In Modern C++, is there a way to do safe navigation?
For example, instead of doing...
if (p && p->q && p->q->r)
p->q->r->DoSomething();
...having a succinct syntax by using some sort of short-circuiting smart pointer, or some other kind of syntax leveraging operator overloading, or something in the Standard C++ Library, or in Boost.
p?->q?->r?->DoSomething(); // C++ pseudo-code.
Context is C++17 in particular.
although it's 2022 now, there's still no language level support for this. a very close simulation I can figure out:
template <typename T, typename F>
auto operator->*(T&& t, F&& f) {
return f(std::forward<T>(t));
}
#define pcall(fn) \
[&](auto&& __p) { \
if constexpr (std::is_pointer_v<std::remove_reference_t<decltype(__p)>>) \
return __p ? __p->fn : decltype(__p->fn){}; \
else \
return __p ? __p.fn : decltype(__p.fn){}; \
}
struct A {
int v;
int foo(int a, int b) { return a + b + v; }
};
struct B {
A* getA() { return &a; }
A a{100};
};
struct C {
B* getB() { return &b; }
B b;
operator bool() { return true; }
};
int main(){
int v = 3;
C by_val;
int t1 = by_val->*pcall(getB())->*pcall(getA())->*pcall(foo(1, v));
C* by_ptr = &by_val;
int t2 = by_ptr->*pcall(getB())->*pcall(getA())->*pcall(foo(1, v));
}