Question

I am looking at someone’s code, I don’t understand how the object getting initialized here:

template <typename String>
void test_numbers()
{
  SampleClass<String> compare;
  String lhs = "abc";
  String rhs = "efg";
  check_equality(compare(lhs, rhs), true);
}

The object compare is created of class type SampleClass and then assigned 2 strings when passed on as a parameter. How this initialization works? Any comments? suggestions?

Answer 1

//I am initialised with my default constructor (no args) 
SampleClass<String> compare;

//I am initialised with my `const char*` constructor (and assignment operator)   
String lhs = "abc";
String rhs = "efg";

//Compare (already initialised) is being invoked by it's `operator()`
check_equality(compare(lhs, rhs), true);

compare is already constructed. It has an operator() implemented that allows it to appear as a function, accepting arguments.

you can make your own easily.

struct op_test{
    int i;
    op_test(int i_) : i(i_){}
    int operator()(int j)const { return j*i; }
};
:::
op_test ot(5);
ot(6); //5*6

The reason this is useful is because we can do thing like this.

std::vector<int> a(700); //700 ints

std::transform(a.begin(), a.end(), b.begin(), op_test(5));
//or
std::transform(a.begin(), a.end(), b.begin(), &my_func); //calls a function
std::transform(a.begin(), a.end(), b.begin(), [](int i){ return i*5; }); //lambda

see here:
http://msdn.microsoft.com/en-us/library/5tk49fh2(v=vs.80).aspx
useful with
http://en.cppreference.com/w/cpp/algorithm