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Hi. Suppose I want to implement a function like this:
void function_name(int length, short *array)
{
array = (short*)malloc(sizeof(short)*length);
....
....
}
This unfortunatelly doesn't work. To work I must place the allocation line (array = (short*)malloc(sizeof(short)*length);)
in my main(), just before calling the function.
Why is this happening?
Thanx
If you are trying to get the function to allocate memory for you, then you either need to pass the array pointer by value, or pass in a pointer to the pointer to the array.
Currently, you are passing the array pointer by reference, therefore the function will not be able to modify it.
Pass by value:
Or, pointer to pointer:Code:void function_name(int length, short* &array)
{
array = (short*)malloc(sizeof(short)*length);
....
....
}
ViggyCode:void function_name(int length, short **array)
{
*array = (short*)malloc(sizeof(short)*length);
....
....
}
int main()
{
short *myShort;
function_name(3, &myShort);
}
Thanx alot MrViggy.
It is being passed as:Quote:
Originally posted by MrViggy
Currently, you are passing the array pointer by reference, therefore the function will not be able to modify it.
Where do you see a reference? However it (the array pointer) is passed by reference in your solution.Code:short *array
By definition of the term "by reference" it should be modifiable if it is passed "by reference".
The terms "by reference" and "by value" can be very confusing when used for C/C++ in the manner they are used for other languages. Those terms have virtually no use in the context of the C language and a C++ reference (& operator) is different from a reference for other languages.
Of course, you probably meant to say that the array pointer was being passed "by value", and if so, then that might be an example of how confusing the terminology is when it is used for C/C++.
mrviggy, you are getting the definitions of "pass by value" and "pass by reference" backwards.
when you pass by value, the function gets a >copy< of the orginial value. thus, whatever changes are made to that var are not seen in the calling scope.
Sam: you have to be careful about what "by reference" and "by value" refer to, but the terms make perfect sense in C and C++.
Actually, it is possible to say that in C and C++ everything is passed by value, including references.Quote:
Originally posted by CornedBee
Sam: you have to be careful about what "by reference" and "by value" refer to, but the terms make perfect sense in C and C++.
No. In every language you can say that SOMETHING is passed by value, but the thing you want to refer to (like a variable) may not be passed by value.
The question is whether the method or the subject is fixed.
Please beleive me that not everyone agrees with that. Many people would say that in:String is passed by reference and many people would say that String is passed by value. As a pointer, String is passed by value, but as a string, String (using non-C++ terminology) is passed by reference.Code:void Func(char *String)
I think I had a similar discussion a while back (I think over a year ago) with Paul McKenzie.
Everything in C is passed by value. When a parameter is char *, the type is pointer to char. Since a pointer is an integral value (and it must be an integral value), it is no different than passing an int, double, float, etc. Now in the called function what you do with the value is another issue. If it's a pointer, the value can be dereferenced.
In C++, you have a pass-by-reference in which reference types are used. A reference is not a true "value", in that it is theoretically dimensionless -- it has no "size" (you can get the sizeof() what is being referred, but you can't get the sizeof() a reference), plus many other things that you can do with true value types that disqualifies references.
Basically, a reference is a C++ invention, and any compiler can come up with any way it wants when representing what a reference really is. For most compiler's, a reference does boil down to a pointer of some sort, but it isn't a hard, fast, rule. This is why it is legitimate to state that pass-by-reference and pass-by-value are two different and well-defined concepts when referring to C and C++. For other languages, then it may mean different things.
Regards,
Paul McKenzie
whats the point of picking things to death? why make such a big fuss over simple terminology that you learn in programming 101?Quote:
Originally posted by Sam Hobbs
Please beleive me that not everyone agrees with that. Many people would say that in:String is passed by reference and many people would say that String is passed by value. As a pointer, String is passed by value, but as a string, String (using non-C++ terminology) is passed by reference.Code:void Func(char *String)
I think I had a similar discussion a while back (I think over a year ago) with Paul McKenzie.
i don't see how or >why< the above c++ terminology should be debated.Code:void f(int x); //pass by value
void f(int &x); //pass by reference
void f(int *x); //pass by reference by pointer
void f(char *str); //the char pointer is passed by value, but the "string" is passed by reference. i'd say its "pass by reference by pointer"
-- C
Then ignore this thread. I assume you don't look at every post in every thread. If you are getting email notifications of replies to this thread, you can usubscribe to getting email notifications of replies to this thread.Quote:
Originally posted by clockworks
i don't see how or >why< the above c++ terminology should be debated.
For those of us that have seen thousands of posts in this forum, we know how important it is to be accurate in terminology. You might not get help when you ask for it if you are not clear and accurate about what you are asking.
In other languages, when an item is passed "by reference", it is modifiable. If it is not modifiable, it is passed "by value". The modifyability of a parameter is the inherent destinction that defines the terms.
In C++, beginners can get confused when something is passed as a "LPSTR". It is not a reference, right? Therefore the parameter is passed "by value" and not modifiable, right? Of course experienced C++ programmers understand, but this could be confusing for beginners.
Again, the terms "by value" and "by reference" are intended to differentiate non-modifiable and modifiable parameters (respectively), yet in C++ many parameters that are not references are often considered to be modifiable. When someone understands C++ well enough to understand that something is modifiable only if the parameter is a pointer to it, then they understand well enough that we don't need to use the terms "by value" and "by reference".
but the member who posted this thread could have got a very simply answer to his question. instead, he gets bombarded with discussion of ultra-technicalities by the seasoned veterans.Quote:
Originally posted by Sam Hobbs
Then ignore this thread. I assume you don't look at every post in every thread. If you are getting email notifications of replies to this thread, you can usubscribe to getting email notifications of replies to this thread.
For those of us that have seen thousands of posts in this forum, we know how important it is to be accurate in terminology. You might not get help when you ask for it if you are not clear and accurate about what you are asking.
i think my post outlined exactly what he needed to know as a novice, without being confused with discussions about "computer philosopy" instead of "computer science".
i agree with you that when you post questions, you must be as articulate as possible or you will receive no help or wrong help. but this discussion of articulation, i believe, is more confusing than helpful...especially for a newbie.
-- christopher
Note that the first reply above says "you either need to pass the array pointer by value, or pass in a pointer to the pointer to the array". However passing an array pointer by value is not what is needed; if the terminology being discussed here is used, then it would be that the array pointer needs to be passed by reference. The sample caaled "Pass by value" is good, but there is no need for the pass "by value" terminology, and instead of helping, it is confusing.Quote:
Originally posted by clockworks
but the member who posted this thread could have got a very simply answer to his question. instead, he gets bombarded with discussion of ultra-technicalities by the seasoned veterans.
Discussions such as this do happen in this forum. That's the way it works. Sometimes the conversations don't help the person that created the thread, but if they result in something that helps others, then it usually is worthwhile.Quote:
Originally posted by clockworks
but this discussion of articulation, i believe, is more confusing than helpful...especially for a newbie.
No...passing by reference does not mean that the parameter is modifiable. References can allow the parameter being changed or not. At least, if the term 'reference' is used in regard to the mechanism itself instead of a description of the general mechanism of passing arguments...Quote:
Originally posted by Sam Hobbs
In other languages, when an item is passed "by reference", it is modifiable. If it is not modifiable, it is passed "by value". The modifyability of a parameter is the inherent destinction that defines the terms.
...
Again, the terms "by value" and "by reference" are intended to differentiate non-modifiable and modifiable parameters (respectively), yet in C++ many parameters that are not references are often considered to be modifiable.
i can dig that...=)Quote:
Originally posted by Sam Hobbs
Discussions such as this do happen in this forum. That's the way it works. Sometimes the conversations don't help the person that created the thread, but if they result in something that helps others, then it usually is worthwhile.
-- C
I readily believe that.Quote:
Originally posted by Sam Hobbs
Please beleive me that not everyone agrees with that. Many people would say that in:String is passed by reference and many people would say that String is passed by value. As a pointer, String is passed by value, but as a string, String (using non-C++ terminology) is passed by reference.Code:void Func(char *String)
However, in your example the question is "what is String?". Is String the pointer to the character(s)? Or is String the character(s) themselves. In the first case, String is passed by value, in the second by reference. This is why I said that the terminology depends on what you refer to.
Paul is right, since references are not DEFINED to be addresses, they don't really have a value and you can't refer to them.
If you pass an argument by reference or value, there is a need for a clear definition of what we refer to. But as this very thread proves, that cannot always be read from the syntax. Maybe with a unified whitespace placement convention, but that would be the first time in history that there was such a thing. ;)
How many times do I need to say that I am refering to the terms "by value" and "by reference" as used in the context of other languages? How many times do I need to say that a C++ reference is different?Quote:
Originally posted by Andreas Masur
No...passing by reference does not mean that the parameter is modifiable. References can allow the parameter being changed or not. At least, if the term 'reference' is used in regard to the mechanism itself instead of a description of the general mechanism of passing arguments...
For example, see Argument Passing ByVal and ByRef. It says:Quote:
Passing an argument by value means the procedure cannot modify the contents of the variable element in the calling code underlying the argument. Passing by reference allows the procedure to modify the contents in the same way that the calling code itself can.
Yes.Quote:
Originally posted by CornedBee
If you pass an argument by reference or value, there is a need for a clear definition of what we refer to.
The only significant point I wanted to make is that in the context of C++ the terms are not defined clearly enough to be useful. In other languages, such as VB, they are clearly defined. In VB, "by value" is the ByVal keyword and "by reference" is the ByRef keyword.
Exactly.