C++ Boolean Function Operands

So I've started working with c++ for my university classes and it's been going really well so far. There's a dilemma I'm having with a current question and I've got the basic code structure all figured out, there's just one problem with my output.

What I'm looking for e.g;

if (bool variable = true){ 

  output

else
  alternate output

I'm aware that this isn't a free debugging service place, but it would really help me in future projects as well and there aren't any bugs, it executes just fine.

My code:

#include "stdafx.h"
#include <iostream>
#include <iomanip>

using namespace std;

//function prototypes
bool calculateBox(double, double, double, double *, double *);

int main()
{
    //defining variables
    double length, width, height, volume, surfaceArea;

    cout << "Hello and welcome to the program.\nPlease enter the dimensions for the box in [cm](l w h): ";
    cin >> length >> width >> height;
    calculateBox(length, width, height, &volume, &surfaceArea);

    if (bool calculateBool = true) {
        cout << "Volume: " << volume << "cm^3" << endl << "Surface Area: " << surfaceArea << "cm^2" << endl;
    }
    else
        cout << "Error, value(s) must be greater than zero!" << endl;

    system("pause");
    return 0;
}

//functions
bool calculateBox(double length, double width, double height, double * volume, double * surfaceArea) {
    if ((length > 0) && (width > 0) && (height > 0)) {
        *surfaceArea = length * width * 6;
        *volume = length * width * height;
        return true;
    }
    else
        return false;
}

*Key, if the values do not meet the requirements, the output displays not the error message, but a strange string for surfaceArea and volume. It appears to skip over the 'else' statement.

My question - does my error lie within the return statements in the function? Or is it a logic problem with my 'if' statement in the main method?