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# overloading the comma operator

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I discovered today that the comma (,) operator can be overloaded in C++. This will solve the last remaining niggle in my vector library, which is assigning arbitry values to the vector using the = operator.
Vector3 myvec;

myvec = 1.2, globalvec.y, (float) rand ();


The above would get translated into
myvec.x = 1.2;
myvec.y = globalvec.y;
myvec.z = (float) rand ();


One problem though - how can I get the comma operator to stack with the previous one? I'm assuming some kind of proxy/flyweight object is involved, but I would like to keep this as lightweight as possible. Any ideas? High amounts of ++ are up for grabs ;).

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You can't overload the comma operator for built-in primitives like float. In order to get it to work, you'll need to 'kick start' the comma chain with a user defined type.

So you're code will look like:

myvec = Something , 1.2 , globalvec.y, (float)rand();

Of course if you do that you won't be saving much typing over just doing

myvec = Vector3(1.2, globalvec.y, (float)rand());

So with that said, do you still want to know how to get the comma operator to work with your assignment?

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Off hand, I'd say

struct Vector3{   float x, y, z;   AssignProxyY operator=(float x_)   {      x = x_;      return AssignProxyY(this);   }   struct AssignProxyZ   {      Vector3* vec3;      explicit AssignProxyZ(Vector3* vec3) : vec3(vec3) {}      void operator,(float z_)      {         vec3->z = z;      }   };      struct AssignProxyY   {      Vector3* vec3;      explicit AssignProxyY(Vector3* vec3) : vec3(vec3) {}      AssignProxyZ operator,(float y_)      {         vec3->y = y;         return AssignProxyZ(vec3);      }   };};

Use at your own risk.

EDIT - except that it won't work due to the low priority of operator=. Listen to SiCrane. a = b, c, d is a = (b, c, d), not (a = b), c, d

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EDIT: above posted just said what I was thinking

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Overloading the comma operator is a bad idea in general because the results are nonintuitive. And you typically have to surround the expression to be overloaded with parentheses to supercede the low precedence of the comma operator, e.g.,

DictionaryEntry* de = /* array */;
de[0] = ("allotrope", Lexicals.Noun, "a structurally distinct form of an element or molecule");
de[1] = ("amorphous", Lexicals.Adjective, "without form or shape");
de[2] = ("tergiversation", Lexicals.Noun, "the act of evasion or escape");
// ... etc.

To use this, your class definition would contain something like:

DictionaryEntry operator,(const std::string& word, const Lexicals& wordType, const std::string& wordDefinition)
{
DictionaryEntry t(word, wordType, wordDefinition);
return t;
}

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Surely assignment is higher in precedence than comma; for example the Blitz++ scientific computing library utilizes the comma operator for initializing things.

C operator precedence table

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yep, the comma is as low as they go.

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Mental: But that is a good thing: it means that Fruny's example works (with Blitz++, they do it the same way, by returning a proxy from the assignment operator).

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Quote:
 Original post by kSquaredOverloading the comma operator is a bad idea in general because the results are nonintuitive. And you typically have to surround the expression to be overloaded with parentheses to supercede the low precedence of the comma operator, e.g.,DictionaryEntry* de = /* array */;de[0] = ("allotrope", Lexicals.Noun, "a structurally distinct form of an element or molecule");de[1] = ("amorphous", Lexicals.Adjective, "without form or shape");de[2] = ("tergiversation", Lexicals.Noun, "the act of evasion or escape");// ... etc.To use this, your class definition would contain something like:DictionaryEntry operator,(const std::string& word, const Lexicals& wordType, const std::string& wordDefinition){   DictionaryEntry t(word, wordType, wordDefinition);   return t;}

The , operator cannot be overloaded to have three parameters unfortunately.

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Quote:
Original post by Mxz
Quote:
 Original post by kSquaredOverloading the comma operator is a bad idea in general because the results are nonintuitive. And you typically have to surround the expression to be overloaded with parentheses to supercede the low precedence of the comma operator, e.g.,DictionaryEntry* de = /* array */;de[0] = ("allotrope", Lexicals.Noun, "a structurally distinct form of an element or molecule");de[1] = ("amorphous", Lexicals.Adjective, "without form or shape");de[2] = ("tergiversation", Lexicals.Noun, "the act of evasion or escape");// ... etc.To use this, your class definition would contain something like:DictionaryEntry operator,(const std::string& word, const Lexicals& wordType, const std::string& wordDefinition){ DictionaryEntry t(word, wordType, wordDefinition); return t;}

The , operator cannot be overloaded to have three parameters unfortunately.

D'oh, of course you're right. The example is still valid, however; just remove one of the other two parameters.

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