What does range-for loop exactly do?
Question
I'm working on a snake game program. I use a deque of Body in class Snake to represent a snake and of course Body is a struct I have defined. Here is part of the code:
struct Body { // one part of snake body
int x, y, direction;
Body() : x(0), y(0), direction(UP) { }
Body(int ix, int iy, int id) : x(ix), y(iy), direction(id) { }
};
class Snake {
protected:
std::deque<Body> body;
// other members
public:
auto begin()->std::deque<Body>::const_iterator const { return body.cbegin(); }
auto end()->std::deque<Body>::const_iterator const { return body.cend(); }
// other members
};
And in another function construct_random_food I need to generate a food and make sure it does not coincide with the snake. Here's the function definition:
Food construct_random_food(int gameSize, const Snake& snake) {
static std::random_device rd;
static std::uniform_int_distribution<> u(2, gameSize + 1);
static std::default_random_engine e(rd());
Food f;
while (1) {
f.x = u(e) * 2 - 1;
f.y = u(e);
bool coincide = 0;
for (const auto& bd : snake) // This causes an error.
if (bd.x == f.x && bd.y == f.y) {
coincide = 1; break;
}
if (!coincide) break;
}
return f;
}
An error is caused at the range-based for-loops line. It says that I'm trying to cast const Snake to Snake& (casting a low-level const away). I fix the problem by rewriting that line like this:
for (const auto& fd : const_cast<Snake&>(snake))
So I'm wondering what exactly a range-for do and what it needs. Does the error have anything to do with the begin() function in class Snake?
1 answers
solution1
6 ACCPTED 2017-08-11 16:47:36
The problem is that your begin and end functions are not const.
auto begin()->std::deque<Body>::const_iterator const { return body.cbegin(); }
// this applies to the return type ^^^^^
You've applied the const qualifier to the return type, not to the calling object. Put the const qualifier before the trailing return type.
auto begin() const ->std::deque<Body>::const_iterator { return body.cbegin(); }
You can see the correct order in which you should place function qualifiers here: http://en.cppreference.com/w/cpp/language/function
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