Error: taking address of temporary [-fpermissive]

Question

I've been looking into this for a few hours, to no avail. Basically I have

struct rectangle {
    int x, y, w, h;
};

rectangle player::RegionCoordinates() // Region Coord
{
    rectangle temp;
    temp.x = colRegion.x + coordinates.x;
    temp.w = colRegion.w;
    temp.y = colRegion.y + coordinates.y;
    temp.h = colRegion.h;

    return temp;
}

// Collision detect function
bool IsCollision (rectangle * r1, rectangle * r2)
{
    if (r1->x < r2->x + r2->w &&
        r1->x + r1->w > r2->x &&
        r1->y < r2->y + r2->h &&
        r1->y + r1->h > r2->y) 
        {
            return true;
        }
    return false;
}

//blah blah main while loop
if (IsCollision(&player1.RegionCoordinates(), &stick1.RegionCoordinates())) //ERROR
{
    player1.score+=10;
    stick1.x = rand() % 600+1;
    stick1.y = rand() % 400+1;
    play_sample(pickup,128,128,1000,false);
}

Any ideas? I'm sure it's something really obvious but for the life of me I can't figure it out.

Boarish

Answer 1

RegionCoordinates() returns an object by value. This means a call to RegionCoordinates() returns a temporary instance of rectangle. As the error says, you're trying to take the address of this temporary object, which is not legal in C++.

Why does IsCollision() take pointers anyway? It would be more natural to take its parameters by const reference:

bool IsCollision (const rectangle &r1, const rectangle &r2) {
if (r1.x < r2.x + r2.w &&
    r1.x + r1.w > r2.x &&
    r1.y < r2.y + r2.h &&
    r1.y + r1.h > r2.y) {
        return true;
    }
        return false;
}
//blah blah main while loop
if (IsCollision(player1.RegionCoordinates(), stick1.RegionCoordinates())) //no error any more
{
player1.score+=10;
stick1.x = rand() % 600+1;
stick1.y = rand() % 400+1;
play_sample(pickup,128,128,1000,false);
}

Oboe

Answer 2

Since IsCollision takes a rectangle * and you are taking the address of the result here:

if (IsCollision(&player1.RegionCoordinates(), &stick1.RegionCoordinates()))

You most likely are returning a rectangle back from RegionCoordinates() which is a temporary variable since it will disappear after the if statement is done. If you assign the result of RegionCoordinates() to a variable then it will no longer be a temporary and you can then take the address of it:

rectangle r1 = player1.RegionCoordinates() ;
rectangle r2 = stick1.RegionCoordinates() ;
if (IsCollision(&r1, &r2))

Alternatively you could take the parameters as const references which would be the more C++ way of doing it:

bool IsCollision (const rectangle &r1, const rectangle  &r2)

Burlington

Is there spec about " If you assign the result of RegionCoordinates() to a variable then it will no longer be a temporary"? – Separative

Answer 3

Given the kind of error you are getting, I must assume RegionCoordinates() is returning an object by value, thus causing the creation of a temporary, and you are taking the address of that temporary.

The address-of operator requires an lvalue as its operand, but you are applying it to an rvalue (temporaries are rvalues).

You could do this (if you are not using C++11, replace auto with the type returned by RegionCoordinates):

auto rcPlayer1 = player1.RegionCoordinates();
auto rcStick1 = player1.RegionCoordinates();
if (IsCollision(&rcPlayer1, &rcStick1)) //ERROR
{
    player1.score+=10;
    stick1.x = rand() % 600+1;
    stick1.y = rand() % 400+1;
    play_sample(pickup,128,128,1000,false);
}

Alternatively, you can change IsCollision so that it accepts references rather than pointers, as suggested by Angew in his answer.

Calvano