The address value indicated by the pointer and the value of the object indicated by the pointer (learning notes)

The address value indicated by the pointer :

         The pointer p As if The condition of the statement , The actual test is the value of the pointer itself , That is, the address value indicated by the pointer . If the pointer points to a real variable , Then its value must not be 0（ Such as pointer in code p1）, At this time, the condition is true ; If the pointer does not point to any object or is an invalid pointer （ Such as pointer in code p2）, On the other hand p The use of will lead to unpredictable results .

The value of the object to which the pointer refers :

         Dereference operator *p As if The condition of the statement , What is actually checked is the content of the object referred to by the pointer , In code is a pointer p1 The value referred to i I.e 0, If it's time to int The value is 0, Then the condition is false ; otherwise , If it's time to int Values are not for 0, The corresponding condition is true .

Code block ：

#include<iostream>
using namespace std;

int main(){
	int i = 0;
	int *p1 = &i;
	int *p2 = NULL;
	if(p1){// Check pointer P1 The value of the indicated address  
		cout<<"p1 pass"<<endl;
	}
	if(p2){// Check pointer p2 The value of the indicated address  
		cout<<"p2 pass"<<endl;
	}
	if(*p1){// Quoting  , Check pointer p1 The value of the content of the object  
		cout<<"*p1 pass"<<endl;
	}
	return 0;
	
}

Output results ：

p1 pass                   // The pointer p1 The address has a real value, so the result is true 
p2 no pass                // The pointer p2 The address has no real value, so the result is false 
*p1 no pass               // Quoting *p1 The content value of the object is 0, So the result is false 
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