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Conversion to reference

General and Gameplay Programming Programming

Started by sQuid August 19, 2003 02:36 AM

10 comments, last by sQuid 20 years, 8 months ago

sQuid

149

Author

August 19, 2003 02:36 AM

Hello again, I am using references to specify function arguments quite a bit lately under the (possibly misguided) impression that it is a converient way to avoid copying of large objects. Anyway this little program:


#include <iostream>

using namespace std;

float Foo(float& arg) {
    return arg;
}

int main(void) {

    float a = 2.0;

    cout << Foo(a) << endl;
    cout << Foo(Foo(a)) << endl;


}

will not compile with g++ 3.2.2 and spits out the error


[tmp]$ g++ test.cpp
test.cpp: In function `int main()'':
test.cpp:14: could not convert `Foo(float&)((&a))'' to `float&''
test.cpp:5: in passing argument 1 of `float Foo(float&)''

It compiles fine with VS C++ 7. Am I doing something wrong here?

glassJAw

128

August 19, 2003 02:50 AM

Weird. Looks like gcc thinks you''re trying to pass a function pointer.

Looks like legal C++ to me.

Wildfire

154

August 19, 2003 02:54 AM

The value returned by foo is only temporarily valid.

For example: You won't be able to pass an intermediate value to Foo ( like: Foo(10) ), because a reference must 'refer' to an object, which it cannot as '10' isn't stored anywhere (It is for the runtime of the function, but after that it's invalid).

For the same reason you can't pass the result returned by foo to another function that expects a reference to a variable. The result of foo isn't stored anywhere, it's temporary. Thus you shouldn't be able to refer to it.

Offending line: Foo(Foo(a))

Edit: Maybe VC7 allows this because it actually 'sees' that you're not modifying the passed parameter, or maybe it has a different 'expiration date' of returned values...

. Are you sure you did not return a float& in the VC code?

[edited by - Wildfire on August 19, 2003 3:58:20 AM]

How do I set my laser printer on stun?

VolkerG

151

August 19, 2003 03:10 AM

Try

float Foo(const float& arg) {    return arg;}

instead.

Wildfire

154

August 19, 2003 03:13 AM

Btw. unless you''re gonna modify the passed parameter you should do the following:

float Foo(const float& input) {...}

The const is there to show the user of your function that whatever (s)he''s passing to you won''t get modified. Plus, using a const float& does allow you to pass intermediate values.

float Foo(const float& x) {return(x);}
float Bar(float& x) {return(x);}

...

Foo(10); // No Problem
Bar(10); // A reference that does not refer to a const value, may not refere to a non L-value
(liberaly tranlated from the german compiler message

)

How do I set my laser printer on stun?

ironpoint

122

August 19, 2003 03:18 AM

This is a nonstandard extension that VC supports. GCC doesn''t support it because of religious reasons.

sQuid

149

Author

August 19, 2003 04:00 AM

OK, that clears things up a bit.

Thanks people.

fizban75

130

August 19, 2003 08:15 AM

Another thing. Unless you are changing the value of the parameter in the function (and want that value reflected in the caller), you don''t need to pass a reference for POD types (i.e. int, float, double, long, char, etc.)

In other words, "Foo(const float &f)" is completely unnecessary and you''re probably better off just doing "Foo(float f)."

Fruny

1,658

August 19, 2003 09:14 AM

quote:Original post by fizban75
you don''t need to pass a reference for POD types (i.e. int, float, double, long, char, etc.)

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

Thunder_Hawk

314

August 19, 2003 09:26 AM

What does POD stand for? Does his comment have any merit beyond the fact that those types are too small to gain much from passing by constant reference?

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