C++ redefinition error for two different function signatures

Question

I have a type, morton_id_t, which is a 64-bit number. I have a second type which is a concatenation of the bits of a Morton ID (morton_id_t) with several bits that contain information about the level in a tree:

typedef uint64_t morton_id_t;
typedef uint64_t morton_id_lvl_t;

#define BITS (sizeof(morton_id_lvl_t) * 8)

// number of bits to store the level
#define LEVEL_BITS 6
#define LEVEL_MASK ((1 << LEVEL_BITS) - 1)

// utility functions to pull the level or morton id out of a morton_id_lvl_t
lvl_t level(morton_id_lvl_t n);
morton_id_t morton_id(morton_id_lvl_t n);

// this just strips off the level bits using the `morton_id` function...
bool isAncestor(morton_id_lvl_t ancestor, morton_id_lvl_t descendent);

// ...and calls this
bool isAncestor(morton_id_t ancestor, morton_id_t descendent);

morton_id_t is just typedef'ed from uint64_t even though I only need the 64 - 6 level bits.

The compiler throws an error saying that the second isAncestor definition redefines the first, even though the type signatures are different.

How can I define two different isAncestor functions, one of which takes morton_id_lvl_t's and the other takes morton_id_t's?

Do I need to use classes instead of typedef's?

Use a strong typedef if you want that behaviour (Boost has one). — chris, Apr 03 '14 at 20:16

score 5 · Answer 1 · answered Apr 03 '14 at 20:15

5

morton_id_t and morton_id_lvl_t are the same, so the isAncestor signatures are the same too.

A typedef does not create a new type, but rather an alias for an existing one.

answered Apr 03 '14 at 20:15

juanchopanza

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that tells me why it's not working, but how do i fix it? – aaronstacy Apr 03 '14 at 20:34
@aaronstacy Do these functions really need to have the same name? – juanchopanza Apr 03 '14 at 20:40
of course they don't need to. i'd just prefer them to have the same name because i find it, combined w/ their type signature, communicates their purpose well. – aaronstacy Apr 04 '14 at 12:52

score 1 · Accepted Answer · answered Apr 03 '14 at 20:44

A typedef only introduces a new name for a type, it doesn't create a new type, unfortunately.

You could wrap them in structs to turn them into different types:

struct morton_id_t
{
    uint64_t id;
};

struct morton_id_lvl_t
{
    uint64_t id_lvl;
};

or possibly (not 100% sure that it's valid):

struct morton_id_lvl_t
{
    uint64_t id:58;
    uint64_t level:6;
};

This would also provide type safety for your conversion functions.

C++ redefinition error for two different function signatures

2 Answers2