Function calling is when you call a function yourself in a program. While function invoking is when it gets called automatically.
For example, consider this program:

struct s
{
  int a,b,s;

  s()
  {
    a=2;
    b=3;
  }

  void sum()
  {
    s=a+b;
  }
};

void main()
{
  struct s obj; //line 1
  obj.sum(); // line 2
}

Here, when line 1 is executed, the function (constructor, i.e. s) is invoked. When line 2 is executed, the function sum is called.

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