Question

I encountered a problem in c++. I read some codes,but there was a very wired usage of pointer. The code is following:

   double* marginalProbability =
   new double [10 * sizeof(marginalProbability[0])];
   memset( marginalProbability, 0, 10 * sizeof(double) );
    //................
            //.................
   if(marginalProbability>0)
        printf("larger");
   else
        printf("smaller");

The question I’m asking is what does it mean that if(marginalProbability>0). It is a pointer greater than zero. I think that in a normal compiler, there are no addresses which will be equal to zero. Or are there any other meanings of that? Otherwise, this line seems meaningless. Thanks.

Answer 1

a pointer with the value of zero is a null pointer. A pointer to nothing. Checking that it is not zero (p!=0 is more common that p>0 although p!=nullptr is most correct) means that it is not null.

The code you have listed is also wrong. if the new operation fails it will not return a null pointer but throw an exceptions which will no be caught.

this is what it should be.

try{
    std::vector<double> marginalProbability (10, 0.0);
}catch(const std::bad_alloc& e){
    std::cerr << "no memory" << std::endl;
}