C++ – What _can_ I use as std::map keys

ckeymapstl

I have:

struct Coord
{
  int row, col ;

  bool operator<( const Coord& other ) const
  {
    return row < other.row && col < other.col ;
  }
} ;

I'm trying to create a map<Coord, Node*>, where you can look up a Node* by Coord.

The problem is, it has bugs. Lookups into the map<Coord, Node*> by Coord are returning the wrong ones.

I'm having difficulty figuring out if this is appropriate or not.

Wikipedia says, map [keys] requires a strict weak ordering. Have I done this wrong? Is there a way to make it work, or should keys for a map be simple values that can be "strictly ordered"?

Basically the question is what is required for a custom struct to work as a key for my std::map?

Best Answer

Yes you could very well have a problem with strict-weak ordering. Odds are its not working like you'd expect. Consider:

  bool operator<( const Coord& other ) const
  {
    return row < other.row && col < other.col ;
  }

obj1 (this) row: 2 col: 3

obj2 row: 3 col: 2

obj1 < obj2? => false

ok well then:

obj2 < obj1? => false

The only conclusion is that they must be equal (based on your < operator). Since this is a map, and keys are unique, both keys reselve to the same spot. This behavior may-or-may not be what you expect, but it sounds like it probably isn't.

What you need is to make a precedence between row/col so that < really works like you'd expect:

  bool operator<( const Coord& other ) const
  {
     // look at row first, if row is equal, check column.
     if (row < other.row)
     {
         return true;
     }
     else if (row == other.row)
     {
         return col < other.col ;
     }
     return false;
  }

Related Solutions

C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

C++ – What are the differences between a pointer variable and a reference variable in C++

Related Topic