Has the new C++11 member initialization feature at

问题:

With C++11, we now have the ability to initialize class members in a header declaration:

class aClass
{
    private:
        int mInt{100};
    public:
         aClass();
        ~aClass();
};

So I'm a bit confused. Traditionally initialization lists in constructors have been used for member initialization:

aClass::aClass()
: mInt(100)
{
    ...
}

Has the new C++11 member initialization feature at declaration made initialization lists obsolete? If not, what are the advantages of one over the other? What situations would make initialization at declaration advantageous, or initialization lists advantageous? When should one be used over the other?

回答1:

No, they are not obsolete as this article Get to Know the New C++11 Initialization Forms says in the Class Member Initialization section (emphasis mine):

Bear in mind that if the same data member has both a class member initializer and a mem-init in the constructor, the latter takes precedence. In fact, you can take advantage of this behavior by specifying a default value for a member in the form of a class member initializer that will be used if the constructor doesn't have an explicit mem-init for that member. Otherwise, the constructor's mem-init will take effect, overriding the class member initializer. This technique is useful in classes that have multiple constructors

So although in class member initialization is a nice convenience it does not remove the need for initialization lists but both features instead work together to give you a nice way to specify default values and override them when needed. This seems to be also how Bjarne Stroustrup sees it too, he says:

This saves a bit of typing, but the real benefits come in classes with multiple constructors. Often, all constructors use a common initializer for a member:

and provides an example of members which have a common initializer:

class A {
  public:
    A(): a(7), b(5), hash_algorithm("MD5"), s("Constructor run") {}
    A(int a_val) : a(a_val), b(5), hash_algorithm("MD5"), s("Constructor run") {}
    A(D d) : a(7), b(g(d)), hash_algorithm("MD5"), s("Constructor run") {}
    int a, b;
  private:
    HashingFunction hash_algorithm;  // Cryptographic hash to be applied to all A instances
    std::string s;                   // String indicating state in object lifecycle
};

and says:

The fact that hash_algorithm and s each has a single default is lost in the mess of code and could easily become a problem during maintenance. Instead, we can factor out the initialization of the data members:

class A {
  public:
    A(): a(7), b(5) {}
    A(int a_val) : a(a_val), b(5) {}
    A(D d) : a(7), b(g(d)) {}
    int a, b;
  private:
    HashingFunction hash_algorithm{"MD5"};  // Cryptographic hash to be applied to all A instances
    std::string s{"Constructor run"};       // String indicating state in object lifecycle
};

Note: disadvantage in C++11

There is one disadvantage to using in class member initialization in C++11 since it makes a class a non-aggregate we can no longer use aggregate initialization which may be rather surprising. This is not the case in C++14 where this restriction was removed. See: C++11 aggregate initialization for classes with non-static member initializers for more details.

回答2:

No, they are not obsolete.

Initialization lists are still the only way to go if you need constructors arguments to initialize your class members.

class A
{
 int a=7; //fine, give a default value
public:
 A();
};

class B
{
 int b; 
public:
 B(int arg) : b(arg) {}

 B(int arg, bool b) : b(arg) { ... }
};

Note that if both are present, the constructor's init will take effect, overriding the class member initialization, which is useful to specify a default value for a class member.

回答3:

The way I look at it, in-class initialization is an ehancement of mem-initializer-lists. In C++03, members not listed in a mem-initializer-list were always default initialised. This means the default constructor for classes, and no initialization for primitive types.

In-class initialization simply allows you to specify your own defaults. There are two ways to look at it.

One: if most/all constructors of your class want to provide the same initial value for a member, use an in-class initializer for that member. For other members, use mem-initializer-lists. You'll of course have to use those whenever the initial value depends on constructor arguments.

The other one: provide an in-class initializer for all members, exactly how the default constructor of your class would initialise them. Then, mem-initializer-lists in non-default constructors get the semantics of "how it differs from a default-constructed object."