头文件导致函数调用后原始数组出现问题

Question

I created a header file called primes.h What I want is to call a function generate_primes() defined in this header file to fill an array with a determined number of prime numbers. So to test if everything works fine, I first printed the elements of the array from 'inside' the header file' wich works just fine. Then I tried printing them from outside the header file, in the program including the header file but It's not working, and the program just stops after printing a few elements. Why is this unproper behaviour? And how can I do this the right way. Thank's for helping.

The header file :

int generate_primes(const unsigned long long n, unsigned long long*prime) 
{

    try
        {
            prime = new unsigned long long[n]; 
        } 
    catch(const std::bad_alloc&e) {
        std::cerr << e.what(); 
        exit(EXIT_FAILURE);  
    }

    prime[0] = 2, prime[1] = 3, prime[2] = 5; 

    unsigned long long p;
    unsigned long long index = 3;

    for (p = 7; index < n; p += 2)
        {
            short isPrime = 1; 
            unsigned long long test_limit = (unsigned long long) sqrt(p); 

            for (int i = 1; prime[i] <= test_limit && i < index; i++)
                {
                    if (!(p%prime[i]))
                        {
                            isPrime = 0; 
                            break;  
                        }   
                } 
            if (isPrime) prime[index++] = p; 
        }

    //everything works fine when I print from inside the header file 
    /*for (int i = 0; i < n; i++)
      {
      if (i && i % 7 == 0) std::cout << '\n'; 
      std::cout << prime[i] << "\t\t"; 
      }
    */
    return 1; 
}

My program :

#include <iostream>
#include <cmath>
#include <vector>
#include <new>
#include <cstdlib>
#include "primes.h"

int main(){
    unsigned long long n = 100;//example  
    unsigned long long *prime;   
    generate_primes(n, prime); 

    //but printing from here causes problems 
    //isn't the array now filled after calling the above function ? 
    for (unsigned long long i = 0; i < n; i++) 
        {
            if (i && i % 5 == 0) std::cout<< '\n'; 
            std::cout << prime[i] << "\t"; 
        } 

    return 0; 
}

Answer 1

The problem is that the parameter prime is being passed by value, not by reference. So when generate_prime assigns:

prime = new unsigned long long[n];

this only assigns to the local variable prime , not the caller's variable. You need to pass the parameter by reference so that the assignment will be visible to main() .

int generate_primes(const unsigned long long n, unsigned long long*&prime) 

Answer 2

That's not a header file, it's a bunch of code. If you want to separate that code it should be in another .cpp file.

An associated .h would look like this:

#include <cmath>
#include <vector>
#include <new>

int generate_primes(const unsigned long long n, unsigned long long*prime);

Then you'd have generate_primes.cpp or something like it:

 #include "generate_primes.h"

 int generate_primes(const unsigned long long n, unsigned long long*prime) {
   // (function body)
 }

To build your application you'd need to compile both .cpp files and link them together with the required libraries.

Answer 3

The value of the uninitialized pointer prime is an argument to the call of generate_primes. There it is not evaluated but the parameter assigned a new (local) value. Therefore inside the method the Array is accesible. But the new value is never returned, so in main the value of prime remains unchanged.

Additionally you should change n from unsingned long long to a smaller type for teh sake of the compiler.

Two possible solutions: a) modify generate_prime so that it return the address of the Array

unsigned long long *prime generate_primes(const unsigned long n) 

b) gíve the address of the pointer as an Argument to the method

int generate_primes(const unsigned long n, unsigned long long **prime) 
{

    try
    {
        *prime = new unsigned long long[n]; 
    } catch (const std::bad_alloc &e) {
    // your code

and

int main(){
    unsigned long n = 100;//example  
    unsigned long long *prime;   
    generate_primes(n, &prime);

    // your Code

    delete[] prime; // free memeory
    return 0; 
}