Magical Mystery constructor?

[R.Stiltskin]

Why is this array initialization legal? The array type is Part. Granted, the components inside the brackets are of the correct types for a Part, but no Part has been constructed, and there is no 2-argument constructor for Part anyway.

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

const int SIZE=5;
struct Part {
  string partNo;
  double cost;
};

int main()
{
  Part parts[SIZE] =
    {
    {"X-101",7.50},
    {"XY-2.04",4.75}
    };
    cout << parts[0].partNo << endl;
    cout << parts[0].cost << endl;
}

[tabstop]

This is ... pretty straightforward. You've always been able, from C days, to initialize a struct using an initializer list.

[R.Stiltskin]

I just thought, incorrectly it turns out, that C++ structs were somehow fundamentally different than C structs and required a (usually default) constructor.

This means that the same thing can be done to instantiate a class member if all member variables are public. I've certainly never seen that before either, but it works:

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

const int SIZE=5;
class Part {
  public:
  string partNo;
  double cost;
};

int main()
{
  Part parts[SIZE] =
    {
    {"X-101",7.50},
    {"XY-2.04",4.75}
    };
    cout << parts[0].partNo << endl;
    cout << parts[0].cost << endl;
    Part part1 = {"abc", 1.23};
    cout << part1.cost <<endl;
}

[matsp]

Yes, as long as your main class has no constructor. struct and class are really the same thing except for whether the default is public or private.

--
Mats

[iMalc]

Aggregate initialization
MFC Programmer's SourceBook : Thinking in C++

[R.Stiltskin]

Aggregate initialization
MFC Programmer's SourceBook : Thinking in C++

Thanks. That's a great explanation.

The really hard question is, how did you find the link to Eckel's book on the CodeGuru site?