Why should I prefer to use member initialization list?

I'm partial to using member initialization lists with my constructors... but I've long since forgotten the reasons behind this...

Do you use member initialization lists in your constructors? If so, why? If not, why not?

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For POD class members, it makes no difference, it's just a matter of style. For class members which are classes, then it avoids an unnecessary call to a default constructor. Consider:

class A
{
public:
    A() { x = 0; }
    A(int x_) { x = x_; }
    int x;
};

class B
{
public:
    B()
    {
        a.x = 3;
    }
private:
    A a;
};

In this case, the constructor for B will call the default constructor for A , and then initialize ax to 3. A better way would be for B 's constructor to directly call A 's constructor in the initializer list:

B()
  : a(3)
{
}

This would only call A 's A(int) constructor and not its default constructor. In this example, the difference is negligible, but imagine if you will that A 's default constructor did more, such as allocating memory or opening files. You wouldn't want to do that unnecessarily.

Furthermore, if a class doesn't have a default constructor, or you have a const member variable, you must use an initializer list:

class A
{
public:
    A(int x_) { x = x_; }
    int x;
}

class B
{
public:
    B() : a(3), y(2)  // 'a' and 'y' MUST be initialized in an initializer list;
    {                 // it is an error not to do so
    }
private:
    A a;
    const int y;
};

---

除了上面提到的性能原因外，如果你的类存储对作为构造函数参数传递的对象的引用，或者你的类有const变量，那么你除了使用初始值设定项列表外没有其他选择。

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Initialization of base class

One important reason for using constructor initializer list which is not mentioned in answers here is initialization of base class.

As per the order of construction, base class should be constructed before child class. Without constructor initializer list, this is possible if your base class has default constructor which will be called just before entering the constructor of child class.

But, if your base class has only parameterized constructor, then you must use constructor initializer list to ensure that your base class is initialized before child class.

Initialization of Subobjects which only have parameterized constructors

Efficiency

Using constructor initializer list, you initialize your data members to exact state which you need in your code rather than first initializing them to their default state & then changing their state to the one you need in your code.

Initializing non-static const data members

If non-static const data members in your class have default constructors & you don't use constructor initializer list, you won't be able to initialize them to intended state as they will be initialized to their default state.

Initialization of reference data members

Reference data members must be intialized when compiler enters constructor as references can't be just declared & initialized later. This is possible only with constructor initializer list.

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