C++ structs: force initialize members? [duplicate]

Question

e.g.

#include <iostream>
using namespace std;


struct Point {
    int x;
    int y;
};

int main() {
    Point p1 {1};
    Point p2 {.y=2};

    cout << p1.x << ", " << p1.y << endl;
    cout << p2.x << ", " << p2.y << endl;
    return 0;
}

https://ideone.com/DlJRfU

Is there any way to force the caller to initialize all struct members when using initializer lists? I want to ensure both x and y are set by the caller in this example.

I also want to allow designated initializers. Many of our structs are very long and that syntax adds much-needed clarity.

Answer 1

Something like what you want is possible in practice by making the aggregate meaninglessly templated:

template<class Bad>
struct PointImpl {
  int x=Bad(),y=Bad();
};
using Point=PointImpl<void>;

void f() {
  Point p0;             // error
  Point p1{1};          // error
  Point p2{.y=2};       // error
  Point p3{.x=1};       // error
  Point p4{.x=1,.y=2};  // OK
}

I won’t say I morally agree with this sort of “defeat the client” approach, and it’s not even clear from the standard that default member initializers should work like default arguments this way ([temp.inst]/3, /11, and /13 don’t mention them at all), but implementations agree that they do.

Answer 2

You could add a 2-parameter constructor. That would force initializing with both parameters.

Point(int x, int y) : x(x), y(y) {}
Point() = default; // to maintain behaviour of default constructor

then

Point p1{1, 2}; // OK
Point p1 {1}; // error: no matching constructor for initialization of 'Point'
Point p2 {.y=2}; // error: no matching constructor for initialization of 'Point'

Note that this means Point is no longer an aggregate, which means you can no longer use designated initializers.