Does copy list initialization invoke copy ctor conceptually?
Before C++11, we can do copy initialization by writing something like A a = 1;
which is more or less equivalent to A a = A(1);
. That is, a temporary is first created and then a copy ctor is invoked. Regardless of copy elision, this must be so conceptually and the copy ctor must be accessible.
With list initialization in C++11, we can do a copy list initialization by writing A a = {1, 2};
. In my opinion, this should be more or less equivalent to A a = A(1, 2);
. However, on GCC and clang, A a = {1, 2}
compiles even when the copy and move ctor are inaccessible (by declaring as private). Still, A a = 1;
does not compile on GCC or clang if the corresponding copy/move ctor is inaccessible. So, A a = {1, 2};
seems more or less equivalent to A a{1, 2};
which is direct list initialization. The difference between this and the real direct list initialization is that A a = {1, 2};
does not compile if the ctor that takes two ints are explicit. In this aspect, A a = {1, 2};
resembles copy initialization.
So, my question is: what is the exact semantics of expressions like A a = {1, 2};
conceptually? By conceptually, copy elision do not stay in the way.
Solution 1:
The standard describes it pretty well; [dcl.init.list]/3:
List-initialization of an object or reference of type
T
is defined as follows:
- [...]
- Otherwise, if
T
is a class type, constructors are considered. The applicable constructors are enumerated and the best one is chosen through overload resolution (13.3, 13.3.1.7). If a narrowing conversion (see below) is required to convert any of the arguments, the program is ill-formed.
[over.match.list] (emphasis mine):
When objects of non-aggregate class type
T
are list-initialized (8.5.4), overload resolution selects the constructor in two phases:
Initially, the candidate functions are the initializer-list constructors (8.5.4) of the class
T
and the argument list consists of the initializer list as a single argument.If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class
T
and the argument list consists of the elements of the initializer list.If the initializer list has no elements and T has a default constructor, the first phase is omitted.
In copy-list-initialization, if anexplicit
constructor is chosen, the initialization is ill-formed.
Hence, if no initializer-list constructor is found (as in your case), the elements of the initializer list constitute the arguments for the constructor call.
In fact, the only difference of direct-list-initialization and copy-list-initialization is covered by the last, bolded sentence.
This is one of the advantages of list-initialization: It doesn't necessitate the presence of a special member function that is not gonna be used anyway.