It is automatically converted to the base type derived class method calls the base class

class A
{
    public:
    int a;
    void showthis()
    {
        printf("from A,this:%p\n", this);
    }
};
class B
{
    public:
    int b;
    void infothis()
    {
        printf("from B,this:%p\n", this);
    }
};
class C:public A, public B
{
    public:
    C(){
        printf("from C:this:%p\n", this);    
    }
};

int main()
{
    C c;
    c.showthis();
    c.infothis();
    
    return 0;
}

Compile and run, output:

from C:this:0x7ffff4321a80
from A,this:0x7ffff4321a80
from B,this:0x7ffff4321a84

Type conversion may be accompanied by changes in this address, this base class and derived classes referred to this deviation, transformed into the base class to this pointer.