cout 指针 c++

Question

I can not understand why cout does not work in this code:

#include<iostream>
using namespace std;

int main() {
    int v = 65;
    int* q = &v;
    char** o = (char**)&q;
    cout <<  o << endl;  // output: 012FFCAC
    cout <<  *o << endl; // output: A
    cout << **o << endl; // output: A
    printf("%c",*o);     // cause an error
    printf("%p",*o);    // works and the output=&v

And cout does not work else in this code

#include<iostream>
using namespace std;

int main() {
    char v = 65;
    cout <<  &v << endl;  // output: A╠╠╠╠▀?╞«4°O 

Answer 1

Because an int is (usually) 4 bytes 65 will fit quite neatly into just the first byte and the rest of the memory allocated for the int will be 0 this byte pattern happens to match up very closely to how strings are stored in memory.

So when the memory is accessed through a char* it will print A most of the time even though most of the prints were ill formed

int v = 65;  // Byte pattern on most machines [65,0,0,0] 
int* q = &v; 
char** o = (char**)&q; // *o now points to [65,0,0,0] ==> ['A','\0','\0','\0'] ==> "A"
std::cout <<  o << std::endl;  
    // input:      char** 
    // printed as: pointer 
    // result:     012FFCAC

std::cout <<  *o << std::endl;  // Undefined Behaviour
    // input:      char*
    // printed as: null terminated string
    // result:     "A"

std::cout <<  **o << std::endl; // Undefined Behaviour 
    // input:      char
    // printed as: single character
    // result:     'A'

printf("%c",*o);    // %c expects a [char] type but is given [pointer to char]      ERROR
printf("%p",*o);    // %p expects a [pointer] type and is given a [pointer to char] OK

Since a char is (usually) 1 byte there is no null terminand to stop the printing once it starts and it keeps printing whatever is around in memory until it runs into a 0 or an access violation

char v = 65;
std::cout << &v << std::endl;  // &v is not a well-formed null terminated string: Undefined Behaviour
    // input:      char*
    // printed as: null terminated string
    // result:     "A<whatever other data is around v in memory>"