Chaining multiple function calls inside an if statement in C++ 17 - c++

Question

Let's say I have this as current code:
if (GetFlag(...)) {
if (auto x = GetX(...)) {
if (auto y = GetY(...)) {
...
}
}
}
and that the method calls and variable definitions need to be made in this order for both correctness and performance reasons (we don't want to calculate x if GetFlag(...) returns false, and we don't want to calculate y if x is nullptr).
Is there a way to refactor this into a "one-liner"? The closest I got to actually compiling would be
if (auto x = GetX(...); auto x = GetX(...) && GetFlag(...)) { ... }
but this does not preserve the order of operations.
Thanks!

Answer 1

You can extract the conditionals into a separate function that returns a tuple, and structured bind in the if
std::tuple<bool, std::optional<X>, std::optional<Y>> getFlagsXY()
{
if (!getFlags())
{
return { false };
}
if (auto x = getX(); !x)
{
return { true, std::move(x) };
}
else
{
return { true, std::move(x), getY() };
}
}
if (auto [f, x, y] = getFlagsXY(); f && *x && *y) { ... }

Answer 2

And yes it is possible.
If statements stop processing terms once they see which branch is going to be taken. Try changing the values of GetFlag, GetX to see when the body of the if gets called.
#include <iostream>
bool GetFlag()
{
std::cout << "GetFlag\n";
return true;
}
int GetX()
{
std::cout << "GetX\n";
return 0;
}
int GetY()
{
std::cout << "GetY\n";
return 3;
}
int main()
{
int x;
int y;
if (GetFlag() && (x = GetX()) && (y = GetY()))
{
std::cout << "Do " << x << ", " << y;
}
return 0;
}