Is there a way to call a function expecting a pointer without creating variable?

Question

I have this function call:

uint32_t func(uint32_t* a, uint32_t b)

I want to call it with an integer literal like this:

func         


        

Answer 1

A helper class:

struct int_ptr {
    int v;

    operator int *() { return &v; }
};

int foo(int *a, int b);

void bar()
{
    foo(int_ptr{0}, 0);
}

This results in a construction of a temporary int_ptr class, initializing its v member to 0. This gets passed as a parameter to a function that takes an int *, and int_ptr provides a suitable operator * method that passes the right pointer to the function.

This entire house of cards hinges on the fact that the int_ptr temporary exists until the end of the function call. You should pick a name for the helper class to underline that fact. If you always use it to pass a pointer to 0 to foo, then spell it out:

struct zero_value_to_foo {
    int v=0;

    operator int *() { return &v; }
};

int foo(int *a, int b);

void bar()
{

    foo(zero_value_to_foo{}, 0);
}

So that using it in other contexts will look to be very much out of place, i.e.

int *p=zero_value_to_foo{};

This compiles, but leaves you with a dangling pointer; but hopefully the "zero_value_to_foo" label gives a honking clue that something is wrong here.

Another little thing you can do to help yourself from misusing this is to use a ref qualifier for the operator:

struct zero_value_to_foo {
    int v=0;

    operator int *() && { return &v; }
};

With this,

foo(zero_value_to_foo{}, 0);

still compiles, but not this:

zero_value_to_foo zero{};

foo(zero, 0);

The more that can be done to make it difficult to use this except in the context is meant for, the fewer opportunities there are for bugs to creep by.