The role of the default constructor for the "C + + Primer seventh chapter"

The role of the default constructor

1. Default constructors

A C + + default constructor is a constructor that provides a default value for a parameter in a class, typically an empty function with no parameter value, or a constructor that provides default values, and if the user does not define a constructor, the compiler gives the class a default constructor. But as long as the user has customized any constructor, then the compiler will not provide a default constructor , in this case, easy to compile error, so the correct way to do is to define the constructor, the user also need to add a default constructor, so that does not cause compilation error.

such as: User-defined default constructors

class test{public:    //  default constructor};

2. Why to add a default constructor

The reasons are as follows:

1, when the user defines an array, and the element type of the array is a Class object 1, then this time will call the default constructor, if this time object 1 does not define the default constructor, will be an error. However, if it is not an object type, it is the built-in data type of C + + and will not error.

such as: Object array[10];

2, when the user defines an array and dynamically allocates object 1 with new, then this time if object 1 does not have a default constructor, it will also be an error, because new will invoke object 1 's parameterless default constructor to initialize the object.

such as: Object *temp = new OBJECT[10];

3, when the user uses the standard library container, when the container is a class, such as objects, this time will call the object's default constructor to initialize, if the object's class does not define a default constructor, then will be an error.

such as:vector<object> Vo;

4. When a Class A is a member of an object of another class B, if a provides a parameterless constructor and B does not, then a cannot use its own parameterless constructor.

Example 1:

The following code causes a compilation error.

class b{    B (int  i) {}}; class a{    A () {}    b b;}; int Main (void) {     //  error C2512: ' B ': no appropriate default Constructor available    return0

Example 2:

1 classNoDefault2 {3  Public:4NoDefault (CONSST std::string&);//cannot synthesize default constructor5     //There are other members, but no other constructors6 };7 8 structA9 {Ten NoDefault My_men;  One }; A  -A;//error: cannot be a composite constructor -  the structB - { -B () {}//error, B_member no initial value - NoDefault B_member; +  -};

5, if the class A defines a copy constructor, but does not define a default constructor, then if B inherits a, a, a, a, a, a, a, the initialization will call A's default constructor, this time will be an error.

class a{    A (const a&) {}}; class  Public a{    }; int Main (void) {     //error C2512: ' B ': no appropriate default Constructor available    return0

Resources

C++ Importance of default constructors

The role of the default constructor for the "C + + Primer seventh chapter"