Question about setting size of a C style string

[EdFab]

In order to make this program work, I had to use a magic number to set the size of a character array that will then have a strcpy_s followed by a couple of strcat_s's done to it. Here is the code, and the part I had a problem with is at the end and bolded...it runs, but only because css4 was declared with a magic dimension number of 30. If I substitute the variable 'size', which is a const size_t, I get a compile error indicating that a constant expression is needed. I'm really baffled by this.

#include <iostream>
#include <cstring>
#include <cstddef>
using std::cout; using std::endl; using std::strcmp; using std::strlen; using std::strcat; using std::size_t;

int main()
{
   // Declare 3 c_style strings from strings and compare them (note: the null character is automatically appended when a c_style string is initialized from a string).
   const char css1[] = "A string.";
   const char css2[] = "A different string.";
   const char css3[] = "A string.";

   // Comparing css1 to css2
   if (strcmp(css1, css2) == 0)
      cout << "css1 and css2 are equal!" << endl;
   else
      cout << "css1 and css2 are not equal!" << endl;
   
   // Comparing css1 to css3.
   if (strcmp(css1, css3) == 0)
      cout << "css1 and css3 are equal!" << endl;
   else
      cout << "css1 and css3 are not equal!" << endl;

   // Properly terminated c_style string.
   const char ca[] = { 'h', 'e', 'l', 'l', 'o', '\0' }; 
   const char *pChar = ca;
   // Improperly terminated c_style string (null is not appended).
   const char ca1[] = { 'h', 'e', 'l', 'l', 'o' };
   const char *pChar1 = ca1;

   // Loop through properly terminated ca.
   while (*pChar) {
      cout << *pChar << " ";
      ++pChar;
   }
   cout << endl;

   // Loop through improperly terminated ca1.
   while (*pChar1) {
      cout << *pChar1 << " ";
      ++pChar1;
   }
   cout << endl;

   // Create a character array with a size that will be able to hold css2 concatenated to css1 with a space in between the two strings plus room for the terminating null.
   const size_t size = strlen(css1) + strlen(css2) + 2; // Includes room for a space between the strings and for the null character.
   cout << "The concatenated string will have length " << size << endl;
   char css4[30]; // Using magic number because won't compile with 'size' as dimension specifier. char css4[size] FAILS!
   strcpy_s(css4, css1);
   strcat_s(css4, " ");
   strcat_s(css4, css2);

   cout << css4 << endl;

   return 0;
}

Edit: follow up question: what's the difference between a const and a constant expression?

[EdFab]

I found a solution to avoiding magic numbers when declaring a character array, using new to dynamically create the array on the heap, and then delete it when done. This code worked:

// Create a character array with a size that will be able to hold css2 concatenated to css1 with a space in between the two strings plus room for the terminating null.
   const int size = strlen(css1) + strlen(css2) + 2; // Includes room for a space between the strings and for the null character.
   cout << "The concatenated string will have length " << size << endl;
   char *css4 = new char[size]; // Using magic number because won't compile with 'size' as dimension specifier.
   strcpy_s(css4, size, css1);
   strcat_s(css4, size, " ");
   strcat_s(css4, size, css2);
   
   cout << css4 << endl;
   delete[] css4;