3(A());

Since most of the answers are pre-c++11 I am adding what c++11 has to say about this:

A simple-type-specifier (7.1.6.2) or typename-specifier (14.6) followed by a parenthesized expression-list constructs a value of the specified type given the expression list. If the expression list is a single expression, the type conversion expression is equivalent (in definedness, and if defined in meaning) to the corresponding cast expression (5.4). If the type specified is a class type, the class type shall be complete. If the expression list specifies more than a single value, the type shall be a class with a suitably declared constructor (8.5, 12.1), and the expression T(x1, x2, ...) is equivalent in effect to the declaration T t(x1, x2, ...); for some invented temporary variable t, with the result being the value of t as a prvalue.

So optimization or not they are equivalent as per the standard. Note that this is in accordance with what other answers have mentioned. Just quoting what the standard has to say for sake of correctness.