What real use does a double pointer have? [closed]

Answer 1

In C++, if you want to pass a pointer as an out or in/out parameter, you pass it by reference:

int x;
void f(int *&p) { p = &x; }

But, a reference can't ("legally") be nullptr, so, if the pointer is optional, you need a pointer to a pointer:

void g(int **p) { if (p) *p = &x; }

Sure, since C++17 you have std::optional, but, the "double pointer" has been idiomatic C/C++ code for many decades, so should be OK. Also, the usage is not so nice, you either:

void h(std::optional &p) { if (p) *p = &x) }

which is kind of ugly at the call site, unless you already have a std::optional, or:

void u(std::optional> p) { if (p) p->get() = &x; }

which is not so nice in itself.

Also, some might argue that g is nicer to read at the call site:

f(p);
g(&p); // `&` indicates that `p` might change, to some folks