Why is Copy Construtor called?

[bistelA0005]

I am using an example from Sams Teach Yourself C++ In One Hour a day. The instructor states that passing sayHello by value to the function UseMyString() around line 68 results in the copy constructor being called. I do not understand why. Could someone explain? Is it because sayHello is a function of MyString type? Thanks

#include <iostream>
#include <string.h>
using namespace std;

class MyString
{
private:
   char* buffer;

public:
   
   MyString(const char* initString) // constructor
   {
      buffer = NULL;
      cout << "Default constructor: creating new MyString" << endl;
      if(initString != NULL)
      {
         buffer = new char [strlen(initString) + 1];
         strcpy(buffer, initString);

         cout << "buffer points to: 0x" << hex;
         cout << (unsigned int*)buffer << endl;
      }
   }

   MyString(const MyString& copySource) // Copy constructor
   {
      buffer = NULL;
     cout << "Copy constructor: copying from MyString" << endl;
      if(copySource.buffer != NULL)
      {
         // allocate own buffer 
         buffer = new char [strlen(copySource.buffer) + 1];

         // deep copy from the source into local buffer
         strcpy(buffer, copySource.buffer);

         cout << "buffer points to: 0x" << hex;
         cout << (unsigned int*)buffer << endl;
      }
   }

   // Destructor
   ~MyString()
   {
      cout << "Invoking destructor, clearing up" << endl;
      delete [] buffer;
   }

   int GetLength() 
   { return strlen(buffer); }

   const char* GetString()
   { return buffer; }
};

void UseMyString(MyString str)
{
   cout << "String buffer in MyString is " << str.GetLength();
   cout << " characters long" << endl;

   cout << "buffer contains: " << str.GetString() << endl;
   return;
}

int main()
{
   MyString sayHello("Hello from String Class");
   UseMyString(sayHello);

   return 0;
}

[keskiverto]

The function takes one parameter by value.

How do you think the parameter str to get its value?

[bistelA0005]

Thanks for the reply. Your question is a good one for me. I realize I am not clear on that also. Does str get its value when str.GetLength() is called or earlier?

[keskiverto]

Lets suppose that it would get its value on line 59 due to member function call. Where would the value come from, and how?

[bistelA0005]

Great point, that practical question makes me understand that part of it good. It is constructed before it hits its first line.

Q: How do you think the parameter str to get its value?
A: By a MyString object calling its constructor.

Therefore the only signature that fits is to use the Copy constructor--?

[keskiverto]

Yes, on line 69 a function is called with an argument. That argument is used to initialize the parameter str. It is like you had written:
MyString str( sayHello );

By value parameters are copies of the values that the function is called with.

[bistelA0005]

That helps a lot. I totally understand now. Thank you so much