C++11 member initializer list vs in-class initializer? - c++

Question

What difference between these ways of initializing object member variables in C++11 ? Is there another way ? which way is better (performance) ?:
class any {
public:
obj s = obj("value");
any(){}
};
Or
class any {
public:
obj s;
any(): s("value"){}
};
Thanks.

Answer 1

No, these are not the same.
The difference between them is the same that applies for direct-initialization vs. copy-initialization, which is subtle but often very confusing.
§12.6.2 [class.base.init]:
The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of 8.5 for direct-initialization. [...]
In a non-delegating constructor, if a given non-static data member or base class is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class (10.4), then
— if the entity is a non-static data member that has a brace-or-equal-initializer, the entity is initialized as specified in 8.5;
§8.5 [dcl.init]:
The initialization that occurs in the form
T x = a;
as well as in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), and aggregate member initialization (8.5.1) is called copy-initialization.
Initializing a non-static data member on a member-initializer-list follows the rules of direct-initialization, which doesn't create intermediate temporaries that need to be moved/copied (if compiled without a copy-elision), neither the type of the data member must be copyable/movable (even if the copy is elided). In addition, a direct-initialization introduces an explicit context, while a copy-initialization is non-explicit (if a constructor selected for the initialization is explicit, the program won't compile).
In other words, the obj s = obj("value"); syntax won't compile if obj is declared as:
struct obj
{
obj(std::string) {}
obj(const obj&) = delete;
};
or:
struct obj
{
obj(std::string) {}
explicit obj(const obj&) {}
};
As a more tangible example, while the below won't compile:
struct any
{
std::atomic<int> a = std::atomic<int>(1); // ill-formed: non-copyable/non-movable
std::atomic<int> b = 2; // ill-formed: explicit constructor selected
};
this one will:
struct any
{
std::atomic<int> a;
std::atomic<int> b{ 2 };
any() : a(1) {}
};
Which way is better (performance) ?
With a copy-elision enabled both have identical performance. With copy-elision disabled, there is an additional copy/move constructor call upon every instantiation when the copy-initialization syntax is used (that obj s = obj("value"); is one of).
Is there another way ?
The brace-or-equal-initializer syntax allows one to perform a direct-list-initialization as well:
class any {
public:
obj s{ "value" };
any() {}
};
Are there any other differences?
Some other differences that are worth mentioning are:
Brace-or-equal-initializer must reside in a header file along with a class declaration.
If both are combined, member-initializer-list takes priority over brace-or-equal-initializer (that is, brace-or-equal-initializer is ignored).
(C++11 only, until C++14) A class that uses brace-or-equal-initializer violates constraints for an aggregate type.
With the brace-or-equal-initializer syntax it's not possible to perform a direct-initialization other than a direct-list-initialization.

Answer 2

Both examples are equivalent.
Though only if the type is copyable or movable (check it for yourself) and NRVO is actually done (any halfway decent compiler will do it as a matter of course).
Though if you had many constructors and constructor-chaining were inappropriate, the first method would allow you not to repeat yourself.
Also, you can use that method to define aggregates with defaults different from aggregate-initialization for (some) members since C++14.

Answer 3

They are the same.
Neither is better than the other in terms of performance, and there is no other way to initialise them.
The benefit of in-class initialisation (the first in your example) is that the order of initialisation is implicit. In initialiser list you have to explicitly state the order - and compilers will warn of out-of-order initialisation if you get the ordering incorrect.
From the standard:
12.6.2.5
nonstatic data members shall be initialized in the order they were declared
in the class definition
If you get the order wrong in your list, GCC will complain:
main.cpp: In constructor 'C::C()':
main.cpp:51:9: warning: 'C::b' will be initialized after
main.cpp:51:6: warning: 'int C::a'
The benefit of initialiser lists is perhaps a matter of taste - the list is explicit, typically in the source file. In-class is implicit (arguably), and is typically in the header file.