Simple understanding of "!"

I saw a code today, there is such a sentence:

pListHead *pListHead,*pToBeDeleted;
if(!pListHead||!pToBeDeleted)

I have never understood, this! The mode of the + pointer indicates that the pointer is not initialized, or the pointer is empty.
I did two experiments, the code is as follows.

int *p；
if(!p){
    cout<<"true";
}

This is a pointer without initialization, and the result is directly displayed.

int *p=new int；
if(!p){
    cout<<"true";
}
 Can run, no executionifcontent.

This indicates that the pointer P as long as initialization allocation memory space,! P is false.

int *p=nullptr；
if(!p){
    cout<<"true";
}
 print:true

This shows that for non-symbols "!", Only pointers are TRUE.
Then I did experiment with the variable INT.

int i
if(!i){
    cout<<"true";
}
 Compile Error

This is not good, as long as it is not initialized, it is not possible to perform non-judgment.

int i=5；
if(i){
    cout<<"true";
}
print:true

Only in this way can be compiled, so for non-pointer variables, as long as it initializes the TRUE in IF.

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