Explain the initialization rules for variables in C + + _c language

Objective

When you define a variable that does not have an initialization, the system sometimes helps us initialize the variable.

How the system initializes depends on the type of the variable and the location of the variable definition.　

Whether the built-in type variable is automatically initialized depends on where the variable is defined.

The variables defined outside the function are initially 0; the variables defined in the function body are not automatically initialized. In addition to the left-hand operand used as an assignment, any other behavior that uses an uninitialized variable is undefined and does not rely on undefined behavior.

Take the int type for example, a simple test code:

#include <iostream>
using namespace std;

int A;

int main ()
{
 int b;

 cout << a << Endl;
 cout << b << Endl;

 return 0;
}

In VS execute this code, output variable a value of 0, while vs will be an error: Run-Time Check Failure #3 — The variable 'b' is being used without being initialized . variable A is automatically initialized to 0, and variable b is not automatically initialized.

When a class type variable is defined, the default constructor is automatically called for initialization , regardless of where the variable is defined, if no initialization is provided. If a type does not have a default constructor, you must provide a display initializer when defining the type object.

A simple test code (the default constructor is automatically generated by the compiler):

#include <iostream>
using namespace std;

Class Testa
{public
 :
 void printf () const
 {
  cout << data << Endl;
 }
 Private:
 int data;

Testa A;

int main ()
{
 testa B;
 
 A.printf ();
 B.printf ();

 return 0;
}

In VS execute this code, get the following results:

The default constructor generated by the compiler initializes the data member using the same rules as the variable initialization. Object A is defined outside the function, and its int type data member is initially 0; object B is defined in the function body, and the composite default constructor is not initialized (conforming to the built-in type variable initialization rules), where all the random values are stored. Similarly, if the data member is a class type, the corresponding default constructor is called to initialize the data member.

If you change the definition of this class A little, define a constructor to prevent the compiler from automatically generating the default constructor:

#include <iostream>
using namespace std;

Class Testa
{public
 :
 Testa (int a)
 {
  data = A;
 }

 void printf () const
 {
  cout << data << Endl;
 }
 Private:
 int data;

Testa A;

int main ()
{
 testa B;
 
 A.printf ();
 B.printf ();

 return 0;
}

This code cannot be compiled: error C2512: “testA”: no suitable default constructor is available.

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